LECTURE IX THE PHYSIOLOGICAL POSITION AND DOMINANCE OF THE BRAIN

We may now attempt to gather from the various notions, however fragmentary, that have occupied us, some general conception of the neural architecture of an animal as a whole; though of course only in its motor aspect, for its truly sensorial aspects we have hardly had before us. The problem is too difficult for me to expect much success. Yet it will repay us if from the attempt we glean something at least of one cardinal feature of the scheme, namely, the dominance attained by one limited set of neural segments, the brain, over all the rest.

We must allow ourselves at certain points some repetition of considerations already urged, in order to draw from them now in new juxtaposition some further significance.

The primitive reflex-arc. — If we seek for a reflex-arc of simplest construction it is true we find in some unicellular organisms, e. g. Vorticella, a mechanism which resembles a nervous arc and is quite simple. This mechanism, composed from a single cell, shows differentiation into three parts respectively, — receptive, conductive, and effective. In Vorticella the receptive element is the ciliated peristome; a stimulus reaching these cilia at the free end of the cell excites contraction of the myoid filament at the fixed end of the cell. Similarly with the individual cells of Poteriodendron (Verworn). In multicellular organisms of low organization like mechanisms occur. In Actinia there are ectoderm cells which have externally a receptive hairlet and internally a contractile fibre, and this latter contracts when the receptive hairlet is stimulated.

In view of such cases it might have seemed likely that in more highly developed organisms examples would have been forthcoming in which the differentiation of the parts of a single cell would have advanced further still and produced something yet more akin to a simple reflex-arc such as is considered typical of the true nervous system itself. That expectation is not realized, What we find as the simplest arc in the organisms which possess a true nervous system is that the conductor mediating between receptor and effector is itself a separate cell intercalated between a receptive cell and an effector cell. At each end this separate conductive cell breaks up into branches. The branching at the receptive end places it in communication not with one but with several receptor cells. This must allow stimuli at a number of receptive points to combine by summation to a conjoint effect. By this means the threshold of reaction will be lowered and the organism in that respect become more sensitively reactive to the environment. At the deep, i. e. effector, end the branching of the conductive stem places it in touch not with one effective cell but with many. Thus, again, there must result lowering of the threshold — of what we may term the effective threshold. The contraction of a single muscle-fibre in a muscle is practically ineffective where the resistance and mass of the muscle and its load are great as compared with the power of a single muscle-fibre. But by its branching the motor neurone obtains hold of many muscle-fibres. This must tend to lower the effective threshold of reaction, and thus again the organism is rendered more delicately responsive to stimulation by its environment.

But — and it is a striking fact — we do not know of any reflexarc in which in fact the nervous conductor connecting receptor to effector is formed from end to end of one single neurone. The length of the conductor seems always to include at least two neurones in succession. A moment’s reflection reminds us that such arrangements as Vorticella, Poteriodendron, and the neuro-muscular cells of Hydra and Actinia do not exhibit the germ of a feature that we have already considered fundamental in the construction of the reflex nervous system. The cases cited do not exhibit even in germ the co-ordinative mechanism which is attained by the principle of the common path. Such cases confine each effector to the use of one receptor only, and confine each receptor to the use of one effector only. But we saw that a great principle in the plan of the nervous system is that an effector shall be at the behest of many receptors, and that one receptor shall be able to employ many effectors. We saw further in respect to this that there are two conditions which the nervous system satisfies. One is that the effector is at the behest of various receptors which can use it simultaneously and use it harmoniously all in more or less the same way. Thus an advantage accrues in that their reactions sum, even though the receptors may be of different modality; and by summation the threshold is lowered and the organism more sensitized to the environment. This arrangement cannot be obtained by the unicellular mechanisms instanced above. It can only be obtained by the formation of a common path, and the formation of a common path can only be rendered possible by having a conductor of pluricellular length. And there is another condition which the nervous system satisfies The unicellular reflex-arc — if reflex-arc it can be called — not only admits no opportunity for pluricellular summation but also none for the second function of the jointed reflex-arc of pluricellular length, namely “interference.” In animals of complex organization the activity of one effector organ may interfere with the function of another, e. g., in the case of muscles which when contracting pull in opposite directions at the same lever. We have seen how this wasteful confusion is avoided by one receptor having power not only to throw a particular effector into action but also to throw the opposed effector out of action. We saw that this action it exercises not peripherally but within the nervous system, at the entrance to a common path. The unicellular reflex-arc allows no common path. It lacks, therefore, the mechanism which renders possible the two great co-ordinative processes of pluri-receptive summation and of interference. Without these the nervous system is shorn of its chief powers to integrate a set of organs or an organism.

It is therefore a significant thing that in the nervous system there is not only no instance of the reflex triune — receptor, conductor, and effector — being formed of one cell only, but also no indubitable instance where the middle link, the conductor, is even itself formed of one cell (one neurone) only. In other words, we know of no instance in the nervous system of a reflex-arc so constructed as not to include a junction between one neurone and another neurone. And the rule is apparently always that at such junctions not only does one neurone meet another, but several neurones converge upon another and make of the latter a common path.

The diffuse nervous system, the gray-centred nervous system, the central nervous system a part of the latter. The term “nerve-centre” is sometimes abused, yet seems in several ways apt. A keynote regarding that part of the nervous system which is termed “the central” seems that it is wholly pieced together into one system. The nervous system in its simplest forms is diffuse — a number of scattered mechanisms performing merely local operations with much autonomy save that they have communication with their immediate neighbors across near boundaries. The co-ordination effected by the diffuse nervous system is not adapted to compass the quickly combined action of distant parts. It is slow, and it throws en route the effectors of intermediate regions into action. It is ill suited, therefore, to produce the integration of a large and complex individual as a whole, or even to integrate large differentiated portions of an individual. Yet the co-ordination it brings about in its own local field may be strikingly effective. A co-adjustment though simple and restricted may be not less perfect than one involving wide and complex neural mechanism. The co-ordination of a peristaltic movement of the bowel is, as shown by Bayliss and Starling, even when managed exclusively by the local diffuse nervous system, capable of the perfect taxis of two muscular coats arranged antagonistically in the viscus. It directs a relaxation of the one co-ordinately with a contraction of the other; it exhibits a primitive but none the less perfect form of “reciprocal innervation.”

This diffuse system seems the only one in such an organism as Medusa. But in higher animals a system of longer direct connections is developed. And this latter is “synaptic,” that is, possesses the adjustable junctions which belong characteristically to “gray matter.” This synaptic system co-existing with the diffuse in various places dominates the latter. Thus it controls and overseers the actions of the local nervous system of the viscera, and heart, and blood-vessels, which even in the highest animal forms remain diffuse.

The synaptic nervous system has developed as its distinctive feature a central organ, a so-called central nervous system; it is through this that it brings into rapport one with another widely distant organs of the body, including the various portions of the diffuse nervous system itself.

That portion of the synaptic system which is termed “central” is the portion where the nervous paths from the various peripheral organs meet and establish paths in common, i. e. “common paths.” It is therefore in accord with expectation that we find the organ in which this meeting occurs situated fairly midway among them all, i. e. centrally. In bilaterally symmetrical animals this organ would be expected to lie where it does, namely, equidistant from the two lateral surfaces of the animal, and to exhibit as it does, laterally symmetrical halves united by a number of nervous cross ties bridging the median line. This central nervous organ contains almost all the junctions existent between the multitudinous conducting arcs. In it the afferent paths from receptor-organs become connected with the efferent paths of effector-organs, not only those adjacent to their own receptors but, through “internuncial” (J. Hunter, 1778) paths, with efferent paths to effector-organs remote. This central “exchange” organ is therefore well called the central nervous system. In the higher Invertebrata it is known as the longitudinal nerve-cord with ganglia, supraoesophageal, suboesophageal, etc.; in Vertebrata it is known as the spinal cord and brain. Under these different anatomical names the same physiological organ is designated. It would be more convenient for the biologist were one general term for it in use. We have seen that it is not merely a meeting place where afferent paths conjoin with efferent, but is, in virtue of its physiological properties, an organ of reflex reinforcements and interferences, and of refractory phases, and shifts of connective pattern; that it is, in short, an organ of co-ordination in which from a concourse of multitudinous excitations there result orderly acts, reactions adapted to the needs of the organism, and that these reactions occur in arrangements (patterns) marked by absence of confusion, and proceed in sequences likewise free from confusion.

By the development of these powers the synaptic system with its central organ is adapted to more speedy, wide, and delicate co-ordinations than the diffuse nervous system allows. Out of this potentiality for organizing complex integration there is evolved in the synaptic nervous system a functional grading of its reflex arcs and centres. Thus, with allied reflexes, the mechanism of the common path knits together by plurireceptive summation not only the separate individual stimuli of similar kind, e. g. tangoreceptive or photoreceptive received from some agent as this latter becomes prepotent in the environment; but it knits together separate stimuli of even wholly different receptive species. C. J. Herrick has shown that in Ameiurus nebulosus (cat-fish) the reaction of the animal to stimulation of the barblets by meat is a reaction to a twofold stimulus, a chemical and a mechanical, and he finds that these two reactions mutually reinforce. Nagel reports a similar case with the tentacle of the Actinian, Aiptasis saxicola. v. Uexkiill finds that the Giftzangen of Echinus acutus react only when a chemical and a mechanical stimulus are combined. The several qualitatively different properties of an object which is acting as stimulus are thus combined and reinforce each other in eliciting appropriate reaction. By this summation reflex complication in Herbart’s sense is made possible. A touchstone for rank of a centre in this neural hierarchy is the degree to which paths from separate loci and of different receptive modality are confluent thither. Indicative of high rank is such functional position as relieves from “local work” and involves general responsibility, e. g. for a series of segments or for the whole body. The “three levels,” of Hughlings Jackson is an expressive figure of this grading of rank in nerve-centres.

