LECTURE VIII SOME ASPECTS OF THE REACTIONS OF THE MOTOR CORTEX

We shall now venture a glance at certain reactions of the cerebral hemisphere itself; our survey must be circumscript for several reasons. By use of such methods as we are employing, artificial excitation and so on, and under such observations as these allow, namely the initiation under narcosis of muscular movements or the recording of their immediate defects from normal movement, little light is given in regard to much that goes on in an organ whose chief function is mentality itself. Our expectation must be modest, for modest assuredly must be the achievement reached by such means in a problem of such a nature. The very poverty of the achievement is itself an indication that the methods pursued by the physiologist successfully in other spheres of his study are here confronted with problems to which they are far less suited. It is not that I esteem lightly the labours of the many distinguished workers in this field. As far as the methods referred to can avail, it is to the skill with which they have been used that we owe what knowledge we have of the topographical representation of movement in the various fields of cerebral cortex. We have only to remember how much more numerous the physiological facts concerning the cerebral cortex are to-day than prior to the experiments of Hitzig and Fritsch⁴⁷ and of Ferrier,⁵¹ following on the observations of Broca,²⁹ Hughlings Jackson,³⁵ and Bastian.⁴⁶ Experiment had failed to get evidence of localization of function in the cortex of the hemispheres, though in microscopic structure that great sheet of gray matter presents such similarity to nervous formations regarded as nerve-centres elsewhere. Progress of knowledge in regard to the nervous system has been indissolubly linked with determination of localization of function in it. This has been so from the time of the Bell¹⁰-Magendie¹¹ discovery of the difference of function in the two spinal roots, and Flourens’¹⁹ delimitation of the respiratory centre in the bulb. The discovery of localization of function in parts of the cortex has given the knowledge which now supplies to the student charts of the functional topography of the brain much as maps of continents are supplied in a geographical atlas. The student looking over the political map of a continent may little realize the complexity of the populations and states so simply represented. We looking at the brain chart of the text-book may never forget the unspeakable complexity of the reactions thus rudely symbolized and spatially indicated.

If we may be allowed an a priori consideration it is this,—that although it is not surprising that such territorial subdivision of function should exist in the cerebral cortex, it is surprising that by our relatively imperfect artifices for stimulation we should be able to obtain clear evidence thereof. The neurone chains that together build up the nervous system are in the architecture of that system so arranged that the longest of them all tend to pass through the cerebral cortex. Every increase in the number of links composing a nerve-cell chain seems to increase greatly the uncertainty of its reactions under artificial excitation. With increase in number of links goes increase in numbers of side branches and connections. The difficulty of getting long chains of nerve-cells to react in a regular way under artificial stimulation seems greatly enhanced by the multiplication of the side connections. The momentary condition of any cell-chain is in part a function of the condition at the moment of all the other cell-chains with which it is connected. The cortex cerebri might therefore well have been expected to yield under artificial stimulation only extraordinarily inconstant results. To Hitzig and Fritsch, and to Ferrier, we owe the pregnant demonstration that as regards the motor region this expectation is not well founded.

It is only of the reactions of the Rolandic area of the cortex that I shall venture to speak. Ferrier showed that the application of faradic currents to that cortex excites with great regularity movements which vary in distribution as the electrodes are moved from place to place, but remain within limits constant under repeated application of the stimulus to any one and the same spot. Ferrier’s mode of indicating the topographical arrangement of the reactions he obtained is seen in his well-known diagrams of the cortex. His motor centres, as he termed them, were marked in his figures by circular areas. “The areas have no exact line of demarcation from each other, and where they adjoin stimulation is apt to produce conjointly the effect peculiar to each.”⁷⁰ He showed these motor centres to extend forward over the frontal lobe, producing there movements of the eyeballs. Regarding their extension round and over the upper edge of the hemisphere and down upon the mesial surface he noted them in the marginal convolution. “This convolution in the parieto-frontal region gave rise to movements of the head and limbs apparently similar to those already obtained by stimulation of the corresponding regions on the external surface.”⁷⁰

This original research by Ferrier ranks among the classics of experimental neurology and physiology. It has been followed by a number of kindred contributions from workers whose names are familiar to us all,—Albertoni, Schäfer, Munk, Luciani, Tamburini, Paneth, Beevor, Horsley, Mott, Ballance, Mann, and others. The detailed knowledge of the localization has been largely based on the cerebral cortex of the common ape, the macaque. It was an interesting step further when Beevor and Horsley¹²⁷ published observations on the localization of the motor functions in the central cortex of an orang-utan. Their experiment long remained the single one for which an anthropoid species had been laid under contribution. It exercised a notable influence on the scheme of motor localization adopted as probably obtaining in the brain of man.

Of the three or four species of anthropoid apes that are known, most authorities agree that it is the gorilla which possesses the most highly developed cerebrum; next to it probably stands the chimpanzee, and a little below the chimpanzee comes Simia satyrus, the orang-utan. But there are great individual differences, and the simpler examples of chimpanzee brains seem inferior in development to well-developed examples of the brain of the orang.

A. S. Grünbaum^{225, 229} and myself have obtained observations on cerebral localization in the several species of anthropoid apes. In the chimpanzee the scheme of topography we find existent is illustrated by the accompanying Figures 72 and 73.

The so-called motor area occupies unbrokenly the whole length of the precentral convolution, and in most places the greater part or the whole of its width. It extends into the depth of the central sulcus, occupying the anterior wall, and in some places the floor, and in some extends even into the deeper part of the posterior wall of the fissure. We have examined more than forty hemispheres, but have never found the motor area extend indubitably to the free face of the post-central convolution. This delimitation agrees remarkably with the original results obtained by Hitzig⁵⁴ on the brain of the monkey, Innuus rhesus.

At the upper mesial edge of the hemisphere the motor area extends round and down upon the mesial face of the hemisphere, but we have not found it reach the calloso-marginal fissure. The anterior limit of the motor region is in great part not coincident with any fissure. The front portion of the region usually dips into and across the upper part of the superior precentral fissure, and lower down it not infrequently dips into the inferior precentral fissure. Occasionally the front edge of the region dips into almost the whole length of the superior precentral sulcus. It is not the extent of the motor area which appears to be variable, the variant is the sulcus itself. The great variety of individual pattern exhibited by the convolutions and sulci in these richly convoluted brains gives opportunity for studying critically the claim of value of these fissures as landmarks in the topography of the cortex. From this point of view their use for strict localization is small. Not only are the extremes of pattern exhibited by the convolutions extraordinarily different one from another, but the frequency of the individual variation is so great that hardly a pair can be found in which the existent convolutions are, when compared with the minuteness applicable to functional centres, really closely alike. Schafer, in his important contribution to the physiology of the motor cortex in 1887,^111a pointed out that the fissures of the cortex do not mark in any sense the boundaries of the functional areas of the organ. Our examination of the anthropoid brains we have worked through convinces us that not only do the fissures of the frontal region not mark physiological boundaries, but that they are not closely reliable even as landmarks for the functional topography. Their relation is too inconstant. v. Monakow²⁴¹ has found the same uncertainty in the calcarine fissure in respect to visual cortex. The degree to which these fissures are subject to individual variation and the frequency of their asymmetry in the two hemispheres stands in contrast with the constancy from individual to individual and greater bilateral symmetry which holds good for the arrangement of the func-tional centres. A practical outcome of this is that it is essential for accurately detailed localization, when the opening through the skull is of moderate size, not to trust to the anatomical details of the exposed cerebral surface, but to obtain orientation in the topography by application of the electrodes and observation of the movement, if any, which is excited. In our early experiments we thought to obtain much help by having at hand a brain of the same species already experimented upon and thought to save time in recording the results of the fresh experiment upon chart outlines prepared from the specimen already worked upon. But the variation of the convolutions from individual to individual has been too great to allow of these expedients. As an exception to the above general rule two landmarks of relative constancy are the genua of the sulcus centralis, the Rolandic fissure. In the chimpanzee and gorilla the genua are two (Fig. 72); the upper, opposite the junction between leg area and arm area, may be termed the cruro-brachial; the lower, between arm area and face area, may be termed the brachio-facial. In the orang there is in addition a third genu, which from its relation to the functional topography may be called the labio-lingual. In the orang the facial area of the cortex is considerably longer from above down than in the chimpanzee or gorilla.

