740 CORTICAL MOTOR REGION. [BOOK in. 



The histories, related in a preceding section, of various ani- 

 mals 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 verte- 

 brata, 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. 



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

 of experimental interference with the cortex. When the sur- 

 face 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 mechani- 

 cally stimulating 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 dor- 

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

 running outwards almost at right angles from the great longi- 

 tudinal fissure ; this is called the crucial sulcus (Fig. 121), the 

 gyrus or convolution in front and behind it, and sweeping round 

 its end being called the sigmoid gyrus. It will hardly be profit- 

 able 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 move- 

 ments 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, 

 inasmuch as stimulation of this region leads to movements car- 

 ried out by skeletal muscles, while stimulation of other regions 

 does not. Further, stimulation of particular districts or areas 