Integrative action of the nervous system in the segment and in the segmental series. In animal organisms of any considerable complexity a division of the body into segments, metameres, is widely found. By the occurrence of separating constrictions or sepiments, or through the regular repetition of appendicular structures, subdivisions of the body are established which severally possess analogues of functions possessed more or less similarly by the other subdivisions but also severally possess functional unity. Such is this functional unity and completeness that in some instances a metamere comes to be independent of the total organism, and able to lead a separate existence. The nervous system it is which largely gives functional solidarity to the composite collection of unit lives and organs composing the individual metamere. Further, the linkage of the several metameres into one functional whole is largely of nervous nature. The integrative function of the nervous system is seen to perfection in the welding together of metameres into the unity of an animal individual. The kind of nervous system employed for this is the synaptic system. Although the nerve-net system is retained even in the highest vertebrates, it is then confined to unsegmentally arranged musculature, e. g. visceral and vascular. In the skeletal musculature, where segmental arrangement holds, the nervous system is synaptic. It is not surprising therefore that in metameric animals the nervous system, especially its synaptic part, should strikingly exhibit that metamerism.

Various schemes of metamerism have been evolved. Where it is radiate so that each segment bears exactly similar relations to the common axis and to the other segments, the opportunity for dominance of one segment over the rest is slight. The conditions of life for each segment are practically those which are the average for all. The mouth, for instance, lies equidistant from them all. Evolution toward higher differentiation of the whole metameric individual and toward more intricate welding of its parts into one, is at a disadvantage in these radiate forms as compared with its opportunity in the great groups of Arthropoda and Vertebrata where the metameres arc ranged serially along a single axis, the longitudinal axis of the organism. With fore and aft arrangement of its segments the animal body has its first opportunity for really high differentiation. Certain of the segments of necessity lie nearer to the mouth than do others; moreover certain segments come to habitually lead, that is to say go foremost, during the animal’s active locomotion.

In the integrating function of the nervous system a segmental arrangement of its functions is frequently apparent. It makes itself felt in two ways. Firstly, the various separate and different elements of the segment are knit together by nervous ties. Secondly, where kindred functions are exercised in successive segments, so that throughout a series of segments one set of organs forms a more or less functionally homogeneous system, these organs are combined by interrelated nervous arcs. But particular systems of organs common to all or many metameres of an individual present special differentiation of their function in particular metameres. In this manner the organism is built up of component segments possessing resemblance one to another, but presenting also specializations peculiar to certain segments. Hence the segmental arrangement forms a convenient basis not merely for anatomical but for physiological description. And in dealing with the special problems of integration by the nervous system, especially those of the synaptic nervous system, analysis can employ two co-ordinate sets of descriptive factors — one the segment, the other the line of organs of analogous function scattered along the series of segments. In the two great animal groups just mentioned the latter ordinate is longitudinally extended, while the individual segment is extended transversely. The analysis thus proceeds formally somewhat in the same way as the analysis of a plane figure by rectangular co-ordinates.

The receptive fields. The central nervous system, though divisible into separate mechanisms, is yet one single harmoniously acting although complex whole. To analyze its action we turn to the receptor-organs, for to them is traceable the initiation of the reactions of the centres. These organs fall naturally into three main groups, distributed in three main fields, each field being differently circumstanced.

Multicellular animals regarded broadly throughout a vast range of animal types are cellular masses presenting to the environment a surface sheet of cells, and under that a cellular bulk more or less screened from the environment by the surface sheet. Many of the agencies by which the environment acts on the organism do not penetrate to the deep cells inside. Bedded in the surface sheet are numbers of receptor cells constituted in adaptation to the stimuli delivered by environmental agencies. The underlying tissues devoid of these receptors are not devoid of all receptor-organs; they have other kinds apparently specific to them. Some agencies act not only at the surface of the organism but penetratively through its mass. Of these there are for some apparently no receptors adapted, for instance, none for the Rontgen rays. For others of more usual occurrence receptors are adapted. The most important of these deep adequate agents seems to be mass acting in the mode of weight and mechanical inertia involving mechanical stresses and mechanical strain. Moreover, the organism, like the world surrounding it, is a field of ceaseless change, where internal energy is continually being liberated, whence chemical, thermal, mechanical, and electrical effects appear. It is a microcosm in which forces are at work as in the macrocosm around. In its depths lie receptor-organs adapted consonantly with the changes going on in the microcosm itself, particularly in its muscles and their accessory apparatus (tendons, joints, walls of blood-vessels, and the like).

There exist, therefore, two primary distributions of the receptor-organs, and each constitutes a field in certain respects fundamentally different from the other. The deep field we have called the proprio-ceptive field, because its stimuli are, properly speaking, events in the microcosm itself, and because that circumstance has important bearing upon the service of its receptors to the organism.

Richness of the extero-ceptive field in receptors; comparative poverty of the intero-ceptive. The surface receptive field is again subdivisible. It presents two divisions. Of these one lies freely open to the numberless vicissitudes and agencies of the environment. That is to say, it is co-extensive with the so-called external surface of the animal. This subdivision may be termed the extero-ceptive field.

But the animal has another surface, its so-called internal, usually alimentary in function. This, though in contact with the environment, lies however less freely open to it. It is partly screened by the organism itself. For purposes of retaining food, digesting and absorbing it, an arrangement of common occurrence in animal forms is that a part of the free surface is deeply recessed. In this recess a fraction of the environment is more or less surrounded by the organism itself. Into that sequestered nook the organism by appropriate reactions gathers morsels of environmental material whence by chemical action and by absorption it draws nutriment. This surface of the animal may be termed the intero-ceptive. At its ingress several species of receptors are met with whose “adequate” stimuli are chemical (e. g. taste organs). Lining this digestive chamber, this kitchen, the intero-ceptive surface is adapted to chemical agencies to a degree such as it exhibits nowhere else. Comparatively little is yet known of the receptor-organs of this surface, though we may suppose that they exhibit refined adaptations. But the body-surface in this recess, though possessed of certain receptors specific to it, is sparsely endowed as contrasted with that remainder of the surface (the extero-ceptive surface) lying open fully to the influences of the great outer environment. The afferent nerve-fibres in the sympathetic system as judged by their number in the white rami are comparatively few; Warrington’s²⁹⁸ recent observations show this conclusively. The poverty of afferent paths from the intero-ceptive field is broadly indicated by the fact that we know no wholly afferent nerve-trunk in the sympathetic (Langley’s “autonomic”) system, though such are common enough in the nervous system subserving the exteroceptive arcs; and that in the latter system we know no wholly efferent nerve-trunk, whereas in the sympathetic such exist, e. g. the cervical sympathetic trunk.

The extero-ceptive field far exceeds the intero-ceptive in its wealth of receptor-organs. This seems inevitable, for it is the extero-ceptive surface, facing outward on the general environment, that feels and has felt for countless ages the full stream of the varied agencies forever pouring upon it from the outside world. Mere enumeration of the different species of receptor-organs recognizable in it suffices to illustrate the importance of this great field. It contains specific receptors adapted to mechanical contact, cold and warmth, light, sound, and agencies inflicting injury (noxa). Almost all these species of receptors are distributed to the extero-ceptive field exclusively; they are not known to exist in the intero-ceptive or in the proprio-ceptive fields.

It is an instructive exercise to try to classify the stimuli adequate for the receptors of the extero-ceptive field. Each animal has experience only of those qualities of the environment which as stimuli excite its receptors; it analyzes its environment in terms of them exclusively. Doubtless certain stimuli causing reactions in other animals are imperceptible to man; and in a large number of cases his reactions are different from theirs. Hence it is impossible for man to conceive the world in terms more than partially equivalent to those of other animals. Humanly, the classification of adequate stimuli can be made with various departments of natural knowledge as its basis. Physics and chemistry can be taken as basis usefully in a number of cases where the sources of stimulation are known to those more exact branches of experimental science. But in several ways a physico-chemical scheme of classification of stimuli lacks significance for physiology. Thus, in the case of the noci-ceptive organs of the skin, those receptors — probably naked nerve-endings — are non-selective in the meaning that they are excitable by physical and chemical stimuli of diverse kind, radiant, mechanical, acid, alkaline, electrical, and so on, so that a classification according to mode of exciting energy on the one hand fails to differentiate them from each of a number of more specialized other groups (tango-receptors, chemo-receptors, etc.) from which biologically they are quite different, and on the other hand that classification apportions them, though physiologically a single group, to a whole series of different classes. A physiological classification deals with them more satisfactorily. Physiological criteria can be applied which at once separate them from other receptors and yet show their affinity one to another. Thus, physiologically the stimulus which excites these end-organs must, whatever its physical or chemical nature, possess, in order to stimulate them, the quality of tending to do immediate harm to the skin. Further, the reflex they excite (i) is prepotent; (ii) tends to protect the threatened part by escape or defence; (iii) is imperative; and (iv) if we include psychical evidence and judge by analogy from introspection, is accompanied by pain.

Here what we may call the physiological scheme of classification proves the more useful at present. And similarly it proves useful with a group of stimuli that may be termed “distance-stimuli,” to which we must turn presently. The key to the physiological classification lies in the reaction which is produced. But the physico-chemical basis of classification also has its uses, and especially with those manifold receptors of the extero-ceptive field which possess highly developed accessory structures that render them selectively receptive — and among these are some of the most highly adapted and important receptors, e. g. photo-receptors, possessed by the organism. It is to the extero-ceptive field that these belong.