It is a general belief that for excitation of the cortex in man there is needed an intensity of faradism much greater than that sufficing for the cortex of the monkey. Actually comparing the excitability of the cortex of the anthropoid with that of the bonnet monkey by employing exactly the same current in each case, we found the excitability as measured by the least intensity of current required to evoke motor reaction practically the same in the anthropoid and in the lower ape.²²⁵ The motor cortex of the anthropoid, though undoubtedly far more complex in many ways than that of the lower ape, remains as readily amenable to electric stimulation. Cushing of Baltimore and Krause of Berlin find this holds good also for the human brain, and that it is not necessary to employ strong faradization. In the majority of the anthropoids upon which we have experimented cortical epilepsy has been quite easily provoked, just as it is in the small monkeys.

In the precentral gyrus, the sequence of representation of the musculature starting from below upward follows broadly that known for the lower apes. The sequence runs—tongue, jaw, mouth, nose, ear, eyelids, neck, hand, wrist, elbow, shoulder, chest, abdomen, hip, knee, ankle, toes, and perineal muscles. It is noticeable that movements of the eyeballs do not occur in this list As did Beevor and Horsley in their orang,¹²⁷ so we in the chimpanzee, gorilla, and orang find a frontal area extending into the middle and inferior frontal convolutions excitation of which gives conjugate deviation of the eyeballs to the opposite side. We find this area separated from the area yielding the other movements by an intervening space. We find this intervening space broken, however, by small areas whence movements of the eyeballs can be elicited, partially bridging between it and the upper facial region on the precentral convolution.

The sequence of representation of movement which we note follows a plan more in accordance with the order of the spinal series of segments than that hitherto obtained. Between the place of representation of shoulder and that of hip is an area which, next to the shoulder, yields unilateral movement of the chest muscles, and, next to the hip, yields unilateral movement of the abdominal muscles, and furthest upward lies a focus for perineal muscles.

In our experience, in accord with the original observations by Hitzig⁵³ on the lower apes, the electrodes when placed upon the surface of the post-central convolution fail to evoke any obvious effect, though when placed with an even weaker current upon the precentral they evoke the regular reaction. In our experience, though small lesions in the precentral convolution caused marked paralyses and descending spinal degenerations, similar and larger lesions in the post-central did not produce even temporary paralysis nor any unequivocal degeneration.

With regard to the degenerations, it is noteworthy that from a hand-area lesion the spinal pyramidal degeneration shows in some chimpanzees a ventral direct pyramidal tract of size not obviously inferior to that of a man. But this ventral direct tract does not appear to be present in all individual chimpanzees,—a fact agreeing with Flechsig’s⁷⁰ discovery of its variability in man. The hand-area lesion gives a heavy degeneration of the homolateral pyramidal tract in the lateral column on the same side as the cerebral lesion.

As regards symptoms resulting from the cortical lesions, extirpation of a great part if not of the whole of the hand area from the right hemisphere caused an immediate severe crossed brachioplegia without the slightest sign of paresis in either face or leg. The paresis affected the fingers most; these were kept helplessly semi-extended, the wrist being dropped. The elbow seemed little if at all affected, but the shoulder seemed distinctly paretic, there being difficulty in raising or abducting the upper arm. The paresis diminished quite rapidly, and in six weeks’ time the animal had in large measure recovered the usefulness of the limb.

A lesion in the leg-area similarly caused temporary paresis of the opposite leg, especially in the toes and at the ankle-joint. The lesion was smaller and the recovery more rapid than with the arm-area lesion. The knee-jerk, which showed no alteration under the arm-area lesion, here showed exaltation immediately, that is, a quarter of an hour after the leg-area lesion. Weeks later, when the paresis to inspection had passed off, the knee-jerk still exhibited greater briskness on the crossed side.

We have seen often confirmed what our predecessors¹²⁷ with the orang have well pointed out, namely, the greater integration of localized representation of movements in the anthropoid as compared with the lower ape. There is not one of the fingers that we have not seen move separately and alone under excitation of certain points of the cortex; again, isolated movement of the pinna of the ear, of the tip of the tongue, rare in the lower monkeys, are easily obtainable in the anthropoid.

As to the extent of the so-called motor area, from our observations we think it probable that in the anthropoid brain as much of that area lies hidden from the surface in the sulci as is actually exposed on the free surface on the convolutions. Nevertheless we indorse the opinion expressed by Beevor and Horsley that the so-called motor area in the anthropoid brain forms a smaller fraction of the total surface than it does in the lower types of monkey. If it has grown in extent—as undoubtedly it seems to have done—other regions belonging to those so-called “silent” fields whence electric stimulation excites no obvious response have increased still more. It is especially with the exploration of that great inexcitable field that research has to deal. The discoveries of Flechsig, v. Monakow, Déjerine, Mott, Campbell, the Vogts, and others yearly advance further in the problem.

The results on the gorilla²²⁵ confirm those we have obtained on the chimpanzee, though in the gorilla and orang in my experience eyeball movements have been elicited from a larger field of the frontal cortex than in the chimpanzee, an upper area yielding eye movement on a level with the hand area being more readily discoverable.

That the free surface of the post-central convolution belongs to the motor cortex we have not found. Brodman^256a and Campbell^272a have since called attention to marked structural differences between the cortex respectively behind and in front of the central sulcus. The arrangement of the fibres and the character of the cells is different. Ramon-y-Cajal,^128a using the Golgi method, and Flechsig^271a following the myelinization, had also previously drawn distinction between the structure of the two convolutions divided by this great fissure; and observations by Mott, A. Tschermak, and others had indicated an especially close connection of the post-central gyrus with ascending presumably afferent paths. Evidence of this last by excitation methods is of course difficult to obtain, but nothing in our experiments is contrary to it, and some occasional results that have come before me in experimenting by excitation lend themselves to such an explanation. To enter upon these here would lead too far from our main interest now.

It is natural to inquire whether reciprocal innervation is exemplified by reactions from the cortex. That inhibition of muscular contraction is obtainable by artificial excitation of the cortex was early noted by Bubnoff and Heidenhain⁹² in the dog and by Exner⁹⁹ in the rabbit. I have myself worked chiefly with the monkey.^{151, 304} In that animal the ocular axes are parallel, and the setting of the eyeball in the orbit is such that the tensions of its connections, apart from unequal activity of its extrinsic muscles, are in equilibrium when the globes are approximately parallel. That is their primary position. This can be shown in various ways. Thus, if the III, IV, and VI nerves are all severed the eyeballs assume this primary position. If one eyeball be then rotated by the finger or fixation forceps to right or left or up and down a considerable resistance is felt, and on letting it go the globus springs back at once to the primary position. So also in chloroformization. In the early stage of chloroformization the eyes enter various positions of squint — in the monkey a very usual position is bilateral divergence upward and outward. When the narcosis has become profound, the eyes revert to approximate parallelism in the primary position. On being then displaced by the finger they at once swing into the primary median position again. So also immediately after death, before rigor mortis has set in.

