886 CENTRAL NERVOUS SYSTEM 



of whose brains most closely resembles that of man. Conversely parts 

 of the cortex may be removed and the distribution of the muscles which 

 are then no longer under voluntary control may be determined. By 

 these methods it is learned that the principal motor area lies in the 

 cortex immediately in front of and extending into the fissure of Rolando. 

 The Representation of Functional Activity in the Motor Area. When 

 this area is explored with a localized electrical stimulus it is found that 

 definite parts of the body are excited by the stimulation of definite 

 parts of the motor area. There is then, just as in the case of sensory 

 areas, a region corresponding to each anatomical part of the body. 

 Movements of the muscles of the head are occasioned by excitation of 

 the lower portion of the area; above it the neck, arms, trunk, and legs 

 are represented in turn. Again, like the sensory areas, the representation 

 is in respect to the functional use of the parts, so that each part of the 

 cortex is the focus for impulses giving rise to an orderly act rather than 

 to the contraction of a single muscle. This is shown by the fact that a 

 weak, sharply localized stimulus gives rise to a coordinated movement 

 such as the animal might make volitionally, in which certain groups 

 of muscles contract while their opponents relax by virtue of a reciprocal 

 inhibition. Thus stimulation of an appropriate spot in the motor area 

 of the monkey will cause the fist to be clenched, an act which involves 

 the setting of the extensors of the wrist, the relaxation of the extensors 

 of the fingers, and the contraction of the flexors of the fingers. Because 

 the thing represented in the motor area is a complete functional act, 

 the areas related to each region of the body vary in size with the num- 

 ber and complexity of the acts performed by these regions. Consequently 

 the head and arm occupy a large part of the cortex because of the 

 intricacy of the muscular acts which may be carried out by the face, 

 tongue, and fingers. The leg likewise has a large representation when 

 compared with the trunk, the functional reactions of which are obviously 

 limited. In the cat the sensory and motor areas of the cortex for the 

 head, limbs and trunk also coincide in their position, but in the higher 

 apes and man they have become separated for the most part. Only 

 those aspects of sensation which are concerned with the recognition of 

 spacial relationships, particularly of the muscles and joints are repre- 

 sented in the part of the cortex which lies in front of the fissure of Ro- 

 lando, and in which the motor functions lie. This is an obvious relation- 

 ship since a very close association is to be expected between the func- 

 tions of coordinated movement and of recognition of the position and 

 movement of the limbs, etc. 



The Visuo-Motor Areas. In addition to the large motor area there are 

 two smaller areas, excitation of which gives rise to movements of the 

 ocular muscles. One of them, the frontal visuo-motor area, is located 