Nervous integration of the segment. The edifice of the whole central nervous system is reared upon two neurones, — the afferent root-cell and the efferent root-cell. These form the pillars of a fundamental reflex arch. And on the junction between these two are superposed and functionally set, mediately or immediately, all the other neural arcs, even those of the cortex of the cerebrum itself. The private receptor paths and the common effector paths are in the Chordata gathered up in a single nerve-trunk for each segment. Close to the central nervous organ, however, there occurs in the segmental nerves of many Vertebrates a cleavage of the private receptor paths from the common effector paths. A dorsal spinal nerve of centripetal conduction and a ventral of centrifugal conduction results. Among the afferent root-cells (afferent spinal root of Vertebrates) in each segment are quota from the extero-ceptive (cutaneous) and from the proprio-ceptive (deep) fields. In many segments there is a third quotum, intero-ceptive, from the visceral field. This visceral constituent of the spinal ganglion is not present in all segments and is probably, even in those segments in which it is present, numerically the weakest of the three components. Cranial, caudal, and other segments exist, therefore, in which the total afferent nerve of the segment is extero-ceptive and proprio-ceptive but not intero-ceptive. In the remaining segments it is intero-ceptive as well as exteroceptive and proprio-ceptive, and in these its function is therefore fundamentally threefold.

The efferent segmental nerve conversely radiates outward from the central end of the afferent nerve and the central nervous organ to the various effector organs at the surface and in the depth of the segment. Function does not, however, strictly respect the ancestral boundaries of segments. Among the efferent fibres in the ventral root are a number that extend quite beyond the boundaries of the segment in which the spinal root is placed. These pass to the viscera and muscles of the skin. They embouch not directly into their effector organs, e. g. intestinal muscle-wall, pilomotor muscle, etc., but into ganglia of the sympathetic system. In these ganglia, although not gray matter in the same sense as spinal cord and brain, axone-endings, perikarya, and dendrites are nevertheless found. By its distribution to the cells in such a ganglion and by being distributed in many cases to more than one such ganglion, a single constituent efferent path in the ventral spinal root obtains access to a very large number of effector organs. These ganglia seem, therefore, mechanisms for the distribution of nerve-impulses. We have seen (p. 310) how by such widening of distribution the threshold of effective reaction is lowered. But though adapted for distribution of nerve-impulses there is no evidence that these ganglia can serve for the regulation of them in the same sense as does the gray matter of the spinal cord with its synapses of variable resistance and connection. Prominent among the integrating connections intrinsic to each segment itself are conducting paths from the extero-ceptive field to the “final common paths” for the skeletal musculature. Thus, in the mammal we laid it down (Lect. V, p. 157) as a general rule that “for each afferent root there exists in immediate proximity to its own place of entrance into the cord, i. e. in its own segment, a reflex motor path from skin to muscle of as low resistance as any open to it anywhere.”

The extero-ceptive arcs appear in most segments less closely connected with the visceral musculature than with the skeletal musculature. The intero-ceptive arcs appear in most segments less closely connected with the skeletal musculature than with the visceral. In physiological parlance a resistance to conduction seems intercalated between the two. But both exteroceptive and intero-ceptive fields easily influence through their nervous arcs the musculature of the blood vascular organs. So also do the receptors of the proprio-ceptive field itself; and these latter arc in particularly close touch with the skeletal musculature, exerting tonic influence on it. In certain segments these general relations are modified in special ways. Thus, in those segments where the intero-ceptive and extero-ceptive fields conjoin, e. g. at the mouth and the cloaca, closer nervous connections exist between the intero-ceptive arcs and the skeletal musculature, and conversely between the extero-ceptive arcs and the visceral musculature. Thus stimuli acting on the pharyngeal receptors evoke or inhibit activity of skeletal muscles subserving respiration and deglutition; stimuli to the cloacal mucosa evoke movements of the caudal skeletal muscles; and so forth.

It is not merely specific difference between the receptors of the extero-ceptive field and those of the intero-ceptive which brings the former into closer relationship with the skeletal musculature. Receptors of the one and the same species, if they lie in the extero-ceptive field, work skeletal musculature; if they lie in the intero-ceptive, work visceral musculature. Thus the chemo-receptors on the outer surface of the head (gustatory of the barblets of fish) excite reflexes which move the body round, bringing the mouth to the morsel; while the similar chemo-receptors within the mouth excite reflex swallowing without outward movement of the animal (C. J. Herrick).

Receptors of the same specific system, where they lie close together, mutually reinforce reaction. On the contrary, where members of two different systems lie close together, e. g. tango-receptor and noci-ceptor, in one and the same piece of skin, they, as mentioned above, often have conflicting mutual relation. One relationship between receptor arcs of the same species may be particularly noted. Receptors symmetrically placed on opposite sides of the segment, especially if distant from the median plane, excite reactions which mutually “conflict.” Thus, when a noci-ceptor is stimulated on the right side of the tail of the spinal dog or cat or lizard, the reaction moves the organ to the left. The symmetrical receptor on the left side does the converse. The two reactions thus conflict. And the like holds true for the many right and left symmetrical receptors which initiate exactly converse reactions.

But a group of special cases is formed by reactions initiated from receptors distributed at or near to the median line. Stimulation of such a small group of receptors at the median line in many cases evokes a bilateral movement which is symmetrical, e. g. a touch on the decerebrate frog’s lip in the median line causes both fore limbs to sweep forward synchronously over the spot. The median overlap of the distribution of the afferent fibres of the dorsal spinal roots may be connected with this.

Special refinements of the receptors of the “leading” segments. As the receptors that are excitable by the various adequate agencies, e.g. mechanical impact, noxa, radiant energy, chemical solutions, etc., are traced along the series of segments, it is found that in one region of the longitudinal segmental series remarkable developments exist.

In motile animals constituted of segments ranged along a single axis, e. g. Vertebrata, when locomotion of the animal goes on, it proceeds for the most part along a line continuous with the long axis of the animal itself, and more frequently in one direction of that line than in the other. The animal’s locomotor appendages and their musculature are favorably adapted for locomotion in that habitual direction. In the animal’s progression certain of its segments therefore lead. The receptors of these leading segments predominate in the motor taxis of the animal. They are specially developed. Thus, in the earthworm, while all parts of the external surface are responsive to light, the directive influence of light is greatest at the anterior end of the animal. The leading segments are exposed to external influences more than are the rest. Not only do they receive more stimuli, meet more “objects” demanding pursuit or avoidance, but it is they which usually first encounter the agents beneficial or hurtful of the environment as related to the individual. Pre-eminent advantage accrues if the receptors of these leading segments react sensitively and differentially to the agencies of the environment. And it is in these leading segments that remarkable developments of the receptors, especially those of the extero-ceptive field, arise. Some of them are specialized in such degree as almost obscures their fundamental affinity to others distributed in other segments. Thus, among the system of receptors for which radiation is the adequate agent, there are developed in one of the leading segments a certain group, the retinal, particularly and solely, and extraordinarily highly, amenable to radiations of a certain limited range of wave-length, These are the photo-receptors, for which light and only light, e. g. not heat, is the adequate stimulus. In like manner a certain group belonging to the system receptive of mechanical impacts attains such susceptibility for these as to react to the vibrations of water and air that constitute physical sounds. The retina is thus a group of glorified “warm-spots,” the cochlea a group of glorified “touch-spots.” Again, a group belonging to the system adapted to chemical stimuli reach in one of the leading segments such a pitch of delicacy that particles in quantity unweighable by the chemist, emanating from substances called odorous, excite reaction from them.

The refined receptors of the leading segments are “distance-receptors.” The after-coming segments form a motor train actuated chiefly by the “distance-receptors.” It is in the leading segments that we find the “distance-receptors.” For so may be called the receptors which react to objects at a distance. These are the same receptors which, acting as sense-organs, initiate sensations having the psychical quality termed projicience. The receptororgans adapted to odors, light, and sound, though stimulated by the external matter in direct contact with them, — as the vibrating ether, the vibrating water or air, or odorous particles, — yet generate reactions which show “adaptation,” e. g. in direction of movements, etc., to the environmental objects at a distance, the sources of those changes impinging on and acting as stimuli at the organism’s surface. We know that in ourselves sensations initiated through these receptors are forthwith “projected” into the world outside the “material me.” The projicience refers them, without elaboration by any reasoned mental process, to directions and distances in the environment fairly accurately corresponding with the “real” directions and distances of their actual sources. None of the sensations initiated in the proprio-ceptive or intero-ceptive fields possess this property of projicience. And with the distance-receptors considered simply as originators of reflex actions, their reflexes are found to be appropriate to the stimuli as regards the direction and distance of the sources of these latter. Thus, the patch of light constituting a retinal image excites a reflex movement which turns the eyeball toward the source of the image and adjusts ocular accommodation to the distance of that source from the animal itself. Even a negative stimulus suffices. The shadow of the hand put out to seize the tortoise excites, as it blots the retinal illumination, withdrawal of the animal’s head to within the shelter of the shell.

How this result of “distance” has been acquired is hard to say. The net effect is reached in various ways, and with very various gain in the degree of “distance” acquired. By long vibrissae certain tango-receptors obtain excitation from objects still at a distance from the general surface of the organism. By reduction of their threshold value of stimulus, certain other receptors akin to tactual, inasmuch as their adequate stimuli are mechanical, become responsive to vibratory movements of water and air so as to react to physical sounds whose sources lie remote from the animal. Certain chemo-receptors acquire so low a threshold that they react not merely to food and other substances in contact with them in mass, but react to almost inconceivably diluted traces of such, traces which drift off from the objects and permeate the environment through long distances, as so-called odors, before impinging upon the delicate receptors in question. The leading segments thus come to possess not only taste, but taste at a distance, namely smell. In such cases it seems chiefly by lowering of their threshold that these receptors of the leading segments have been brought to react to objects still remote from the organism.