If III and IV cranial nerves of one side, e. g. left, have been severed, so that rectus externus remains the only unparalyzed ocular muscle, appropriate excitation of the cortex cerebri produces conjugate movement of both eyes towards the opposite side, i. e. from left toward right, the left eye travelling however only so far as the median line. Inhibition of the tonus and of the active contraction of rectus externus can thus be elicited from the cortex. The reaction is obtainable from all that portion of the cortex which on excitation gives conjugate lateral deviation of the eyes, i. e. from the area discovered by Ferrier⁷⁰ in the frontal region, and from that discovered by Schäfer^117a in the occipital region.

This inhibition is obtainable from the frontal area after complete removal of the occipital lobe. It is conversely obtainable from the occipital area after complete removal of the frontal area. After a deep frontal section across the hemisphere and into the lateral ventricle (partly entering the internal capsule) so as to sever occipital from frontal cortex in the manner practised by Munk and Obregia,¹²⁸ the reaction is obtainable undiminished from both the frontal and from the occipital areas separately.

The cortex is not essential to the reaction. It is obtainable from the corona radiata underlying the frontal cortex after complete ablation of the frontal cortex itself. It is obtainable from the corona radiata running downwards and forwards from the occipital cortex after free removal of the latter. It is obtainable by direct excitation of the internal capsule itself. From the internal capsule it is elicitable at two distinct places, one in front of the other behind the genu of the capsule. It is obtainable by excitation of the cross-section of corpus callosum about 3–5 millimeters behind the genu; also from corpus callosum at the splenium. The section was laid bare as in Mott and Schäfer’s^121a second method. The reaction as obtained from corpus callosum has in my hands proved comparatively irregular. It is evident that the action of arrest may take place, in centres which are subcortical.

E. H. Hering and myself¹⁷³ made observations on limb movements elicited from the cortex and found evidence of a similar co-ordination in regard to them. As in the experiments of Bubnoff and Heidenhain,⁹² the degree of narcotization formed an important condition for the observations. The narcosis must not be too profound. It is best, starting with the animal in a condition of deep etherization, to allow that condition gradually to diminish. As this is done it almost constantly happens that at a certain stage of anaesthesia the limbs, instead of hanging slack and flaccid, assume and maintain a position of flexion at certain joints, notably at elbow and hip. This condition of tonic contraction having been assumed, the narcosis is as far as possible kept at that particular grade of intensity. The area of cortex ccrebri previously ascertained to produce under faradization extension of the elbowjoint or hip-joint is then excited.

For clearness of description let us suppose the left hemisphere excited, and the limb affected the right. The result of excitation of the appropriate focus in the cortex, e. g. that presiding over extension of the elbow, is an immediate relaxation of the biceps with active contraction of the triceps. As regards the condition of the biceps, the relaxation is usually so striking that merely to place the finger on it is enough to convince the observer that the muscle relaxes. The following is however a good mode of studying the phenomenon: in a monkey with strongly developed musculature the fore arm, maintained by the above-mentioned steady tonic flexion at an angle with the upper arm of somewhat less than 90° is lightly supported by the one hand of the observer, while with the finger and thumb of the other the belly of the contracted biceps is felt through the skin. On exciting the cortex the contracted mass becomes suddenly soft, melting under the observer’s touch. At the same time the observer’s hand supporting the animal’s fore arm tends to be pushed down with a force unmistakably greater than that which the mere weight of the limb would exert. If the triceps itself be felt at this time, it is easy to perceive that it enters contraction, becoming increasingly hard and tense, even when its points of attachment are allowed to approximate, and the passive tensile strain in it should lessen. If the limb be left unsupported the movement is one of simple extension at the elbow-joint. On discontinuing the excitation of the cortex the fore arm usually immediately, or almost immediately, returns to its previous posture of flexion, which is again as before steadily maintained.

Conversely, when, as not unfrequently occurs in conditions of narcosis resembling that above referred to, the arm has assumed a posture of extension and this is tonic and maintained, the opportunity may be taken to excite the appropriate focus in the cortex for flexion of fore arm or upper arm. Triceps is then found to relax, and biceps at the same time to enter into active contraction. If the biceps be hindered from actually moving the arm, the prominence at the back of the upper arm due to the contracted triceps is seen simply to sink down and become flattened. When examined by palpation the muscle is felt to become more or less suddenly soft, and the biceps at the same time to become more tense than before. The movement of the limb, when allowed to proceed unhindered, is flexion with some supination. It is noteworthy that in this experiment not every part of the large triceps mass becomes relaxed; a part of the muscle which extends from the humerus to the scapula does not in this experiment relax with the rest of the muscle. This part, if the scapula be fixed, acts as a retractor of the upper arm, and is not necessarily an antagonist of the flexors of the elbow. This part of the triceps we observed sometimes enter active contraction at the same time as the flexors of the elbow. Under use of currents of moderate intensity we found that not from one and the same spot in the cortex can relaxation and contraction of a given muscle be evoked at different times, but that the two effects are provocable at different, sometimes widely separate, points of the cortex, and are there found regularly.

We obtained analogous results in the muscles acting at the hip-joint. In the narcotized animal the hip-joint is being maintained in flexion, the thighs being drawn up on the trunk, excitation of the region of the cortex previously ascertained, when the limbs hang slack, to evoke extension of the hip, produces relaxation of the flexors of the hip and at the same time active contraction of the extensors of the thigh. We examined particularly the psoas-iliacus, and the tensor fasciœ femoris, also the short and long adductor muscles. Each of these was found to relax under appropriate cortical excitation. If the knee were held by the observer it was found at the time of relaxation of the flexors of the hip to be forced downward by active extension of the hip.

Similarly with other groups of antagonistic muscles, both those of the small apical joints of the limb, e. g. flexors and extensors of the digits, and those of the large proximal joints, e. g. adductors and abductors of the shoulder. At these also instances of reciprocal innervation were obtained. By antagonistic muscles I mean only what are termed true antagonistics; I do not include the cases where one muscle fixes a joint enabling another muscle to thus act better on another joint — H. E. Hering’s pseudoantagonists.¹⁶⁸ Hering has carefully analyzed¹⁶⁹ the co-ordination of such pseudoantagonists in the action of clenching the fist. He has shown that when that movement is evoked in the monkey by excitation of the cortex cerebri the extensors of the wrist are thrown into action simultaneously with the long flexor of the fingers. But there was no evidence that the true antagonists were ever thrown into simultaneous activity.

That a part of the triceps brachii (that retracting the upper arm) should actively contract exactly when another part (that extending the elbow) becomes relaxed is exactly comparable with a phenomenon which can be noted in the limb under spinal reflexes, both in triceps itself and in quadriceps femoris. Beevor and others have shown that different parts of what in gross anatomy is denominated one single muscle are used separately in various movements. And Hering and I similarly saw in the quadriceps femoris, on exciting the cortical region yielding extension of the hip, a relaxation of a part of the quadriceps (a part which flexes the hip) with contraction of another part (which extends the knee). I have also noted that in the monkey by stimulating the appropriate cortical area for flexion of the knee the kneejerk is temporarily depressed or suppressed completely.

The results obtained from the internal capsule were as striking as those obtained from the cortex itself. From separate points of the cross-section of the capsula, relaxation of various muscles was evoked. Among the muscles whose inhibition was directly observed were supinator longus and biceps brachii, the triceps, the deltoid, the extensor cruris, the hamstring group, the flexor muscles of the ankle-joint, and the sternomastoid.

