732 FUNCTIONS OF THE CEKEBRUM [CH. L. 



results, and no doubt the human brain would give identical results 

 also if it could be examined. 



The method used is to expose the brain in an anaesthetised animal, 

 and thoroughly explore it with a weak faradic current, one electrode 

 being placed on the brain, and the other attached to an indifferent 

 part of the animal's body. This allows of finer localisation than is 

 possible with the ordinary double-point electrodes. 



The motor area includes continuously the whole length of 

 the ascending frontal, or as it is sometimes called, the precentral 

 convolution. It never extends behind the central sulcus, or, 

 as it is sometimes called, the fissure of Eolando. On the mesial 

 surface it extends but a short distance, and never as far as the 

 calloso-marginal fissure. The motor area extends also into the depth 

 of the Eolandic and other fissures; the part of the excitable area 

 thus hidden equals or may even exceed that on the free surface of 

 the hemisphere. The arrangement of the various regions of the 

 musculature follow the segmental sequence of the cranio-spinal series 

 to a remarkable extent ; in fact, the excitable area may be compared 

 to the spinal cord upside down. The accompanying figure indicates 

 this better than any verbal description. 



It cannot fail to strike even a superficial observer how large 

 the cortical area is that deals with movements of the head and arm 

 regions when compared with that of the lower limb, and still more 

 with that of the trunk. The trunk itself has a larger mass of 

 muscular tissue, but it is in the head region (which includes the 

 complex movements of the tongue and such structures as the vocal 

 cords) and in the arm and hand that the movements are most varied 

 and most delicate. No doubt this is the explanation of the greater 

 size of their cortical representation. 



It is these finer movements which are most affected by a cortical 

 injury, and which exhibit least recovery; in the upper limb, for 

 instance, the shoulder muscles will be the least, and the hand the 

 most paralysed. 



In experiments on unilateral extirpation in animals, and in 

 destructive lesions of one side of the brain in man, it is the muscles 

 which act normally unilaterally which are most paralysed. The 

 muscles which normally move bilaterally, e.g., the chest muscles in 

 breathing, the trunk muscles in maintaining an erect position, are 

 comparatively little affected ; the spinal centres of such muscles are no 

 doubt connected by commissural fibres, and therefore can be affected 

 from both sides of the brain. 



The marginal convolution on the mesial surface of the hemisphere was first 

 investigated by Schafer and Horsley, in the lower monkeys. They found in these 

 animals that it contained a considerable extension of the " motor " area, including 

 the cortical centres for the trunk muscles. This, at any rate, is not the case for the 

 higher apes, and therefore probably is not true for man. 