The “distance-receptors” seem to have peculiar importance for the construction and evolution of the nervous system. In the higher grades of the animal scale one part of the nervous system has, as Gaskell insists, evolved with singular constancy a dominant importance to the individual. That is the part which is called the brain. The brain is always the part of the nervous system which is constructed upon and evolved upon the “distance-receptor” organs. Their effector reactions and sensations are evidently of paramount importance in the functioning of the nervous system and of the individual. This seems explicable, at least partly, in the following manner.

An animal organism is not a machine which merely transforms a quantum of energy given it in potential form at the outset of its career. It has to replenish its potential energy by continued acquisition of suitable energy-containing material from the environment, and this material it has to incorporate in itself. Moreover, since death cuts short the career of the individual organism, the species has to be maintained, and for that in most higher organisms there is required accession of material (gametic) from another organism (of like species) to rejuvenesce a portion of the adult, which portion then cast off leads a new individual existence. To satisfy, therefore, the primary vital requirements of an animal species, actual material contact with certain objects is necessary; thus, for feeding, and in many cases for sexual reproduction.

In these processes of feeding and conjugation the non-distance-receptors play an important and essential part. But ability on the part of an organism to react to an object when still distant from it allows an interval for preparatory reactive steps which can go far to influence the success of attempt either to obtain actual contact or to avoid actual contact with the object. Thus, we may take in illustration the two sets of selective chemo-receptors, the gustatory and the olfactory. Both are responsive to certain chemical stimuli which reach them through solution in the moist mucous membranes of the mouth and nose. No odorous substance appears to be tasteless, and if the threshold value for olfaction and for taste be measured respectively, the threshold for the former as determined in weight of dissolved material is lower than for the latter. The former is the distance-receptor. Animal behavior shows clearly that in regard to these two groups of receptors the one subserves differentiation of reaction, i. e. swallowing or rejection, of material already found and acquired, e. g. within the mouth. The other, the distance-receptor, smell, initiates and subserves far-reaching complex reactions of the animal anticipatory to swallowing, namely, all that train of reaction which may be comprehensively termed the quest for food. The latter foreruns and leads up to the former. This precurrent relation of the reaction of the distance-receptor to the non-distance receptor is typical.

The “distance-receptors” initiate anticipatory, i. e. precurrent, reactions. I ventured above to use the word “attempt.” Just as a salient character of most of the reactions of the non-projicient receptors taken as sense-organs is “affective tone,” i. e. physical pain or physical pleasure, so “conative feeling” is salient as a psychical character of the reactions which the projicient or distance-receptors, taken as sense-organs, guide. As initiators of reflex movements the action of these latter is characterized by tendency to work or control the musculature of the animal as a whole, — as a single machine, —to impel locomotion or to cut it short by the assumption of some total posture, some attitude which involves steady posture not of one limb or one appendage alone, but of all, so as to maintain an attitude of the body as a whole. Take, for instance, the flight of a moth toward a candle, the dash of a pike toward a minnow, and the tense steadiness of a frog about to seize an insect. These reactions are all of them excited by distance-receptors. Though in the one case the musculature is impelled to locomotion toward the stimulus (positive phototropism), in the other restrained (inhibited) from locomotion. Whether the reaction be movement toward or movement away from (positive or negative) or whether it be motion or its restraint (excito-motor or inhibitomotor) does not matter here. The point here is that in both reactions the skeletal musculature is treated practically as a whole and in a manner suitably anticipatory of a later event. That is far less the case with the non-projicient receptors. The decerebrate frog changes the whole direction of its path of locomotion when a visual obstacle is set in its way, but a skin impact excites a movement in a small field of musculature only, e. g. the eyelid blinks on corneal contact, the foot flexes at a digital noxa; where the part itself cannot well move itself musculature accessory to it but distant from it is moved. Thus the hind limb is swept over the flank on irritation there, or the fore limb over the snout on irritation there. But in these cases the movement induced is merely local and does not affect the body as a whole. Sufficient intensity (we may include summation under intensity) of a stimulus can of course impel the whole creature to movement even through a non-projicient receptor. A decerebrate frog touched lightly between the scapulae will lower its head at first touch, and again more so at a second; at a third will, besides lowering the head, draw the front half of its trunk slightly backward; at a fourth the same movement with stronger retraction; at a fifth give an ineffectual sweep with its hind or fore foot; at a sixth a stronger sweep; at a seventh a feeble jump; at an eighth a free jump, and so forth Considerable intensity or summation is required to evoke a reflex reaction of the skeletal musculature as a whole from these cutaneous receptors. The projicient receptors and their reflexes once gone, even intense stimuli do not readily move or arrest the creature as a whole. It is relatively difficult to get the “spinal” frog to spring or swim. Co-ordinate movement of the creature as a whole is then obtained by general stimulation (i. e. plurireceptive summation), or if by localized stimulation the stimulus must be intense. Thus the spinal frog will swim when placed in water at 36° C. The warm water forms a nociceptive stimulus to the receptors of the immersed body-surface generally.

Extensive internuncial paths of “distance-receptors.” Conformably with the power of the “distance-receptors” to induce movements or postures of the individual as a whole we find the neural arcs from these receptors particularly wide and far-reaching. The nerve-fibre that starts from the receptor does not in many of these cases itself extend to, or send processes to, the mouths of the “final common paths.” Instead of doing so it ends often far short of them, and forms connection with other nerve-fibres (internuncial paths), which in their turn reach distant “final common paths.” This arrangement involves an intercalation of gray matter between the “private receptor” path and the “final common path” not only at the mouth of the latter, but also where the internuncial path itself commences. The significance of this seems that the internuncial path is itself a “common path, and therefore a mechanism of accommodation.” Its community of function is not so extensive as that of a “final common path,” not co-extensive for instance with all the receptors of the body, as would appear the case with a motor-nerve to a skeletal muscle. Yet it furnishes a path for use by certain sets of receptors in common. In Mustelus the nerve paths from the retinal and from the olfactory receptors converge toward the roof-nucleus of the mid-brain, whence passes the long mesencephalo-spinal path to the spinal motor nuclei. The inference is that conjoint stimulation of eye and nose exert a combined influence and impinge together on the spinal motor machinery. Similarly the Reissner fibre²⁹⁰ may serve as an internuncial path between paths coming in from olfactory and visual receptors on the one hand and the spinal motor common paths from the spinal cord to the muscles on the other. Another instance of an internuncial path is the so-called “pyramidal tract” characteristic of the mammalian nervous system. It furnishes a path of internuncial character common to certain arcs that have arisen indirectly from various receptors of various species and are knitted together in the cerebral hemisphere. Another instance is the path from the thalamus to the post-central convolution (Mott, Tschermak, and others).

Precurrent reactions. Consummatory reactions. It might seem at first that all motor reflexes may be grouped into those that tend to prolong the stimulus and those that tend to cut it short. Consideration shows that such a grouping expresses the truth but partially. We argued above that the “distance-receptors” induce anticipatory or precurrent reactions, that is, precurrent to final or consummatory reactions. The reflexes of certain non-projicient receptors stand in very close relation to “consummatory” events. Thus the tango-receptors of the lips and mouth initiate reflex movements that immediately precede the act which for the individual creature viewed as a conative and a sentient agent is the final consummatory one in respect to nutriment as a stimulus, namely, swallowing. Similarly with the gustato-receptors and their reactions. The sequence of action initiated by these non-projicient receptors is a short one: their reflex leads immediately to another which is consummatory. Those receptors of the chelae of Astacus, Homarus, etc., which initiate the carrying of objects to the mouth, or again the tango-receptors of the hand of the monkey when it plucks fruit anc carries it to the lips, give reactions a step further from the consummatory than those just instanced. These reactions are all steps toward final adjustments, and are not themselves end-points. The series of actions of which the distance-receptors initiate the earlier steps form series much longer than those initiated by the non-projicient. Their stages, moreover, continue to be guided by the projicient organs for a longer period between initiation and consummation. Thus in a positive phototropic reaction the eye continues to be the starting place of the excitation, and in many cases guides change in the direction not only of the eyeball but of the whole animal in locomotion as the reflex proceeds. The mere length of their series of steps and the vicissitudes of relation between bodies in motion reacting on one another at a distance conspire to give to these precurrent reflexes a multiformity and complexity unparalleled by the reflexes from the non-projicient receptors. The reaction started by “distance-receptors” where positive not only leads up to the consummatory reactions of the non-projicient, but on the way thither associates with it stimulation of other projicient receptors, as when, for instance, a phototropic reaction on the part of a Selachian brings the olfactory organs into range of an odorous prey, or, conversely, when the beagle sees the hare after running it by scent. In such a case the visual and olfactory receptor arcs would be related as “allied” arcs (Lecture IV), and reinforce each other in regard to the mesencephalo-spinal path, or in higher mammals the “pyramidal” or other pallio-spinal path. It is easy to see what copious opportunity for adjustment and of side connection such a reaction demands, consisting as it does of a number of events in serial chain, each link a modification of its predecessor.