The spots in the cross-section of the capsula which yielded the inhibitions were constant, that is, the position of each when observed remained constant throughout the experiment. The area of the capsular cross-section at which the inhibition of the activity of, e. g. the triceps, muscle can be evoked is separate from (that is to say not the same as) that area whence excitation evokes contraction of the triceps (or of that part of the triceps inhibition of which is now referred to). On the other hand, the area of the section of the internal capsule, whence inhibition of the muscle is elicited, corresponds with the area whence contraction of its antagonistic muscles can be evoked. Yet synchronous contraction of such pairs of muscles as gastrocnemius and peroneus longus is obtainable from the cortex. The observations make it clear that “reciprocal innervation” in antagonistic muscles is obtainable by excitation of the fibres of the internal capsule. Topolanski¹⁸⁴ observed them on exciting the corpora quadrigemina (rabbit). It is probable therefore that the inhibition elicitable from the cortex cerebri is not in these cases chiefly or at all due to an interaction of cortical neurones one with another.

Exner drew from his observations on the rabbit a similar inference that the inhibitory phenomena had their chief seat in the spinal mechanisms though elicited from the cortex. My own inference has been that the seat of inhibition in these reactions from the “motor” cortex lies probably at the place of confluence of conducting channels in a common path, likely enough at their confluence upon the “final common path,” the motor neurone, that is at the ultimate synapse. But it may well be, indeed is in the highest degree likely, that in other fields of action one cortical element inhibits another cortical element.

In “willed” movements of the eyeballs of the monkey the same kind of co-ordination was revealed in some observations I obtained on this point. When the III and IV cranial nerves had been resected intracranially and the animals in ten days or so had recovered completely from the surgical interference, the eye movements were examined.

In these animals if the gaze was attracted to an object, e. g. food, held level with the eyes and to the right of the median plane (left III and IV nerves cut) the left eye looked straight forward, the right looked to the right. If the object was then shifted more to the right or less to the right, the right eye followed it, moving as the object was moved, while the left eye remained motionless, looking straight forward all the time. But when the object was held to the left of the median plane both eyes were directed upon it, apparently quite accurately. When the object was shifted farther and farther to the left both eyes followed it with a steady conjugate movement not detectably different from the normal. When the object was carried from the left-hand verge of the field back toward the median plane both eyes followed it as accurately as before. If the object was moved suddenly from the extreme left-hand edge of the field up to the median plane both eyes immediately and apparently equally quickly reverted to parallelism with that plane. Or, if the object were suddenly brought back from the left edge of the visual field to some point intermediate between that and the median plane both eyes at once shifted apparently equally to a correspondingly diminished deviation from the primary position. These actions must mean that in the left eye relaxation of rectus externus kept accurate time and step with contraction of rectus externus of right eye. And the action of the left rectus externus gives presumably a faithful picture of a synchronous process going forward in the right rectus internus.

It is interesting to recall that in the seventeenth century Descartes in his De Homine,² discussing willed movements, suggested for the mechanism of the lateral movements of the eyeball, with which he deals in some detail, a co-ordination much resembling reciprocal innervation. He urged that the vital spirits were conducted into the external rectus by valved channels in that muscle, and at the same time were from the internal rectus led out by valved channels, so that as the one muscle became tense by distension the other became flaccid by emptying. He furnished with his own pencil a figure illustrating the mechanism as he conceived it (Fig. 74). There is an essential resemblance between his scheme and that of “reciprocal innervation,” except that he imagined the mechanism a peripheral one, that is to say that the “inhibition,” as we now term it, had its scat in the muscle, not in the nerve-centres themselves.

Again, early last century (1823) Charles Bell, in a footnote to a paper in the Philosophical Transactions,¹³ argued a similar kind of co-ordinate mechanism in the execution of willed movements. He wrote: “The nerves have been considered so generally as instruments for stimulating the muscles, without thought of their acting in the opposite capacity, that some additional illustration may be necessary here. Through the nerves is established the connection between the muscles, not only that connection by which muscles combine to one effort but also that relation between the classes of muscles by which the one relaxes and the other contracts. I appended a weight to a tendon of an extensor muscle which gently stretched it and drew out the muscle; and I found that the contraction of the opponent flexor was attended with a descent of the weight, which indicated the relaxation of the extensor.” “If such a relationship be established, through the distribution of the nerves, between the muscles of the eyelids and the superior oblique muscles of the eyeball, the one will relax while the other contracts.” But like Descartes he pictured a peripheral inhibition, for he says: “If we suppose that the influence of the 4th nerve is, on certain occasions, to cause a relaxation of the muscle to which it goes, the eyeball must be then rolled upwards.” Descartes and Bell, therefore, with remarkable prescience imagined the existence of an action of nerve on muscle just such as was later actually discovered by the Webers²¹ in the vagal inhibition of the heart — an inhibition which Volkmann¹⁷ previous to the Webers met in the course of experiment, but unexpectant of it, rejected¹⁸ as illusory and due to some experimental error.

As for salient objective differences observable between movements elicited from the so-called “motor” cortex and those of spinal reflexes, these are for the most part less clear than might at first be supposed. The general statement that the coordination is of a “higher” kind in the former has doubtless truth, but it proves vague when details are demanded. A coordination though simple may yet be perfect. The co-ordination by which the leg is drawn up in the spinal “flexion-reflex” seems as perfect as when the limb is drawn up by stimulation of the cortex. It is true that in the dog’s scratching movement elicited as a spinal reflex after transection of the cord the foot in my experience practically never attains accurately the site of the stimulation, although broadly directed toward it. Were the movement elicited from the cortex one would expect it to be more accurate in this respect. But I have never succeeded in eliciting this action from the cortex, and so am unable to institute the comparison.

If by higher co-ordination it be meant that larger groups of reflex systems are simultaneously thrown into — or out of — action under cortical excitation than in merely spinal reflexes, that is more than probable. Yet we must admit that the field of musculature thrown into action by a focal stimulation of the cortex seems in some cases extremely limited. Thus A. S. Grunbaum and myself have seen that in the chimpanzee and gorilla any single manual digit can be moved isolatedly by stimulation of the cortex. We must not forget, however, that with even a small movement the field of inhibition may yet be wide, for I have on occasion noted inhibition of muscles of the shoulder when the thumb was moved under cortical excitation, the shoulder previously being unrelaxed.

There is the well-known clonic after-discharge following cortical stimulation. But a marked after-discharge is also usual in spinal reflexes, rhythmic in rhythmic reflexes, tetanoclonic in tetanic reflexes e. g. in the “flexion-reflex,” and is sometimes enormously prolonged (Figs. 49 and 57). So that even this difference is less marked than is customarily thought. Certain other differences appear to me more significant. When a spinal reflex is prolonged under strong stimulation its discharge spreads, developing what Dr. Hughlings Jackson³⁵ has termed a “march.” The “march” of the spinal reaction tends to transgress the median line more than does that of a cortical reaction, which tends rather to spread unilaterally. Also the progress of the spinal “march” runs a more rapid course than does the cortical.

