662 THE BEAIN. 



voluntary and involuntary, not in the coordination of the muscular and 

 nervous components of a movement, but in the nature of the process which 

 starts the whole act. 



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

 of their cerebral hemispheres, while they have further shown 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 vertebrate, 

 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. 



567. With this view we may at once turn to the results of experi- 

 mental 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 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 inter- 

 rupted current, very marked results follow. It is found that certain move- 

 ments follow upon electric stimulation of certain regions or areas. The re- 

 sults, moreover, differ in different animals. It will be convenient to begin 

 with the dog, on which animal the observations of this kind were first con- 

 ducted. 



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

 short but deep sulcus it seen toward the front, running outward almost at 

 right angles from the great longitudinal fissure ; this is called the crucial 

 sulcus (Fig. 147), the gyrus or convolution in front and behind it, and 

 sweeping around 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 men- 

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

 which may be broadly described as that of the neighborhood 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 neighborhood of the crucial sulcus may 

 then be termed an " excitable " or " motor " region, inasmuch 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. For instance, stimulation of the more median 

 parts of the gyrus behind the crucial sulcus (Fig. 147, C) leads to move- 

 ments of the hind limb, whereas stimulation of the lateral part or outer end 

 of the same gyrus leads to movements of the fore limb, and we may here 

 distinguish between an area, stimulation of which (Fig. 147, E) leads to 

 flexion of the fore limb, and an area (Fig. 147, A), stimulation of which 

 leads to extension of the same limb. In a similar way stimulation of other 

 areas within the " motor " region leads to movements of this kind or of that 

 kind of the tail, of the eyes, of the mouth, of other parts of the face, of the 