Strong affective tone an accompaniment of consummatory reactions. We may venture to turn briefly to the psychical aspect of such sequences. To consummatory reactions affective tone seems adjunct much more than to the anticipatory, especially the remotely anticipatory of the projicient sense-organs. Thus the affective tone of “tastes” is strong. The reaction initiated by a noci-ceptor (pp. 226–231) is to be regarded as consummatory. The application of an irritant to the flank of a frog evokes a movement of the leg adapted to at once remove that stimulus from the skin of the flank. Or again, an irritant applied to the skin of the foot evokes a movement of the foot away from that stimulus. In both cases the reaction is a consummatory one, because it is calculated of itself to be final. To judge by our own introspection the affective tone adjunct to these reactions is strong. They instance strong affective tone pertaining to consummatory reactions. The affective tone of the reactions of the projicient receptors is less marked: physical pleasure or pain can hardly be said to accompany them. Not of course that they are wholly unrelated to affective tone. The relative haste with which an animal when hungry approaches food offered to the visual field suggests that conation attaches to the visual reaction by association through memory with affective tone. By associative, memory a tinge of the affective tone of the consummatory reaction may suffuse the anticipatory. The latter becomes indirectly a pleasure-pain reaction. The neutral tango-receptive reactions of the feet of the tortoise hastening stumblingly towards its food may in this way be imbued with a tinge of affective tone derived from the affective tint of the leading reflex, namely the visual, which itself has thus memorial association with a consummatory reflex of strong affective tone. Examples of this type of reaction furnished by new-born animals are given by Lloyd Morgan.¹⁶⁵ When “after a few days the new-born chick leaves ladybirds unmolested while he seizes wasp-larvae with increased energy” he affords evidence that reactions of his projicient receptors have acquired a new value, and that value is made up mediately of affective tone. How they have acquired it or what exact nature their new attribute has is not our question. It is enough here that in regard to certain stimuli the new value—the meaning—which the projicient sensation has obtained has reinforced greatly the conative intensity of the reaction to the stimulus. It has given the stimulus increased force as a spring of precurrent actions aimed at a final consummatory one. It has given this not by altering the external stimulus, nor the receptor-organ, but by, among other alterations, altering internal connections of the receptor arc. Thus it is that, be it by associative memory or other processes, the reactions of the “distance-receptors” come in higher animals to reveal a conative driving force which is perhaps the end for which these psychoses exist.

Nor are the series of reactions, short though they be, which the non-projicient receptors initiate wholly devoid of conative appearance. They show adaptation as executive of steps toward an end. Food, sexual consummation, suitable posture, preservation from injury, are ends to which their direction leads, as with the longer series of actions due to projicient receptors reacting to objects at a wider horizon. It is rather that the latter afford a freer field for the winning more subtle adjustments with wider application of associative memory. In the latter there is more scope for the play of mind,—mind it may be of such elementary grade as to be difficult for us to picture in its operations.

We may suppose that in the time run through by a course of action focussed upon a final consummatory event, opportunity is given for instinct, with its germ of memory however rudimentary and its germ of anticipation however slight, to evolve under selection that mental extension of the present backward into the past and forward into the future which in the highest animals forms the prerogative of more developed mind. Nothing, it would seem, could better ensure the course of action taken in that interval being the right one than memory and anticipatory forecast: and nothing, it would seem, could tend to select more potently the individuals taking the right course than the success which crowns that course, since the consummatory acts led up to are such—e. g. the seizure of prey, escape from enemies, attainment of sexual conjugation, etc.—as involve the very existence of the individual and the species. The problem before the lowlier organism is in some slight measure shadowed to us by the difficulties of adjustment of reaction shown by the human child. The child, although his reactions are perfect within a certain sphere of his surroundings, shows himself at the confines of that sphere a little blunderer in a world of overwhelming meaning. Hence indeed half the pathos and humour derivable from childhood.

It is the long serial reactions of the “distance-receptors” that allow most scope for the selection of those brute organisms that are fittest for survival in respect to elements of mind. The “distance-receptors” hence contribute most to the uprearing of the cerebrum. Swallowing was above termed a consummatory reaction. Once through the maw, the morsel is, we know by introspection, under normal circumstances lost for consciousness. But it nevertheless continues to excite receptors and their nervous arcs. The significant point is that the object has passed into such a relation with the surface of the organism that “conation” is no longer of advantage. The naïve notion that when we have eaten and drunken we have fed is justified practically. No effort can help us to incorporate the food further. Conation has then done its all and has no further utility in respect to that food taken. It is significant that all direct psychical accompaniment of the reactions ceases abruptly at this very point. The immediately precedent reactions that were psychically suffused with strong affective colour pass abruptly over into reactions not merely affectively neutral but void—normally—of psychical existence altogether. The concomitance between certain nervous reactions and psychosis seems an alliance that strengthens the restless striving of the individual animal which is the passport of its species to continuance of existence.

Receptive range. The ascendency of “distance-receptors” in the organization of neural function may be partly traceable to the relative frequency of their use. Although it would be incorrect to assess the value of an organ by the mere frequency with which it is of service, yet caeteris paribus that seems a fair criterion. The frequency with which a receptor meets its stimuli is, other things being equal, proportionate to the size of the slice of the external world which lies within its “receptive range.” Although in a fish, for instance, the skin with its tango-receptors is much larger in area than are the retinae with their photo-receptors, the restricted “receptive-range”—the adequate stimulus requiring actual proximity—of the former gives a far smaller slice of the stimulus-containing world to the skin than pertains to the eyes. In the case of the eye not only is the slice of environment pertaining to it at even a short distance more wide and high than that of the skin, but it is at each moment multiplied by the third dimension. There arise in it, therefore (caeterisparibus), in unit of time many more stimulations, with the result that the receptor-organ of “distant” species receives many more fresh stimuli per unit of time than does the receptor-organ of restricted receptive range. The greater richness of the neural construction of the photo-receptive system than of the tango-receptive accords with this. Thus in the photo-receptive system the so-called “optic nerve” (which since it is the second neural link and therefore to some extent a “common path,” presents numerical reduction from the first or private path in the retina itself) contains more conductive channels (nerve-fibres) in man (1,000,000, Krause) than are contained in the whole series of afferent spinal roots of one side of the body put together (634,000, Ingbert^{264, 265}), and of these latter the cutaneous afferent fibres form only a part, and of that part the tango-receptive fibres themselves form only a fraction. The large number of the channels in the retinal path is no doubt primarily indicative of spatial differentiations of the receptive surface, but that spatial differentiation is itself indicative of the numbers of the stimuli frequenting that receptive field.

Locomotion and “receptive range” Locomotive progression and distance receptivity are two phenomena so fundamentally correlated that the physiology of neither can be comprehended without recognition of the correlation of the two. Evidence is forthcoming from ontogeny and phylogeny. The elaborateness of the photo-receptive organs of the flying Insecta corresponds with the great power of these forms to traverse space. When the Brachiopod passes from a motile wandering life to a fixed sedentary one its “eyes” degenerate and go. The free-swimming Ascidia with fin-like motor organs and semi-rigid axial notochord, affording elasticity and leverage, bears at its anterior end a well-formed photo-receptor organ (eye) and a well-formed otocyst (head proprio-ceptor). Connected with the nerves of these, the anterior end of its truly vertebrate central nervous system has a relatively large “brain.” Thence extends backward along the body a spinal cord. Suddenly its free-swimming habit is exchanged for a sedentary; by adhesive projections from its head, it attaches itself permanently to some fixed object. At once there ensues a re-adaptive metamorphosis. Degeneration sets in concurrently in its locomotive musculature, its eye, its otocyst, its brain, and its cord. These vanish as by magic save that a fraction of the brain remains as a small ganglion near the mouth. The sessile creature retains, so far as can be judged from their microscopic structure, only some gustatory (?) receptors round the mouth, and some tango-receptors (? noci-ceptors) in the tegument, connected doubtless with an irregular diffuse subtegumental layer of unstriped muscle-tissue. Experimental observations seem wanting on the point, but we may presume that in this metamorphosis the receptive range of Ascidia dwindles from dimensions measurable by all the distance through which its free motile individual floats and swims, to a mere film of the external world, say a millimeter deep, at its own surface, especially round its mouth, and unextended by succession of time, save passively by the mere flowing of the water. Such instances illustrate the fundamental connection between the function of the skeletal musculature and that of the “distance-receptors.” Did we know better the sensual aspects of these cases the more significant doubtless would be the comparison.

The “head” as physiologically conceived. As regards the objects acting on the organism at any moment through its receptors, the extension of environmental space—the animal’s receptive range—is not equal in all directions as measured from the organism itself. The extension is greater in the direction about the “leading” pole. Thus, the reactions initiated at the eye forerun reactions (cf. Loeb’s Ketten-reflexe) that will in due time come to pass through other receptor-organs. The visual receptors are usually near the leading pole, and so placed that they see into the field whither progression goes. And similarly with the olfactory receptors. The motor train behind, the elongated motor machinery of the rest of the body, is therefore from this point of view a motor appendage at the behest of the distance-receptor organs in front. The segments lying at the leading pole of the animal, armed as they are with the great “distance” sense-organs, constitute what is termed the “head.”

The proprio-ceptive system and the head. We may now attempt to enquire whether this dominance of the leading segments which is traceable in the receptors of the extero-ceptive field applies in the field of reception which we termed the proprio-ceptive. We arrived earlier at the notion that the field of reception which extends through the depth of each segment is differentiated from the surface field by two main characters. One of these was that while many agents which act on the body surface are excluded from the deep field as stimuli, an agency which does act there is mass, with all its mechanical consequences, such as weight, mechanical inertia, etc., giving rise to pressures, strains, etc., and that the receptors of this deep field are adapted for these as stimuli. The other character of the stimulations in this field we held to be that the stimuli are given in much greater measure than in the surface field of reception, by actions of the organism itself, especially by mass movements of its parts. Since these movements are themselves for the most part reactions to stimuli received by the animal’s free surface from the environment, the proprio-ceptive reactions themselves are results in large degree habitually secondary to surface stimuli. The immediate stimulus for the reflex started at the deep receptor is thus supplied by some part of the organism itself as agent.