In the synthesis of movement in the animal it is obvious that these reactions elicitable from the motor cortex fall into three groups, like the three groups above distinguished in spinal reflexes (Lect. IV). In one group the movement evoked from the cortex of one hemisphere seems a fraction of a natural movement, the natural movement requiring in its completeness the co-operation of the symmetrical area of the cortex of the opposite hemisphere. Opening of the jaw as elicited from one hemisphere, e. g. the left, is seen after the jaw is split at the symphysis to be executed by the muscles of the crossed, i. e. right, half of the jaw, the muscles of the left half being very slightly activated or not at all. This cortical movement is evidently incomplete and fractional. The inference is unavoidable that in the natural undeviated opening of the mouth the actions of symmetrical areas of the right and left cortices are coupled as “allied” reactions. In a second group, instanced by conjugate lateral deviation of the eyeballs toward the opposite side, it is equally obvious that the reactions of symmetrical areas of the right and left cortices are related one to another as “antagonistic” reactions. Such reactions have inhibitory relation one to another (151, 304). They must have this inhibitory relation even when combined, as Mott and Schäfer showed they can be, to yield convergence of the ocular axes under bilateral excitation of the right and left hemispheres. In a third group of cases the reactions of symmetrical cortical areas right and left seem neutral one to another. Thus, with the area which yields movement of the thumb that reaction seems neither to reinforce nor to interfere with the similar reaction evoked from the twin area of the opposite hemisphere. That the reactions are really wholly neutral one to another is of course difficult to say because the experimental observations are carried out under narcosis, and the narcosis probably sets in abeyance many coordinating mechanisms of the brain itself. It is clear, however, that the same broad groups of interrelationship (alliance, interference, neutrality) as were traced in bulbospinal reflexes reappear also between motor reactions of symmetrical areas of the cortex of the hemispheres.

It is striking that the complete, i. e. perfectly balanced, bilaterality of motor representation with which the Broca motor speech centre is credited, no doubt justly, is exceptional in the motor cortex. Beevor and Horsley pointed out that movements of perfectly balanced bilaterality are of much rarer distribution in the cortex of the hemisphere than was generally supposed. I incline to think that even the small category of such movements which they admit will have to be reduced further by the removal from it of “mastication.” Certain it is that for a group of movements to be perfectly bilaterally represented in an area of one hemisphere and not equally in the corresponding area of the other hemisphere, the state of things generally supposed for the Broca centre, is an arrangement wholly unknown in the motor cortex. It shows how different must be the operations of the Broca area from those of the areas of the so called motor cortex. Grunbaum and myself in excitation experiments could get no evidence of a Broca centre in the anthropoid apes.

One broad resemblance between the movements elicited from the motor cortex and spinal reflexes is striking, and yet is, I think, not insisted on by writers. We have seen that the movements elicitable in the various regions as local reflexes by stimulation of the afferent paths of those regions present regular and characteristic quality. Thus stimuli to a fore limb induce lifting of that limb with flexion at elbow and retraction-flexion at shoulder; stimuli to a hind limb induce drawing up of that limb with flexion at knee, hip, and dorsi-flexion at ankle; stimuli to the mouth induce opening of the jaws, and so on. While these movements are emphatically evidenced as local spinal reactions the overwhelming predominance of their occurrence equally emphasizes the scarcity of occurrence of certain other movements as local spinal reactions. Extension of the hind limb can, it is true, be evoked from that limb as a spinal reflex by a certain special form of stimulus, but that stimulus has to be applied to a special part of the foot and is successful only after “spinal shock” has passed off; while the flexion-reflex can be evoked by various forms of stimuli applied practically to any point of the limb surface, and is elicitable almost from the very hour of spinal transection onward. So also I have occasionally succeeded in evoking closure instead of opening of the jaw by stimulation of a certain part of the lip in the decerebrate animal — but even then the reflex is not regularly elicitable. On the other hand reflex opening of the jaws is easily and regularly elicitable from various points of the oral surface. Similarly extension of the elbow as a local reflex elicitable by stimuli applied to the fore limb itself is a reflex practically unknown to me.^*

Now those movements that are practically wanting as local bulbo-spinal reflexes and strike the observer of the spinal or decerebrate animal by their default, are likewise practically absent, comparatively infrequent, or only limitedly and irregularly elicitable from the motor cortex itself.³⁰⁴ On the other hand, the movements regularly and widely elicitable as local reflexes are liberally represented in the motor cortex.³⁰⁴

In the light of the observations mentioned above, which show that reciprocal innervation is a mode of co-ordination widely exhibited in the reactions elicitable through the motor cortex, this sparse occurrence of certain movements, e. g. extension of the knee or closure of the jaw, does not mean that the extensor muscles of the knee or the muscles which close the jaws are unrepresented in the cortex. It does not mean that this cortex is in touch with the flexors alone and not with the extensors. It means that the usual effect of the cortex on these latter is inhibition. It means not that the extensors and the jaw-closers are unrepresented cortically, but that their normal representation in the cortex under the ordinary conditions of experiment has the form of inhibition, not excitation, and thus unless specially sought escapes observation.

Since, as above shown, strychnine and tetanus toxin transform certain inhibitions into excitations, we have a means of further testing this point. It is in my experience quite exceptional to obtain primary extension of the opposite knee as a motor reaction from the cerebral cortex of the cat — or even, indeed, as a secondary movement. In exploring the cortex with unipolar faradization I have often failed to elicit the movement at all throughout a series of observations. Flexion, on the other hand, is regularly obtainable. After exhibition of strychnine the extension of knee can be regularly excited from the cortex, and from the very points of it that yielded flexion previously. This conversion is not so facile as the conversion of the spinal reflex. The dose of strychnine has to be larger, or to operate longer. With doses additively given, there seems, early in the experiment, a period when reflex spinal inhibition of the extensors has been converted into excitation, but the cortex of the brain still yields knee-flexion, not knee-extension. The cortical reversal has required in my hands doses that evoke convulsive seizures from time to time. I have seen, immediately after a severe convulsion, the cortex either unable to evoke any movement of the knee or produce knee-flexion, though a short while before it gave knee-extension.

Tetanus toxin likewise converts the cortical flexion into extension. The effect is in its case the more marked, because, if the cortical examination be at an early stage of the progressive malady ensuing on inoculation by a moderate dose, or where the dose has been quite small, the tetanus is “local” and confined to the inoculated limb, and then, if the tetanus be “local” in one hind limb, e. g. the left, the appropriate area of the right hemisphere yields knee-extension, whereas the corresponding of the left hemisphere yields knee-flexion.

But these effects are better studied in the monkey. There, in my experience, to obtain primary extension of the crossed knee from the cortex is, as in the cat, extremely unusual. A number of experiments can be made without obtaining it at all. Even as a secondary movement it is extremely poorly represented in the cortex. For twenty instances of flexion at knee it is, in my experience, often difficult to find one of extension at that joint. But after tetanus toxin or strychnine the whole “legarea” of the cortex, from all points of its surface, may yield nothing but leg-extension, in which extension of knee is prominent as an evident part of a primary combined movement. This is especially striking when the tetanus is still merely “local,” and confined to one hind limb, e. g. left. The “leg-area” of the right cortex then yields knee-extension everywhere; the “leg-area” of the left cortex yields the normal flexion results. The “legarea” of the right cortex provokes moreover from many of its points extension of right knee and ankle, as well as of left, though less strongly. The “leg-area” of left hemisphere does this little, if at all. Under moderate faradization the “leg-area” in the monkey, in my experience, moves the homonymous hind limb, in addition to the crossed, very slightly and rarely, much less easily than in the cat, though in both the movement is the same, namely, “extension.” So localized may be the toxic influence in its early stage that reversal of the usual cortical effect at knee may obtain while in the same hemisphere that on hip and ankle still remain flexion as usual.

