OBSERVATIONS ON THE EXCITABLE CORTEX OF THE CHIMPANZEE, ORANG‐UTAN, AND GORILLA

1. The “motor” area of the cortex in the three species of anthropoid examined (chimpanzee, orang‐utan, and gorilla), as determined by faradisation, embraces almost all of the free surface and a large part of the sulcal surfaces of gyrus centralis anterior; it also extends over the mesial border upon gyrus marginalis for a distance about half‐way toward sulcus cinguli, in agreement with Campbell's delimitation of his “precentral” type of cortex in chimpanzee and orang. 2. The proportion of motor area buried in the sulci is probably usually about one‐third of the whole area. 3. Although the broad “localisation” of the responses of the various main motor parts of the opposite half of the body follows a well‐fixed topographical scheme in this cortex, the minuter localisation, as examined by faradisation, is subject to temporal instability. 4. This instability is largely the expression of mutual influences exerted transiently by the physiological states for the time being of different points of the motor cortex, and of the sub‐cortical centres they connect with, one upon another. These influences make themselves felt as “deviation of response,” “reversal of response,” and “facilitation,” phenomena all seemingly akin. 5. Subject to this temporal instability, details of localisation of various movement groups in chimpanzee, orang, and gorilla are described. Differences in the smaller details of the localisation were met with from individual to individual of the same species, and between the right and left motor areas of the same individual. Making all allowance for experimental error, these differences, in some particulars, seem too large to be accounted for fully by that or by temporal instability of the cortex; they may represent, as Franz urges for analogous differences he found in macaque, permanent individual differentia existing from hemisphere to hemisphere. 6. The motor responses obtained by faradisation from a baby chimpanzee equalled in differentiation, as far as could be seen, the average of those obtained from the adult of any of the three anthropoid species examined. And “epilepsy” was produced neither more nor less easily than in those. 7. The anterior edge of the motor area seems to fade away somewhat gradually into inexcitable cortex. Farther forward still is a large diffuse field, from scattered points of which, in the middle and inferior frontal convolutions not extending to their more anterior parts, conjugate deviations of the eyeballs and opening of both eyes are elicitable. But stronger faradisation is required for these, nor are the results so regular as with the responses of the motor area proper. 8. Eyeball movements similar to those just mentioned are likewise obtainable from the occipital pole and from the calcarine region. 9. The motor area for face and tongue movements seems, relatively to the rest of the motor area, more extensive in orang than in chimpanzee. Apart from that distinction, there seemed no clear difference between the motor area from species to species of the anthropoids examined. The largest and most highly developed brain we examined was that of a gorilla, and the motor area in that specimen appeared to be, on the whole, the most extensive and differentiated of those experimented upon. 10. The motor cortex may be regarded as a synthetic organ for compounding and re‐compounding in varied ways movements of varied kinds of scope from comparatively small, though in themselves well co#x2010;ordinate, fractional movements. For this synthesis the motor cortex is provided with, i.e. has at call, these partial or fractional movements and postures. The cortex obtains these partial movements, perhaps by analytic powers of its own, from the bulbo‐spinal mechanisms, but the higher of the synthetic results of the bulbo‐spinal mechanism exhibit, as judged from cat and dog, certain only of the kinds of compound movements which the motor cortex gives. From the recomposition of these partial movements into wholes of varied pattern and sequence there result motor acts which, taken in their entirety, making use of the same fractional pieces, attain with them aims of varied scope by varying the spatial and temporal combinations of them. 11. The free surface of gyrus centralis posterior was found to differ from that of gyrus centralis anterior in not being similarly excitable by faradisation. Faradisation behind the sulcus centralis can, under certain circumstances, evoke reactions from the cortex, but these are doubtful for acceptance as equivalent to “motor‐area” reactions. They appear as “echo‐responses” when the faradisation is made to follow directly and quickly on faradisation of gyrus centralis anterior in the near neighbourhood, i.e. about the same horizontal level and not far from sulcus centralis. The “echo‐response” thus obtained from gyrus centralis posterior repeats a response just previously given froma centralis anterior, and soon dies out under repetition of the stimulus to gyrus centralis posterior, unless stimulation of gyrus centralis anterior is repeated to renew it. 12. Stimulation of the middle and posterior parts of the inferior frontal convolution of left hemisphere failed in chimpanzee, orang, or gorilla to evoke any vocalisation. Ablation of a large portion of that area in one chimpanzee, chosen because it was a noisy and vociferous animal, produced no obvious impairment or change in vocalisation. 