In many forms of animals, e. g. in Vertebrates, there lies in one of the leading segments a receptor-organ (the labyrinth) derived from the extero-ceptive field, but later recessed off from it; and this is combined in action with receptors of the proprioceptive field of the remaining segments. This receptive organ, like those of the proprio-ceptive field, is adapted to mechanical stimuli. It consists of two parts, both endowed with low receptive threshold and with refined selective differentiation. One part, the otolith organ, is adapted to react to changes in the incidence and degree of pressure exerted on its nerve-endings by a little weight of higher specific gravity than the fluid otherwise filling the organ. The other part, the semicircular canals, reacts to minute mass movements of fluid contained within it. These two parts constitute the labyrinth. The incidence and degree of pressure of the otoliths upon their receptive bed change with changes in the position of the segment in which the labyrinth lies, relatively to the horizon line. Movements of the segment likewise stimulate the labyrinthine receptors through the inertia of the labyrinthine fluid and the otoliths. By the labyrinth are excited reflexes which adjust the segment (and with it the head is usually immovably conjoined) to the horizon line. And other parts are similarly reflexly adjusted by it. Thus, the refined photo-receptive patches in the head—the retinae—which conduct reflexes delicately differential in regard to space, appropriate for stimuli higher or lower or to right or to left in the photo-receptive patch, depend in their conduct of these upon a more or less constant standardization of their own normals of direction in regard to the horizon line. These photo-receptive patches are set movably in the head; by the action of muscles they can retain their bearing to the horizon, although the head itself shifts its relation to the horizon. The control of these muscles lies largely with the labyrinth. The labyrinth produces a compensatory eyeball reflex. Thus in the head segments the labyrinth effects reflex movements analogous to that which the proprio-receptive nerves from the extensor muscles of the knee excites in the leg segments, reflexes restoring an habitual posture that has been departed from.

And from the above it seems clear that there is another feature of resemblance between the labyrinthine receptor and the proprio-ceptors of the limb. Stimulation of the labyrinth must in preponderant measure be given not by external agents directly but by the reaction of the organism itself. Posture and movement of the head are the immediate causes which stimulate the labyrinth, whether or not they be part of a total movement or posture of the whole individual. Such movement is most frequently an active one on the part of the animal itself. Thus, when Ascidia becomes sedentary and its locomotor muscalature atrophies its otocyst disappears. But an animal’s active movement is in its turn usually traceable as a reaction to an environmental stimulus affecting the receptors at the surface of the animal. Thus the labyrinthine receptors like the proprio-ceptors in other segments, are stimulated by the animal itself as agent, though secondarily to stimulation of the animal itself via some extero-ceptor.

And there is another point of likeness between labyrinth reflexes and those of the proprio-ceptors of the limb and other segments. The proprio-ceptors of the limbs appear productive of certain continuous, that is tonic, reflexes. Thus, in the decerebrate dog the tonic extensor rigidity of the leg appears reflexly maintained by afferent neurones reaching the cord from the deep structures of the leg itself. Similarly, if the knee-jerk be accepted as evidence in the spinal animal of a spinal tonus in the extensor muscle, this tonus seems maintained by afferent fibres from the extensor muscle itself, since the knee-jerk is extinguished by severence of those fibres. Again, the rapidity of onset of rigor mortis in a muscle is speedier when its tonus prior to death has been high. Section of the afferent roots of the limb prior to death delays onset of rigor mortis³⁰⁴ in that limb as judged by stiffness at the knee; but that delay is not observable when skin-nerves only have been severed. The labyrinthine receptors appear likewise to be the source of certain maintained, that is tonic, reflexes. Destruction of the labyrinth also delays the onset of rigor-mortis in the muscles to which its field of tonus can be traced. Ewald has shown that each labyrinth maintains tonus especially in the neck and trunk muscles and in the extensor-abductor limb-muscles of the homonymous side.

In regard to these tonic reflexes it is difficult to see how a steady mechanical stimulus can continue to elicit a reflex constantly for long periods. If we take sensation as a guide, a touch excited by constant mechanical pressure of slight intensity fades quickly below the threshold of sensation. It is said that a spinal frog may even be crushed by mechanical pressure without exciting from it a reflex movement provided that the pressure be applied by very slowly progressive increments. The office of a receptor would seem to be, placed across the line of a stream of energy, to react under the transference of energy across it, as for instance from the environment to the organism, or vice versa. We have many instances in which the living material adapts itself to, and maintains its own equilibrium under, different grades of environmental stress, treating each fairly continuous or slowly altering grade as a normal zero. The slow changes of barometric pressure on the body surface originate no skin-sensation, though they are much above the threshold value for touch. There streams constantly from the body through the skin a current of thermal energy much above the threshold value of stimuli for warmth sensations; yet this current evokes under ordinary circumstances no sensation. It is the stationary condition, the fact that the transference of energy continues at constant speed, which makes it unperceived. The receptor apparatus is not stimulated unless there is a change of rate in the transference, and that change of rate must occur in most cases with considerable quickness, otherwise there is a mere unperceived shift in the stationary equilibrium which forms the resting zero of the sensual apparatus. Over and over in the elicitation of reflexes as well as in the artificial excitation of nerve or muscle we meet this same feature. Both for sensation and for reflex action a function in the threshold value of stimulus is time as well as intensity and quantity. If a weak agent is to stimulate, its application must be abrupt. But in the tonic reflexes whose source lies at the proprio-ceptors and the labyrinth a weak stimulus, although apparently unchanging, seems to continue to be an effective stimulus.

The proprio-ceptors and the labyrinthine receptors seem to have in common this, that they both originate and maintain tonic reflexes in the skeletal muscles. And they, at least in some instances, reinforce one another in this action. Thus the tonus of the extensor muscle of the knee in the cat and dog appears to have a combined source in the proprio-ceptors of that muscle itself and in the receptors of the homonymous labyrinth. The tonus of skeletal muscles is an obscure problem. Its mode of production, its distribution in the musculature, its purposive significance, are all debateable. The steadiness and slight intensity of the contraction constituting the tonus render its detection difficult. Part of the discrepancy between the experimental findings may be traced to the supposition that a reflex tonus if present is present in all muscles at all times. A single muscle examined for reflex tonus has been taken to represent all muscles under all conditions, although the answer has been sometimes positive and sometimes negative.

It appears to me likely that reflex tonus is the expression of a neural discharge concerned with the maintenance of attitude. In many reflex reactions the effect is movement and the muscles are dealt with as organs of motion. In these cases the stimuli and the reactions both of them are short-lived events. But much of the reflex reaction expressed by the skeletal musculate is postural. The bony and other levers of the body are maintained in certain attitudes both in regard to the horizon, to the vertical, and to one another. The frog as it rests squatting in its tank has an attitude far different from that which gravitation would give it were its musculature not in action. Evidently the greater part of the skeletal musculature is all the time steadily active, antagonizing gravity in maintaining the head raised, the trunk semi-erect, and the hind legs tautly flexed. Innervation and co-ordination are as fully demanded for the maintenance of a posture as for the execution of a movement. This steady co-ordinate innervation antagonizes gravitation and other forces, e.g. as in currents of water. In these tonic as in other reflexes antagonistic muscles co-operate co-ordinately. There is nothing to show that reciprocal innervation does not obtain in the one class of reflex as in the other. If so, it becomes easily intelligible that the slight reflex contraction termed skeletal tonus should under given conditions be found in some muscles and not in others. The slight reflex contraction will be accompanied by reflex inhibition of the antagonistic muscles. For reflex tonus to be the expression of a neural discharge which maintains attitude accords well with the ascription of its source to the proprio-ceptors, including the labyrinth. Those are exactly the receptors which, functioning as sense-organs, initiate sensations of posture and of attitude (Bonnier). And it accords also with the share in the production and regulation of skeletal tonus which the cerebellum has (Luciani’s atonia) and the cerebrum.

Naturally, the distinction between reflexes of attitude and reflexes of movement is not in all cases sharp and abrupt. Between a short lasting attitude and a slowly progressing movement the difference is hardly more than one of degree. Moreover, each posture is introduced by a movement of assumption, and after each departure from the posture, if it is resumed, it is reverted to by a movement of compensation. Hence the taxis of attitude must involve not only static reactions of tonic maintenance of contraction, but innervations which execute reinforcing movements and compensatory movements. In all this kind of function the proprio-ceptors of the body generally and of the labyrinthine receptors in the head appear to co-operate together and form functionally one receptive system.

This system as a whole may be embraced within the one term “proprio-ceptive.” Our inquiry regarding it is now, whether that part of it which is situate in the leading segments, namely its labyrinthine part, exerts preponderance in the system as in the extero-ceptive system do the extero-ceptors situate in the leading segments. It must be remembered of the extero-ceptive system that even in the segments which are not the leading segments its receptors considered as sense organs produce sensations that have some projicience; and that in animals provided with outstanding skin appendages, e. g. hair, the tango-reflexes are to a slight extent reactions to objects at a distance. This germ of distance reaction and projicience of sensation in the extero-ceptors of the ordinary body-segments is developed in the extero-ceptors of the leading segments into the vast distance-reactions of the eye and the absolute projicience of vision. But the proprio-ceptors of the limb and body segments exhibit no germ of distance-reaction nor of projicience of sensation. And the specialized proprio-ceptor organ of the leading segment (the labyrinth) is similarly not a distance-receptor; although some of its sensations seem projected into the environment as well as referred to the organism itself, to the “material me.” Any predominance this proprio-ceptor in the leading segments may exhibit in the proprio-ceptive system is not therefore in virtue of the quality of reaction at a distance. If preeminently important to the organism as a whole its pre-eminence of importance rests on other grounds than does the importance of the great distance-receptors,—the olfactory, the visual, and the auditory.