Similarly with the “arm-area.” In the cat, it is in my experience quite infrequent to obtain primary extension of the crossed elbow from the cortex. Flexion is readily and regularly obtained. Strychnine changes this: the very surface that yielded flexion then provokes extension, and. strongly. But the dose of strychnine seems to be larger than for conversion of the spinal reflex, and the conversion shows the phases before mentioned in regard to the knee-inhibition, and its conversion in the case of the hamstring nerve. In the monkey, in my experience, the effect of strychnine and of tetanus toxin when pushed to the general convulsive stage is often contrary to the effect in that stage in so many other animals. I have seen them, though producing extension at elbow at first, later produce flexion at elbow. In one case, in a monkey, in which the tetanus had become general in the sense that only one limb was unaffected, the affected arm was strongly extended and rigid at elbow with some retraction at shoulder. But in all my instances, where by introduction of the toxin into the trunk of the median or ulnar a “local” tetanus of the arm has been produced, the limb has been extended rigidly at elbow and retracted at shoulder. In these cases faradic examination of the cortex showed that the small field of the “arm-area” to which extension at elbow is restricted, was enlarged so as to include the whole “arm-area.” Under the toxin the cortex that normally in the cat yields flexion of the crossed fore limb and extension of the uncrossed, will yield extension of both when there is local tetanus in the crossed limb. Extension at elbow sometimes alone, more often with retraction at shoulder, or with extension at wrist or fingers, sometimes as a leading movement, sometimes rapidly ensuent on retraction at shoulder or extension in the hand, according as higher or lower points in the area were stimulated, was prominently exhibited at all points of the entire surface of the “arm-area.” That area, with this as its salient reaction, seemed particularly in evidence, for its extreme limits appeared traceable further than usual, and to encroach on or overlap more than is usual under the feeble or moderate stimulation employed, the “leg-area” above and the “face-area” below, and to run exceptionally far forward above the precentral sulcus, though remaining undemonstrable in the free surface of the ascending parietal convolution. From no point in all this extensive “arm-area” was, despite repeated trials, any flexion at elbow or shoulder or hand obtained (Fig. 75). Various intensities of faradization were employed, and points known normally to yield it most regularly were tried: but extension, not flexion, always resulted.

This condition of the “arm-area” can in tetanus exist in one hemisphere or even in both hemispheres and the “leg-area” of each hemisphere yet yield flexions at knee and hip and ankle, and its other normal forms of reaction. Tetanus produced by introduction of the toxin into the arm (e. g. median or ulnar trunk) affects subsequently to the inoculated limb, the fellow fore limb first, and the jaw before the hind limbs, although the knee-jerk on the homonymous side to the inoculation may be brisk.

Under decerebrate rigidity, e. g. in the cat, the closing muscles of the jaw are kept in tonic action, holding the mouth somewhat shut.¹⁸² By stimulation of any point of a large “skinarea” appropriate for the reflex, reflex opening of the mouth, including depression of the lower jaw, is easily and regularly elicited, or by faradization of an afferent twig of the trigeminus; or as was shown by Woodworth and myself,²⁷⁵ even by stimulation of distant afferent nerves, e. g. plantar or saphenous. Here the action of the powerful closing muscles is reflexly inhibited while the weaker opening muscles are reflexly excited — it seems, in fact, a case of Astacus claw, except that the inhibition is central, not peripheral. This reflex “opening” is in the decerebrate animal converted into reflex closure by tetanus toxin and by strychnine, the inhibition of the predominantly powerful closing muscles being converted into excitation of them.

Similarly, when the “face-area” of the monkey’s cortex is tested by faradization after exhibition of strychnine the points of surface that previously yielded regularly the free opening of the jaw, yield strong closure of the jaw instead. Now closure of the jaw is a movement of very limited representation in the cortex of the monkey, even of the anthropoid. On the other hand, opening of the jaw is always readily and regularly elicitable from a large field of the “face-area.” And adjoining and overlapping this large area whence steady opening of the jaw is obtained, is found an area whence, as Ferrier⁷⁰ first pointed out, “rhythmic alternating opening and closing of the jaws,” as in feeding, can be evoked. Under tetanus toxin (Fig. 75) and strychnine the whole of this combined area not only ceases to yield opening of the jaws, either maintained or rhythmic, but yields closing of them instead — often with visible retraction of the tongue. For this conversion larger doses of strychnine have, in my hands, been required than for conversion of knee-flexion into extension. With tetanus toxin the conversion appears the more striking when examined early in the progress of the intoxication, because it may be found at a stage preceding altogether the occurrence of any general convulsions, and also because it can then sometimes be found to be unilateral, that is, to be present in the “face-area” of one hemisphere^* without or almost without any affection of the “face-area” of the other hemisphere.’ The reactions of the normal field thus remain for comparison in the same individual with the reactions of the abnormal field.

Tetanus toxin shows marked predilection for the closure mechanism of the jaw. After inoculation in the hind leg, even before the “local” tetanus has obviously invaded the fellow limb of the opposite side, a slight tightness of jaw and an immobile pursing of the lips has several times given warning that general tetanus had really set in, before any trace of general convulsive seizures or any involvement of the arms was detected. Tetanus toxin has also certainly intensified the reactions of the cortical areas that give retraction of the neck and retraction of the abdominal wall (Fig. 75).

The progress of the change wrought by these agents in converting these reactions of the cortex from their usual form to the diametrically opposed seems to involve the same kind of steps as that noted above in their conversion of the inhibitory hamstring nerve effect on the knee-extensor. Stages can be found in which the inhibitory effect is less than normal, yet is not replaced by excitatory. With the cortical opening of jaw, in early tetanus a grade is discoverable when faradization of the cortex produces a slight opening of the jaw — a mere “loosening” of the jaws, so to say — distinctly less than normal, and hardly effectively opening the mouth. Also with the “leg-area” of the cortex, at an early stage of the tetanus it would seem that an undue but far from exclusive preponderance of plantar extension at ankle over dorsal flexion at that joint exists, while the symptomatic knee-extension is as yet not excitable though flexion is almost in abeyance. Neither under tetanus toxin or strychnine have I at present observed conversion of the abducens inhibition into excitation.

The foregoing observations appear to give an insight into at least a part of the essential nature of the condition brought about by tetanus and by strychnine poisoning. These disorders work havoc with the co-ordinating mechanisms of the central nervous system because in regard to certain great groups of musculature they change the reciprocal inhibitions, normally assured by the central nervous mechanisms, into excitations. The sufferer is subjected to a disorder of co-ordination which, though not necessarily of itself accompanied by physical pain, inflicts on the mind, which still remains clear, a disability inexpressibly distressing. Each attempt to execute certain muscular acts of vital importance, such as the taking of food, is defeated because from the attempt results an act exactly the opposite to that intended. The endeavour to open the jaw to take food or drink induces closure of the jaw, because the normal inhibition of the stronger set of muscles — the closing muscles — is by the agent converted into excitation of them. Moreover, the various reflexarcs that cause inhibition of these muscles not only cause excitation of them instead, but are, periodically or more or less constantly, in a state of super-excitement, and yet attempt on the part of the sufferer to restrain, to inhibit, their reflex reaction, instead of relaxing them, only heightens their excitation further, and thus exacerbates a rigidity or a convulsion already in progress.

It seems to me not improbable that the virus of rabies may similarly upset reciprocal innervation, though its field of operation, at least in man, lies not in the same group of mechanisms as are affected by strychnine and tetanus toxin but in an allied one, namely, that inter-regulating (by co-ordinations involving inhibition, as Meltzer and Kronecker showed) the acts of deglutition and respiration.

Little has met me in the course of observations on the reactions of the cortex under strychnine or tetanus toxin to indicate that the transformation of the motor effects of the reactions is due to action of these agents on the cortex itself. The change of result seems quite explicable by alteration produced in lower centres, e. g. spinal and bulbar, on which the cortex acts. This seems especially shown by the toxin when injected into the right arm and producing extensor rigidity at that elbow and rigid torticollis to the right, converting the flexion of arm-area of the left hemisphere into extension under arm-area excitation, and in the right hemisphere torticollis movement to the right.

