The Excitable Cortex of the Chimpanzee, Oran<;-Utan, and Gorilla 179 



must have, lying at their hand, the numerous amino-acid constituents of 

 proteins; for that purpose the food proteins, split up into constituent 

 chemical sub-groups more or less freed one from another, are presented to 

 the syntiietic organs for varied re-grouping in the re-synthesis which follows. 

 The motor cortex appears to be par excellence a synthetic organ for motor 

 acts. How does the motor cortex obtain these fractional and partial move- 

 ments on which work its powers of varied synthesis ? Simpler co-ordinated 

 elements, such as ilexion of a single joint, e.g. knee or elbow, can be safely 

 assumed to lie ready to its hand in the bulbo-spinal mechanisms. But 

 the higher of the compounded movements which those mechanisms give 

 tend, if judged from the spinal and decerebrate dog and cat, to be 

 compounds exhibiting total flexion or total extension of a whole limb. In 

 the limb movements evoked from the anthropoid motor cortex, Ilexion of 

 one joint may go either with flexion or with extension of another. The 

 motor cortex may therefore obtain the partial and fractional movements 

 it so variously weaves together by, to a certain extent, breaking up com- 

 pounds already constructed by lower centres. Such analytic power may 

 be a property of its own, or of some other, perhaps subcortical, organ with 

 which it keeps close touch. Such synthesis involves time adjustments 

 as well as spatial adjustments. The bulbo-spinal axis also, of course, syn- 

 thesises motor acts. But the diflerence between the constructive planes 

 of the two is considerable. Bulbo-spinal synthesis constructs in the main 

 those locally restricted but co-ordinate movements which the cortical 

 synthesis finds ready to hand as elements for it to work with. The bulbo- 

 spinal organ taken as a whole does, even in types so high as dog and cat, 

 synthesise in addition to the local elementary movements a not inconsider- 

 able number of more complex ones, such as respiratory, defensive, and even 

 locomotory. But comparison of the synthetic capacity of the bulbo-spinal 

 organ with that of the motor cortex reveals a great excess of synthetic 

 capacity in the latter, as evidenced by the variety and multiform scope of 

 the motor acts and sequences it builds up. Especially is this so when it 

 is borne in mind that many acts which, when naturally performed, are 

 bilateral, are, when excited by stimulation of one motor cortex, essentially 

 unilateral, indicating that the two motor cortices have to be regarded 

 as in many respects a single organ when in natural operation. Together 

 they form, in such an animal type as the anthropoid ape, an organ for 

 synthesis of movements — and of postures — on a vast scale. Phenomena, 

 such as " reversal of response," " facilitation," and " deviation of response," 

 prominent in cortical responses, and accounting for the functional instability 

 of cortical motor points, are indicative of the enormous wealth of mutual 

 associations existing between the separable motor cortical points, and those 

 associations must be a characteristic part of the machinery by which the 

 synthetic powers of that cortex is made possible. The motor cortex 

 seems to possess, or to be in touch with, the small localised movements as 

 separable units, and to supply great numbers of connecting processes 



