6o2 TEXT-BOOK OF PHYSIOLOGY 



The Motor Areas. The motor areas especially those associated with 

 the execution of volitional movements have been assigned to the pre-central 

 convolution, the contiguous portions of the base of the medi- and sub-frontal 

 convolutions and the para-central lobule. 



The exclusion of the post-central convolution from the motor area, to 

 which it belongs in the monkey brain, is in accordance with the embryologic 

 researches of Flechsig, which indicate that the efferent fibers which compose 

 the pyramidal tract come from the region anterior to the central fissure, and 

 with the experiments of Sherrington and Grunbaum on the brain of the 

 chimpanzee, which demonstrate that the post-central convolution is abso- 

 lutely inexcitable to electric stimulation. It is quite probable that with the 



FIG. 249. SCHEME OF THE MOTOR AREA OF THE HUMAN BRAIN AND ITS SUBDIVISIONS. (After 



Mills.) 



growth of the brain in size and complexity, the motor area has come to occupy 

 a position somewhat farther forward in the human brain than in the monkey 

 brain. 



This general area is capable of subdivision into areas of variable size, e.g., 

 head and eyes, face, arm, trunk, and leg areas which are related through the 

 efferent nerve-fibers composing in part the pyramidal tract to groups of 

 muscles, e.g., head and eye muscles, face, arm, trunk and leg muscles, on 

 the opposite side of the body and the activities of which they initiate and 

 control. (See Figs. 247 and 248.) 



These large subdivisions of the cortical motor area are in turn capable 

 of a further subdivision into smaller areas which are in relation with one or 

 more muscles of the larger groups (Fig. 249). These smaller areas indi- 

 cated in the diagram by compressed italics contain groups of nerve-cells 

 which excite to action, through their efferent axons and their medullary and 

 spinal connections, the muscles which impart movements to the regions 

 corresponding to the words in compressed italics. 



