CHAP. IL] THE BRAIN. 1035 



The histories, related in a preceding section, of various animals 

 deprived of their cerebral hemispheres, while they have further 

 shewn the difficulty of drawing a sharp line between the presence 

 and absence of volition, such as when we appeal to our own 

 consciousness we seem able to draw, have taught us that in a 

 broad sense the presence of volition is, in the higher vertebrata, 

 dependent on the possession of the cerebral hemispheres ; and we 

 have now to inquire what we know concerning the way in which 

 the cerebral cortex, for this, as we have seen, is the important 

 part of the cerebral hemisphere, by the help of other parts of the 

 nervous system carries out a voluntary movement. 



654. With this view we may at once turn to the results of 

 experimental interference with the cortex. When the surface of 

 the brain is laid bare by removal of the skull and dura mater, 

 mechanical stimulation of the cortex produces little or no effect, 

 thus affording a contrast with the results of mechanically stimu- 

 lating other portions of the brain, or other nervous structures. 

 And for a long time the cortex was spoken of as insensible to 

 stimulation. When, however, the electric current is employed, 

 either the make and break of the constant current, or the more 

 manageable interrupted current, very marked results follow. It is 

 found that certain movements follow upon electric stimulation of 

 certain regions or areas. The results, moreover, differ in different 

 animals. It will be convenient to begin with the dog, on which 

 animal the observations of this kind were first conducted. 



When the surface of the dog's brain is viewed from the dorsal 

 surface a short but deep sulcus is seen towards the front, running 

 outwards almost at right angles from the great longitudinal 

 fissure; this is called the crucial sulcus (Fig. 124), the gyrus or 

 convolution in front and behind it, and sweeping round its end 

 being called the sigmoid gyrus. It will hardly be profitable to 

 discuss here either the homology of this sulcus or the names of 

 the other sulci and convolutions of the dog's brain. We mention 

 this sulcus because it is found that stimulation of the cortex in a 

 region which may be broadly described as that of the neighbourhood 

 of this crucial sulcus gives rise to movements of various parts of 

 the body, whereas no such movements result from stimulation of 

 the extreme frontal region in front of the area around the crucial 

 sulcus, or from stimulation of the occipital region behind this 

 area. Certain exceptions may be made to this broad statement, 

 but these it will be best to discuss in reference to the more 

 highly developed monkey. 



The region of the cortex in the neighbourhood of the crucial 

 sulcus may then be termed an ' excitable ' or ' motor ' region, inas- 

 much as stimulation of this region leads to movements carried out 

 by skeletal muscles, while stimulation of other regions does not. 

 Further, stimulation of particular districts or areas of the region 

 leads to particular movements carried out by particular muscles. 



