436 



PHYSIOLOGY 



B. 



front part of the hemispheres in dogs with weak currents. The results of 

 their experiments are shown in Fig. 225. These experiments were soon 

 after repeated and confirmed by Ferrier, who extended his observations 

 to the monkey, and more lately by Horsley, Schafer, Beevor, Sherrington, 

 and others in the higher apes and man. It was formerly a subject of dispute 

 whether the movements resulting from stimulation of the cortex were due 

 to the excitation of the grey matter or of the underlying white matter. The 

 following facts show that the seat of the excitation is in the grey matter : 



(1) A smaller strength of current is required to excite the grey matter 

 than the underlying white matter, after removal of the grey matter. 



(2) In animals poisoned by chloral the grey matter is inexci table, though 

 movements can still be aroused on stimulating the white matter. A 



similar inexcitability of the grey 

 matter can be produced by paint- 

 ing it with cocaine. 



(3) The latent period elapsing 

 between the beginning of the stimu- 

 lation and the occurrence of the 

 movement in the corresponding limb 

 is longer when the grey matter is 

 excited than when the stimulus is 

 applied to the white matter. The 

 results obtained by Franois Franck 

 give a latent period of -065 sec. for 

 the grey matter and -045 sec. for 

 the white matter (Fig. 226). 



Whether the stimulus acts directly on 

 the pyramidal cells of the cortex, or 



Tracings to show latent periods whether, as seems more likely, it is the 



endings of the afferent nerves to the 



FIG. 226. 



of movements obtained by stimulating : 



A, grey matter ; B underlying white matter cortex which are really excited by the 



stimu l us , we cannot at present determine. 



of cortex. Time-markmg = 



When we compare different animals, such as the dog, monkey, and man, 

 we find there is a much finer differentiation of movements evoked by stimula- 

 tion of the cortex in the higher than in the lower type. Whereas in the dog 

 the excitable areas shade into one another, in the higher ape and man the 

 areas are much more circumscribed and are often separated from adjoining 

 areas by an inexcitable zone. The localisation of motor functions in the 

 cortex of the chimpanzee is indicated in the accompanying diagrams by 

 Sherrington (Figs. 227, 228). It will be seen that the motor cortex is limited, 

 on the convex side of the brain, to the precentral convolution, or ascending 

 frontal convolution, situated immediately in front of the fissure of Eolando. 

 On the inner aspect of the hemispheres only the corresponding part of this 

 convolution gives motor responses on excitation. We may say broadly that, 

 from above downwards, by stimulation of the precentral convolution we get 

 movements of the leg, arm, and face ; though, as is shown in the diagram, 