A posture of the animal as a whole—a total posture—is as much a complex built up of postures of portions of the animal—segmental postures (Bonnier)—as is the total movement of the animal—its locomotion—compounded of segmental movements. With the hinder part of its spinal cord alone intact the frog maintains a posture in its hind limbs. These limbs are kept flexed at hip, knee, and ankle. When displaced from that posture they return to it. But if the animal be rolled over on its back it makes no attempt to right itself. The decerebrate frog with its labyrinths intact and their arcs still in connection with the skeletal musculature maintains the well-known attitude before mentioned. If inverted it at once reverts to that. The labyrinth keeps the world right-side up for the organism by keeping the organism right-side up to its external world. The cranial receptors control the animal’s total posture as do receptors of the hinder musculature the segmental posture of the hind limbs when but the hind end of the spinal cord remains.

Thus the labyrinthine proprio-ceptors are largely the equilibrators of the head, and since the retinal patches are movably attached (in mobile eyeballs) to the head, and since each retina has its normals of direction conforming with those of the head, these equilibrators of the head are closely connected by nervous arcs with the musculature maintaining the postures of the eyeballs. The posture of the head in many animals is dependent on the musculature not of the head segments themselves but of a long series of segments behind the head. In many forms the motor organs that steadily maintain or passingly modify the position of the head in regard to the external world—conveniently indexed by the line of direction of gravitation—are contributed by the skeletal musculature of many post-cranial segments. Hence the labyrinthine receptor is in touch with all the segments of the body, and these in a measure may be regarded as appended to the otic segment. Destruction of the labyrinth in the fish, the frog, the pigeon, the dog produces not only malposture of the eyeball and the head, but of the limbs and body as a whole. The “knock-out blow,” where the lower jaw conveys concussion to the otocyst, reduces in a moment a vigorous athlete to an unstrung bulk of flesh whose weight alone determines its attitude, if indeed a reactionless mass can be described as possessing attitude at all.

The labyrinthine receptors and their arcs give the animal its definite attitude to the external world. The muscular receptors give to the segment—e. g. hind limb—a definite attitude less in reference to the external world than in reference to other segments, e. g. the rest of the animal. Our own sensations from the labyrinth refer to some extent, as said above, to this environment, that is. have some projected quality; our muscular sensations refer to the body itself, e. g. contribute to perceptions of the relative flexions or extensions of our limbs. The arcs of the proprio-receptor of the leading segments control vast fields of the skeletal musculature, and deal with it as a whole, while the arcs of the proprio-ceptors of the other segments work with only limited regions of the musculature. Hence, in conformity with this the proprio-ceptor of the leading segments possesses long internuncial paths, for instance, bulbo-spinal from Deiter’s nucleus proceeding to all levels of the spinal cord.

We traced the reactions of proprio-ceptors of the limb to bear habitually a secondary relation to the reactions of the extero-ceptors of the limb. Similar secondary relation is evident also between the reactions of the proprio-ceptor of the leading segments (the labyrinth) and the reactions of the extero-ceptors of those segments. These latter extero-ceptors were seen to be distance-receptors, and the reactions of distance receptors were seen to be signalized by their anticipatory character. From secondary association with these distance-receptors the reactions of the labyrinth come in their turn to have anticipatory character. They retain, however, their own special features of equilibration and tonus. The locomotion of an animal impelled by its eye toward its prey involves co-operation of the labyrinth with the retina. And the tonic labyrinthine reflex which maintains an attitude may be just as truly an anticipatory reaction as any movement is. The steady flexed posture of the frog directed toward a fly seen on the aquarium wall is a co-ordinate innervation securing preparedness for the seizure of the food. Its character is as truly anticipatory as is that of any movement. We might speak of the animal as “at rest,” but it is the tense quietude of the hunter watching quarry rather than rest, such as supervenes in sleep and other conditions where active innervation is actually relaxed or reflex action is truly in abeyance.

Nervous integration of a segmental series. By longitudinal integration short series of adjoining segments become in respect to some one character combined together, so as to form in respect to that character practically a single organ. It is convenient to speak of such reflex reactions, confined from start to finish to a single integrated set of segments, as “short reflexes” giving “local reactions.” Thus the vertebrate appendages called limbs are plurisegmental, but the individual segments constituting the limb form in respect of the limb a functional group of such solidarity that their reactions in the limb are at any one time unitary.

The reflexes that extend beyond the limit of such a group are on the other hand conveniently termed “long reflexes.” And it is in the integration of long series, or of the whole series, of segments one with another, that, apart from psychical phenomena, the nervous system seems to reach its acme of achievement. Here it is that we see eminently what Herbert Spencer has insisted on, namely, that integration keeps pace with differentiation.

In the segmental series the nervous concatenation of the segments repeats broadly the kind of association evidenced within each segment taken singly. Broadly taken, each segment has on the one hand a piece of the extero-ceptive field, a piece of the proprio-ceptive field, and a piece of the interoceptive field, though this last is wanting in not a few segments. On the other, it has fractions of the skeletal, of the vascular, and of the visceral effector organs. Each segment has musculature and glands on its outer and visceral surfaces. Some segments have also secretors discharging into body spaces. Each of these sets of features of the segments has in the series of segments a nervous system of some functional homogeneity. With these plurisegmental systems as with their unisegmental pieces in the single segment the same harmonies of interconnection are observable. Thus, the nervous arcs embouching into the skeletal musculature start chiefly in the extero-ceptive field in so far as concerns execution of passing movements, in the proprio-ceptive field in so far as concerns tonic postures; and so on, as sketched above. If the receptors of the extero-ceptive field are regarded from the point of view of the nature of the agency adequate for each of their species, representatives of each species are found in almost every segment. In this way the functional properties of the extero-ceptive field form not one but several multisegmental organs or systems of organs. In each segment exist receptors responsive to mechanical, chemical, and radiant agencies respectively. There is thus formed a tango-ceptive system to which practically every segment contributes, a thermo-ceptive system, a noci-ceptive system; so also a musculo-ceptive system, and probably the receptors of the intero-ceptive surface similarly constitute a homogeneous system, prominent among their adequate agencies being those of chemical quality. These systems of receptive arcs present, though more or less compound, a solidarity of action in each system that gives each some rank as a physiological entity.

Restriction of segmental distribution a factor in integration. The impulse to nervous integration given by regional restriction of a peculiar species of organ to a single segment has especial force where that organ is of especial importance. This is the case with effector organs subserving important actions of consummatory (v. s. p. 326, 329) type, e. g. a sexual appendage, or the mouth. Such organs as these are of restricted regional distribution and subserve important reactions of consummatory type. With the mouth is associated differentiation of organs around it. Many postures and movements of the organism are advantageous or disadvantageous to the animal’s existence mainly inasmuch as they improve or disimprove the position or attitude of the mouth in regard to objects in the external world. Much of the long series of movements and other reactions initiated and guided by “distance-receptors” themselves is by-play on the way to a consummatory reaction which requires an appropriate placing and attitude of the mouth. That there is only one mouth and that of limited segmental extent involves co-ordination of the activities of many other segments with the oral. Integration of pluri-segmental activity is effected here, as in the other cases, mainly by the synaptic nervous system. The fact that the mouth is usually placed near the leading segments of the anterior pole is therefore a further factor in differentiating the segments at that end from the after-coming train. Thus it comes about that in many cases the animal consists of two portions broadly different in character but complemental the one to the other, the head and the trunk.

It is noteworthy that the increase of susceptibility instanced by the distance receptors is in each case restricted to a special patch, quite limited in area. Given a synaptic nervous system, no single item of functional arrangement more enforces integration of an individual from its segments than the restriction of a special kind of receptor to a single area or segment in the whole series. The motor apparatus of many segments has then to subserve a single segment, since that segment is provided with a receptor of a species not otherwise possessed by the individual at all. For integrative co-ordination of that kind the synaptic nervous system affords in the animal economy the only instrument. Only by the formation of common paths can due advantage be reaped from a specially refined recipient path (private path) of locally restricted situation.

Further, the condensed setting of a group of specialized receptors favors their simultaneous stimulation in groups together. Stimuli even of small area then cover a number of receptive points in the receptive sheet. Thus, ocular images of various two-dimensional shape tend to be better differentiated by the photo-receptors the more closely the individual photo-receptors lie together. More data are thus gained as a basis for differential reaction.

Further, the juxtaposition of groups of specially refined receptors in one set of segments, the leading or head segments, conduces toward their simultaneous stimulation by several agencies emanating from one and the same environmental object. Thus, the property of brightness and the property of odor belonging to an object of prey may then better excite in unison a reaction in the distant reagent, or excite more potently than would either property alone. And movements of the reagent itself are then more apt to intensify simultaneously the reactions of its two kinds of receptors. The collocation of the disparate receptors in one region will favor that which psychologists in describing sensations term “complication,” a process which in reflex action has a counterpart in the conjunction of reflexes excited by receptors of separate species but of allied reaction. This alliance of reaction we have seen finds expression as mutual reinforcement in action upon a final common path. Thus a reaction is synthesized which deals with the environmental object not merely as a stimulus possessing one property but as a “thing” built up of properties. A reflex is attained which has its psychological analogue in a sense percept.