The vast rôle of inhibition in cerebral processes as evidenced by mental reactions, and the slightness of mental disorder in strychnine poisoning or tetanus indicates a difference between inhibition as it occurs in the bulbo-spinal arcs and in the arcs of purely sensual and perceptual level, a difference presumably of physicochemical nature.

We find, therefore, these reactions changed in a like manner by strychnine and tetanus whether we excite them from the cortex or from reflex spinal arcs. And a further similarity between the representation of movement in the motor cortex and in the bulbo-spinal axis as a mechanism for local reflexes is the following. When the induced movement embraces both hind limbs or both fore limbs it is in an opposite sense in the two limbs. Thus the crossed accompaniment to the flexion-reflex of the limb is extension: and so also when cortical stimulation evokes (e. g. in cat) flexion e. g. of the right fore limb, not rarely it evokes movement, weaker it is true, in the left, and that movement, as Exner⁹⁹ noted in the rabbit, is extension.

The local reflex movements obtainable from the bulbo-spinal animal and the reactions elicitable from the motor cortex of the narcotized animal fall into line as similar series. Both consist of the same group. But in striking contrast to this group stands the motor innervation active in “decerebrate rigidity.”

Decerebrate rigidity¹⁸² is a condition which ensues on removal of the fore-brain by transection at any of the various levels in the mesencephalon or the thalamencephalon in its hinder part.

If in a monkey or cat transection below or in the lower half of the bulb has been performed, the animal when suspended, artificial respiration if necessary being kept up, hangs from the suspension points with deeply drooped neck, deeply drooped tail, and its pendent limbs flaccid and slightly flexed. The fore limb is slightly flexed at shoulder, at elbow, and very slightly at wrist. The hind limb is slightly flexed at hip, at knee, and at ankle. On giving the hand or foot a push forward and then releasing it, the limb swings back into and somewhat beyond the position of its equilibrium under gravity; and it oscillates a few times backward and forward before finally settling down to its original position.

To this condition of flaccid paralysis supervening upon transection in the lower half of the bulb the condition ensuing on removal of the cerebral hemispheres offers a great contrast. In the latter case the animal, on being suspended just as after the former operation, hangs with its fore limbs thrust backward, with retraction at shoulder joint, straightened elbow, and some flexion at wrist. The hand of the monkey is turned with its palmar face somewhat inward. The hind limbs are similarly straightened and thrust backward; the hip is extended, the knee very stiffly extended, and the ankle somewhat extended. The tail in spite of its own weight, and it is quite heavy in some species of monkey, is kept either straight and horizontal or often stiffly curved upward. There is a little opisthotonus of the lumbosacral vertebral region. The head is kept lifted against gravity and the chin is tilted upward under the retraction and backward rotation of the skull on the neck. The mouth is kept closed and there is some stiffness in the elevators of the jaw. When the limbs or tail or head or jaw are pushed from the pose they have assumed considerable resistance to the movement is felt, and unlike the condition after bulbar section, on being released they spring back at once to their former position and remain there for a time more rigid than before.

The rigidity is immediately due to prolonged spasm of certain groups of voluntary muscles. The chief of these are the retractor muscles of the head and neck, the elevators of the jaw and tail, and the extensor muscles of the elbow and knee, and shoulder (i. e. deltoids) and hip. In the dog and cat, just as spinal shock is more severe in the fore limbs than in the hind, so decerebrate rigidity is more marked in the fore than in the hind limb. This prolonged spasm may be maintained, with intermissions, for a period of four days. It is increased, and even when absent or very slight, may be soon developed by passive movements of the part. There is no obvious tremor in the spasm in the earlier hours of its continuance; later it does sometimes become tremulant.

Administration of chloroform and ether, if carried far, quite abolishes the rigidity. On interrupting the administration the rigidity again rapidly returns.

Section of the dorsal columns of the spinal cord does not abolish the rigidity. Section of one lateral column of the cord in the upper lumbar region abolishes the rigidity in the hind limb of the same side as the section. Section of one ventrolateral column of the cord in the cervical region destroys the rigidity in the fore and hind limbs of the same side.

The rigidity develops either very imperfectly or not at all in a limb the afferent roots of which have been severed some days prior to carrying out the operation which produces the rigidity.

If after ablation of both cerebral hemispheres, even when the rigidity is being maintained at its extreme height, the afferent roots previously laid bare and prepared, are carefully severed, the limb at once falls flaccid. The result is quite local, that is, confined to the one limb the afferent roots of which are severed.

Decerebrate rigidity exhibits reflex excitation in those very groups of muscles which the local reflexes and the motor cortex when stimulated excite but little. Not that the muscles exhibiting the rigidity are absolutely unamenable to transient spinal reflexes. The extensor-thrust and certain crossed reflexes are witness to the contrary. And they are not absolutely unamenable to cortical excitation. The extension of the elbow obtainable from the cortex refutes that. But these instances do not efface the broad fact that a wide system of musculature, including the extensors of the hip, knee, shoulder, and elbow, and the elevators of the tail, neck, and jaw, is inhibited by the overwhelming majority of local spinal reflexes and of reactions from the motor cortex, but on the contrary is excited in a set of reflex reactions which employ the local deep afferents (proprioceptive) and some cranial mechanism seated between cerebrum and bulb. The cerebellum at once rises to mind. But I found ablation of the cerebellum did not abolish the rigidity. It is significant that a vertical posture favors the appearance and development of the rigidity. The muscles it predominantly affects are those which in that attitude antagonize gravity. In standing, walking, running, the limbs would sink under the body’s weight but for contraction of the extensors of hip, knee, ankle, shoulder, elbow; the head would hang but for the retractors of the neck; the tail and jaw would drop but for their elevator muscles. These muscles counteract a force (gravity) that continually threatens to upset the natural posture. The force acts continuously and the muscles exhibit continued action, tonus. We seem to have here a field of muscles combined as a physiological entity. A characteristic reaction yielded by muscles of this field is the “jerk,” the “tendon phenomenon,” itself a sign of highly maintained reflex tonus.

Two separable systems of motor innervation appear thus controlling two sets of musculature: one system exhibits those transient phases of heightened reaction which constitute reflex movements; the other maintains that steady tonic response which supplies the muscular tension necessary to attitude. Starting from the tonic innervation as initial state the first step in movement tends to be flexion and involves under “reciprocal innervation” an inhibition of the extensor excitation then in process. This will be involved whether the excitation be via local reflex or via the motor cortex. Hence the very muscles that to the observer are most obviously under excitation by the tonic system are those most obviously inhibited by the phasic reflex system. And the tonic system will, on inhibition of it passing off, contribute toward a return movement to the pre-existing pose, thus having its share in alternating movements and in compensatory reflexes. These two systems, the tonic and the phasic reflex systems, co-operate exerting influences complemental to each other upon various units of the musculature. Drugs and other agents that act in a selective way upon nervous processes might be expected in some cases to throw into relief the operation of one or other member of this paired system. Strychnine and tetanus toxin administered to an animal in decerebrate rigidity increase that rigidity. The posture assumed by the limbs, neck, tail, head, etc., in strychnine poisoning and in tetanus resembles closely in many respects the attitude of decerebrate rigidity. There are differences, — for instance, the ankle is often rigidly extended in tetanus whereas it is little affected in decerebrate rigidity; nevertheless there is much general resemblance.

