MOTOR AREAS OF THE HUMAN BRAIN 6oi 



conditions is not wanting to support the general idea of the existence of cor- 

 tical motor centers in the human brain, figure 402. 



So far, however, it has been possible to localize motor functions only 

 in the precentral convolutions and the walls of the adjacent sulci. 



The relative position of the centers is probably much the same as in the 

 monkey's brain, those for the leg above, those of the arm, face, lips, and 

 tongue from above downward. Destruction of these parts causes paralysis, 

 corresponding to the district affected, and irritation causes contractions of 

 the muscles of the same part. Again, a number of cases are on record in 



ANUS A VAGINA 



OPENING VOCAL 



OFJAW CORDS 



MASTICATION 



FIG. 402. Scheme Showing the Motor Areas of the Brain. (Adapted from Griinbaum 

 and Sherrington by Cunningham.) 



which aphasia, or the loss of power of expressing ideas in words, has been 

 associated with disease of the posterior part of the lower or third frontal con- 

 volution on the left side. This condition is usually associated with motor 

 paralysis on the right side of the body, right hemiplegia. 



This district of the brain, particularly the convolutions bounding the 

 fissure of Rolando anteriorly, is now generally known as the motor area. 

 There is now no doubt whatever that this area gives origin to the nerve fibers 

 which proceed to the spinal cord, and are there represented as the pyramidal 

 tracts. 



This is the reason that movements are produced on stimulation of the 

 white matter after the superficial gray matter of the animal's brain has been 

 sliced off. 



These motor fibers are those which arise from the pyramidal cells of the 

 cortex. From the motor area of the cortex they converge to the internal cap- 