The cerebellum is the head ganglion of the proprio-ceptive system. If the basis taken for classification of receptors be a physiological one with, as its criterion, the type of reaction which the receptors induce, separate receptive systems may be traced running throughout the whole series of segments composing the total organism. We have seen that such separate receptive systems may be treated as functional unities, extending through the segmental series. In any such system there is evident a tendency for its central nervous mechanisms, that is to say, the components of the central nervous organ which specially accrue to the system in question, to be gathered chiefly where the most important contribution to its receptive paths enters the central nervous system. The receptive system in question has as it were its focus at that place. Thus receptive neurones which can influence respiratory movement enter the central nervous organ at various segments, but the chief respiratory centre lies in the bulb where the receptive neurones from the lung itself make entrance and central connection, the vagal receptors being preponderantly regulative in that function. And we have seen that a proprio-ceptive organ (the labyrinth) in the head segments seems preponderantly regulative in those functions which the proprio-ceptive system subserves. The central neural mechanism belonging to the proprio-ceptive system is preponderantly built up over the central connections of this proprio-ceptive organ (the labyrinth) belonging to the head. Thither converge internuncial paths stretching to this mechanism from the central endings of various proprio-ceptive neurones situate in all the segments of the body. There afferent contributions from the receptors of joints, muscles, ligaments, tendons, viscera, etc., combine with those from the muscular organs of the head and with those of the labyrinthine receptors themselves. A central nervous organ of high complexity results. Its size from animal species to animal species strikingly accords with the range and complexity of the habitual movements of the species; in other words, with the range and complexity of the habitual taxis of the skeletal musculature. This central organ is the cerebellum.

The symptoms produced by its destruction or injury in whole or in part in many ways resemble, therefore, the disturbances produced by injury of the labyrinth itself. It also influences tonus very much as do the simple proprio-ceptive arcs themselves. It is closely connected structurally and functionally with the so-called motor region of the cerebral hemisphere, just as the simpler proprio-ceptive arcs and reflexes are closely associated with the mechanisms of extero-ceptive reactions. Knowledge is not ripe as yet for an adequate definition of the function of the cerebellum. Many authorities have defined it as the centre for the maintenance of the mechanical equilibrium of the body. Others regard it as the organ for co-ordination of volitional movement. Spencer suggested that it was the organ of co-ordination of bodily action in regard to space, the cerebrum he suggested being the organ of co-ordination of bodily action in respect of time. Lewandowski considers it the central organ for the “muscular sense.” Luciani, the universally acknowledged authority on the physiology of the cerebellum, describes it as the organ which by unconscious processes exerts a continual reinforcing action on the activity of all other nerve-centres.

It is instructive to note how all these separate pronouncements harmonize with the supposition that the organ is the chief co-ordinative centre or rather group of centres of the reflex system of proprio-ception. The cerebellum may indeed be described as the head-ganglion of the proprio-ceptive system, and the head ganglion here, as in other systems, is the main ganglion.

The cerebrum is the ganglion of the “distance-receptors.” By the “distance-receptors” are initiated and guided long series of reactions of the animal as a whole. Other receptive reactions integrate individual segments; the reactions of the distance-receptors integrate the whole series of segments. It is in the sphere of reactions of these “distance-receptors” that the most subtle and complex adjustments of the animal therefore arise. In their neural machinery not only short arcs but long arcs, involving extensive internuncial tracts, figure largely. Chains of reaction conducive to a final reaction relatively remote are more evident with them than with other arcs. If appeal to psychical evidence be ventured on it is to the field of operation of the arcs of these distance-receptors that higher feats of associative memory accrue, and, though the phrase is hardly permissible here except with curtailed scope, conation becomes more intelligent. Finally, in harmony with the last inference, it is over these “distance-receptors” and in connection with their reflexes and arcs that the cerebrum itself is found. The cerebrum constitutes, so to say, the ganglion of the “distance-receptors.” Langendorff⁷⁶ has pointed out that a blinded frog resembles in its reactions a frog with the cerebrum removed: the elasmo-branch without its olfactory lobes behaves as if it had lost its fore brain. Edinger traces the genesis of the cerebral cortex to a distance-receptor, namely the olfactory organ.

The integration of the animal associated with these “distance-receptors” of the leading segments can be briefly with partial justice expressed by saying that the rest of the animal, so far as its motor machinery goes, is but the servant of them. We might imagine the form of the individual and the disposition of the sense-organs as primitively very simple; for instance, a spheroid with a digestive cavity and sense-organs distributed especially over the external surface. Such an imaginary form we should expect under evolution to become modified. If a motile organism, its contractile mechanisms would obtain mechanical advantage (leverage) by its elongation in certain directions. The lengthwise extension of the vertebrate body and of its lateral motor appendages, e. g. limbs, are in so far such as might be argued a priori. Under evolution in motile animals adaptations securing appropriate leverage for the contractile apparatus appear, and length along certain axes is always a consideration in them. In animals with segments ranged along a single axis, the animal for the greater part of its length comes to be one great motor organ, complex and able to execute movements in various ways, but still a unity. The pole at which the great “distance-receptors” (visual, olfactory, auditory) lie is that which, in the habitual locomotion of the animal under the action of the motor train attached, “leads.” The animal therefore moves habitually into that part of environmental space which has been already explored by the distance-receptors of its own leading segments.

The head is in many ways the individual’s greater part. It is the more so the higher the individual stands in the animal scale. It has the mouth, it takes in the food, including water and air, it has the main receptive organs providing data for the rapid and accurate adjustment of the animal to time and space. To it the trunk, an elongated motor organ with a share of the digestive surface, and the skin, is appended as an apparatus for locomotion and nutrition. The latter must of necessity lie at the command of the great receptor-organs of the head. The co-ordination of the activities of the trunk with the requirements of the head is a cardinal function of the synaptic nervous system. Conducting arcs must pass from the cephalic receptors to the contractile masses of the body as a whole. The spinal cord contains these strands of conductors in vertebrates and is from this point of view a mere appendage of the brain. A salient feature of these conducting arcs is that the nerve-fibres from the cephalic receptors do not run, as might perhaps a priori have been thought natural, direct from their cephalic segment backwards to reach the common effector paths upon which they embouch. Instead of having that arrangement, these fibres, starting in the cephalic receptors, end in the gray matter of the central nervous axis not far from their own segment. Thence the conducting arc is continued backward by another strand of fibres, and these reach (perhaps directly) the mouths of the final common paths in the gray matter of segments of the spinal cord. This is the arrangement exemplified by the pulmono-phrenic and other respiratory arcs, the depresso-splanchnic arcs, the olfacto-phrenic respiratory arcs, the arcs between the otic labyrinth and the muscles maintaining posture in the trunk, and practically that of the retino-motor arcs connecting the retina with the muscles of the neck. It gives at least one synapsis more than the first alternative would do. And each synapse is an apparatus for co-ordination; it introduces a “common path.” And it is in the exercise of the distance-receptors with their extensive range overlapping that of other receptors that the reflexes which relate to “objects” in the sense that they are reflexes synthesized from receptors of separate species become chiefly established. The ramifications of the central neurones attached to these receptors are so extensive and the reactions they excite are so far spreading in the organism that their association with the reactions and central mechanisms of other receptors is especially frequent and wide.

The distance-receptors are the great inaugurators of reaction. The reduced initiation of action which ensues on ablation of the cerebrum seems explicable by that reason. The curtailment which ensues is indicative of damage which their removal inflicts on reactions generated by the distance-receptor organs. By a high spinal transection the splendid motor machinery of the vertebrate is practically as a whole and at one stroke severed from all the universe except its own microcosm and an environmental film some millimeters thick immediately next its body. The deeper depression of reaction into which the higher animal as contrasted with the lower sinks when made spinal signifies that in the higher types more than in the lower the great distance-receptors actuate the motor organ and impel the actions of the individual. The deeper depression shows that as the individual ascends the scale of being the more reactive does it become as an individual to the circumambient universe outside itself. It is significant that spinal shock hardly at all affects the nervous reactions of the intero-ceptors (visceral system); and that it does not affect the intero-ceptive arcs appreciably more in the monkey than in the frog. Its brunt falls, as we have seen before, on the reactions of the skeletal musculature. Not that in the highest animal forms the “distance-receptor” merely per se has necessarily reached more perfection or more competence than in the lower. In lower types, as in fish, are found “distance-receptors” of high perfection, but their ablation does not in lower types cripple in the same way as in higher types. It is that in the higher types there is based upon the “distance-receptors” a relatively enormous neural superstructure possessing million-sided connections with multitudinous other nervous arcs and representing untold potentialities for redistribution of so-to-say stored stimuli by associative recall. The development and elaboration of this internal nervous mechanism attached to the organs of distance-reception has, so far as we can judge, far outstripped progressive elaboration of the peripheral receptive organs themselves. Adaptation and improvement would seem to have been more precious assets in the former than in the latter. And, as related to the former rather than to the latter, must be regarded the parallelism of the ocular axes and the overlapping of the uniocular fields of photo-reception which in mammals has gradually reached its acme in the monkey and in man. This overlapping yields, in virtue one would think of some process akin to Herbart’s “complication,” an important additional datum for visual space. This, together with promotion of the fore limb from a simple locomotor prop to a delicate explorer of space in manifold directions, together also with the organization of mimetic movement to express thoughts by sounds, have with the developments of central nervous function which they connote and promote been probably the chief factors in man’s outstripping other competitors in progress toward that aim which seems the universal goal of animal behavior, namely to dominate more completely the environment. Remembering these conditions, it need not surprise us that the distance-receptors more and more exert preponderant directive influence over the whole nervous system. To say this is to say no more than that the motile and consolidated individual is driven, guided, and controlled by, above all organs, its cerebrum. The integrating power of the nervous system has in fact in the higher animal, more than in the lower, constructed from a mere collection of organs and segments a functional unity, an individual of more perfected solidarity. We see that the distance-receptors integrate the individual not merely because of the wide ramification of their arcs to the effector organs through the lower centres; they integrate especially because of their great connections in the high cerebral centres. Briefly expressed, their special potency is because they integrate the animal through its brain. The cerebrum itself may be indeed regarded as the ganglion of the distance-receptors.