And just as certain agents display their action more obviously in one member of these paired systems than in the other so processes of disease may be expected to deal with the two systems unequally and to reveal more obviously and affect more deeply one of them than the other. Hughlings Jackson^{75, 78, 198} with characteristic penetration of thought argued nearly thirty years ago that rigidity ensuing in hemiplegia (hemiplegic contracture) is not owing to the cerebral lesion nor to the lateral sclerosis. He said: “Whilst the primary cerebral lesion can account for the paralytic element it cannot (nor can the sclerosis of the lateral column) account for the tonic condition of the muscles. My speculation is that the rigidity is owing to unantagonized influence of the cerebellum. Whilst the cerebrum innervates the muscles in the order of their action from the most voluntary movements (limbs) to the most automatic (trunk), the cerebellum innervates them in the opposite order. This is equivalent to saying that the cerebellum is the centre for continuous movements and the cerebrum for changing movements. Thus in ‘walking’ the cerebellum tends to stiffen all the muscles; the changing movements of walking are the result of cerebral discharges overcoming in a particular and orderly way the otherwise continuous cerebellar influence. When the influence of the cerebrum is permanently taken off by disease of the cerebrum, as in hemiplegia, from the parts which it most specially governs (arm and leg) the cerebellar influence is no longer antagonized; there is unimpeded cerebellar influx and hence rigidity of the muscles which in health the cerebrum chiefly innervates. The spinal muscles are those which the cerebrum influences least and the cerebellum most. In health the whole of the muscles of the body are doubly innervated—innervated both by the cerebrum and cerebellum: there being a co-operation of antagonism between the two great centres.”

This view of Hughlings Jackson seems supported and amplified by Luciani and Stefani’s work on the cerebellum. Wernicke,^119a Mann,^153a and Lewandowski³¹² also point out that cerebral paresis selects one group of antagonistic groups of muscles in the limbs. We may very likely have to seek in the afferent nerves of muscles — especially of those antagonizing gravity — and in the nerve of the otic labyrinth — the “tonus-labyrinth” of Ewald — the sources of the influence to which Hughlings Jackson refers as “cerebellar,” but that does not radically affect in its main feature the scheme he draws of a changeful “clonic” (I would prefer to say “phasic”) innervation and a relatively unchanging tonic innervation as two systems in co-operative antagonism. Although we must also admit that the cortical innervation, pre-eminently phasic though it be, also is to some extent tonic; Lewandowski’s³¹² study of hemiplegic contracture seems to make this certain.

And here arises a question concerning the neural tonus of the skeletal musculature. Since Brondgeest’s experiment neural tonus has been demonstrated to exist in various muscles and to be of reflex origin. It has however remained a question whether all skeletal muscles habitually exhibit a reflex tonus or only some of them. Various experiments (Heidenhain, Wundt) failed to discover reflex tonus in the muscles examined by them. If the reciprocal innervation of antagonistic muscles which obtains in so many reflexes obtains also in the tonic reflexes maintaining neural tonus in muscles, it is obvious that when one muscle of an antagonistic pair exhibits reflex tonus its antagonist will not exhibit reflex tonus, but on the contrary a slight degree of reflex inhibition. We have as yet no clear evidence on this. The feeble steady excitation which is the sign of reflex tonus is often difficult to demonstrate. Feeble steady inhibition would be even less easy to detect. But the selective distribution of the jerk-phenomena, under the ordinary conditions employed for their elicitation, to single members of antagonistic couples e. g., glutaeus, vasto-crureus, masseter, and their absence, under those conditions, from the opposite members of the couples, is suggestive that, under the condition taken, reflex tonus may be confined to one member of each antagonistic pair, namely to that member which is then in reflex tonic operation, e. g. counteracting gravity for the preservation of an habitual pose of the animal.

I have laid some stress on the broad resemblance between the movements elicitable from the motor cortex and those of local spinal reflexes. There are broad differences as well.

Spinal reflex movements suggest fairly obviously protective, procreative, or visceral functions on the one hand, and on the other the main movements of the progression habitual to the animal. They seem to refer to stimulation of noci-ceptive or sexual skin nerves or visceral afferent fibres, as though initiated by these. They carry little unequivocal reference to “touch.” The existence of spinal reflexes elicited by pure “touch” — apart from that noxious touch evoking scratching-reflexes or eye-blinking — appears to me not established in respect to the normal spinal cord. Similarly in the cat and dog after decerebration no purely auditory stimulus in my experience excites a reflex,^* nor do visual, though the optic tracts and their midbrain connections have been spared in the decerebration. On the other hand various movements elicitable from the motor cortex carry the significance of possible responses to tactual, auditory, or visual stimuli; for instance, the closure of the hand, the pricking of the ear, the opening of the eyes, and turning of the head in the direction of the gaze.

Combination of cortical reaction with spinal reflex seems patent in certain reactions of the dog. Thus, the normal dog can be seen to, as it were, release, direct, and cut short a scratching reflex (v. s. p. 289). Darwin⁵⁰ draws attention to a phase of canine behavior in regard to defaecation. “Dogs after voiding their excrement often make with all four feet a few scratches backward, even on a bare stone pavement, as if for the purpose of covering up their excrement with earth, in nearly the same manner as do cats.” In the spinal dog defaecation is similarly followed by a number of vigorous backward kicks with the hind limbs. The fore limbs I have not been able to observe because the spinal transection has not lain far enough headward to liberate those limbs for free reflex action. But this movement in the hind limb follows as a reflex in the spinal dog practically invariably in immediate sequence to reflex evacuation of the faeces. In the normal dog it is, as Darwin remarked, not invariable; and it is often not in immediate sequence to the evacuation. The reflex evidently shows modification by cerebral direction and control.

Finally, it seems to me that the number of reflex actions which are “neutral” to each other, in the sense expressed in Lecture II, is less with the cerebral cortex present than without it. This amounts to expressing concretely an inference that the cerebral cortex augments the motor solidarity of the creature. Since there is more solidarity as well as more diversity in those movements of an animal which are directed to its outer environment than to its inner — meaning by this latter the fraction of environment embraced within its own pulmono-digestive cavity — the representation of visceral movement in the cortex will be relatively slight and chiefly concern parts where alimentary canal opens on outer surface.

The reactions of receptor-organs which respond to stimuli from a distance tend especially to have large cortical representation. These receptors tend more than others to control the skeletal musculature of the creature as a whole. The contribution made by the cerebral hemispheres to the solidarity of the motor creature is largely traceable to their bringing to bear on other reflexes the unifying influence of the reactions of the “distance-receptors.” This statement may in its baldness appear doctrinaire; of that character I hope to relieve it somewhat in the next following Lecture.

As to the meaning of this whole class of movements elicitable from the so-called “motor” cortex, whether they represent a step toward psychical integration or on the other hand express the motor result of psychical integration, or are participant in both, is a question of the highest interest, but one which does not seem as yet to admit of satisfactory answer. In regard to the relatively restricted problem in view in these lectures, namely, the simpler elements of the nervous integration of animal reaction, the motor reactions elicitable from the socalled “motor” cortex furnish evidence confirmatory of points mentioned before in regard to lower reflex action. This is interesting, since they must be admitted to be movements of higher order than any of those others. Nevertheless they are to my thinking merely fractional movements; movements which represent but parts of the nervous discharge which emanates from the brain under the normal working of its unmutilated whole. The results before you must appear a meagre contribution toward the greater problems of the working of the brain; their very poverty may help to emphasize the necessity for resorting to new methods of experimental inquiry in order to advance in this field. New methods of promise seem to me those lately followed by Franz, Thorndyke, Yerkes, and others; for instance, the influence of experimental lesions of the cortex on skilled actions recently and individually, i. e. experientially, acquired. Despite a protest ably voiced by v. Uexküll, comparative psychology seems not only a possible experimental science but an existent one. By combining methods of comparative psychology (e. g. the labyrinth test) with the methods of experimental physiology, investigation may be expected ere long to furnish new data of importance toward the knowledge of movement as an outcome of the working of the brain.