494 PHYSIOLOGY 



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 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. 224). 



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 determine. 



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

 man, we find there is a much finer differentiation of movements evoked 

 by stimulation 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 locali- 

 sation of motor functions in the cortex of .the chimpanzee is indicated 

 in the accompanying diagrams by Sherrington (Figs. 225, 226). 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 Kolando. On the inner 

 aspect of the hemispheres only the corresponding part of this convolu- 

 tion 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, within these larger areas smaller areas can be distinguished 

 for definite co-ordinated movements of the different parts of the 

 body. 



NATURE OF MOVEMENTS EXCITED. The movements obtained 

 by excitation of these areas resemble in every respect the co-ordinated 

 movements observed during the normal willed or spontaneous activity 

 of the animal. Like the movements evoked by stimulation of a 

 sensory surface, they involve therefore the reciprocal inner vation of 

 antagonistic muscles. Never do we find simultaneous contractions of 

 antagonists, even where two opposing centres are excited simul- 

 taneously ; one reaction is prepotent, as is the case with cutaneous 

 excitation, and this reaction is attended and brought about by ordered 

 contraction of certain muscles accompanied by an ordered relaxation 

 of their antagonists. Thus the movement of opening the jaw, which 

 can be excited from a fairly large area of the cortex, involves a relaxa- 

 tion of the normal tone of the masseter muscle. Flexion of the leg 

 demands relaxation of the extensor muscles. As in the case of the 

 spinal reflexes, this relaxation, or inhibition, can be abolished under 