13. Faradisation of the surface of the insula failed to evoke any detectable results. 14. Unipolar faradisation of the cut cross‐face of the crusta (orang) evoked responses separately in toes and ankle, hip, trunk, arm, and face from a series of points taken in order from without inward (mesially). From the cut cross‐face of the pyramidal bundles in the pons (gorilla), unipolar faradisation evoked toe movement predominantly from the most lateral, face‐tongue movement predominantly from the most mesial, and finger movements predominantly from the intermediate. 15. Ablation of the cortex of the larger portion of an arm or leg area in gyrus centralis anterior produced heavy paresis of the corresponding limb, but this paresis quickly lessened, and the limb soon regained in large measure its volitional motility, and became successfully used for climbing, picking up food, picking the teeth, etc. Ablation further of the greater part of the arm area of the second hemisphere, after previous ablation of the greater part of that area from the other hemisphere, induced no recrudescence of paresis in the already paretic and partly recovered arm. After the double lesion considerable recovery of the volitional use of both limbs somewhat rapidly ensued, the hands, for instance, being used freely in climbing, picking up food, etc. 16. The degenerations in the spinal cord following on limb‐area lesions exhibited a large crossed pyramidal tract, extending more to the edge of the lateral column than in man, and in this respect resembling a feature seen in the macaque cord. There was obvious also a slight ipsilateral pyramidal tract in the ipsilateral lateral column, derived from a small portion of the fibres of the pyramid passing not to the lateral column of the crossed side, but to that of the ipsilateral side. There was also evident in two of the chimpanzees an ipsilateral ventral pyramidal tract similar in position and relative size to that (“direct Py. T.”) commonly seen in man; this is not existent in the macaque. The pyramidal‐tract degeneration after the arm‐area lesions descended beyond the brachial enlargement of the cord, but did not reach the lumbo‐sacral enlargement. The pyramidal‐tract degeneration ensuing on the leg‐area lesions descended the whole length of the cord. Many fine degenerate fibres (collaterals) were visible in the ventral horn of grey matter among the motor perikarya on the side contralateral to the cortical lesion in the brachial segments of the arm‐area lesion and in the lumbosacral enlargement in the leg‐area lesion. 17. Ablations of portions of the free surface of gyrus centralis posterior gave rise to no obvious symptoms of paresis, nor, in the one case whose bulb and spinal cord were examined, to degeneration of the pyramidal tract. 18. The threshold of faradic excitability of the motor cortex, as tested in the arm area, seems to be practically similar in cat, macaque, and chimpanzee. 19. In some chimpanzees occlusion of the two carotid arteries renders the motor cortex inexcitable to faradism, and does so rapidly, e.g. 2 minutes. After release of the arteries, responses to faradic stimulation reappear in about 1½ minutes. In one animal occlusion of one carotid alone reduced the excitability of the motor area of the corresponding hemisphere almost to extinction, but in another animal occlusion of one carotid alone did not markedly depress the excitability. The anthropoid brain, unlike the brain of the smaller monkeys, has frequently a Circle of Willis of human pattern (20). 20. In a chimpanzee with a cranial vault of about the thickness of the average human, the application of local cold (ice‐bag) and warmth over the parietal scalp rapidly affected the temperature of a thermometer bulb lying under the dura against the cerebral surface beneath the region of application of the local cold or warmth outside. It is a pleasure to express here our thanks to Professor Harvey Cushing of Boston, to Dr A. W. Campbell of Sydney, and to Dr Alfred Fröhlich of Vienna for valued co‐operation and kind assistance in many of the experiments. To Dr Besredka, Dr Weinberg, and the late Professor Metchnikoff of the Institut Pasteur we are indebted for one of the orangs used. We have much pleasure too in recording our acknowledgment of the energy of Mr E. G. Cox, assistant in the laboratory, in obtaining the animal material, and of his skill and care in all matters pertaining to the management of it.