FUNCTIONS OF THE CEREBRAL HEMISPHERES 435 



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 inexcitable, though 

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

 inexcitability of the grey matter 



can be produced by painting it 

 with cocaine. 



(3) The latent period elapsing 

 between the beginning of the 

 stimulation and the occurrence of 

 the movement in the correspond- 

 ing limb is longer when the grey 

 matter is excited than when the 

 stimulus is applied to the white 

 matter. The results obtained by 

 Francois Franck give a latent 

 period of -065 sec. for the grey 

 matter and '045 sec. for the 

 white matter (Fig. 226). 



A. 



B. 



Whether the stimulus acts directly 

 on the pyramidal cells of the cortex, or 

 whether, as seems more likely, it is the 

 endings of the afferent nerves to the 

 cortex which are really excited by the 

 stimulus, we cannot at present deter 

 mine. 



FIG. 226. Tracings to ehow latent periods 

 of movements obtained by /stimulating: 

 A, grey matter ; B, underlying white matter 

 of cortex. Time-marking = T Q sec. 



(F. FRANCK.) 



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 Rolando. 

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

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



