THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 2QI 



descend to the motor nuclei of the cerebrospinal nerves. It is subdivided into 

 areas, each of which controls the muscles moving a given part of the opposite 

 half of the body; and these are arranged in inverted order, beginning with the 

 center for movement of the toes near the dorsal border of the hemisphere, and 

 ending with that for the face at the lower end of the anterior central gyrus (Fig. 

 236). 



The structure of the motor cortex is characteristic. Here the gray matter 

 reaches the maximum thickness, the lines of Baillarger are broad and diffused 

 (Fig. 218). The fifth layer contains the giant pyramidal cells of Betz, from 

 which arise the fibers of the corticospinal and corticobulbar tracts. These 

 cells undergo chromatolysis when these motor tracts are cut; and when the motor 

 cortex is destroyed the tracts degenerate (Holmes and May, 1909). 



Motor projection center 



Somesthetic area 



Auditory re- 

 ceptive center 



Motor projection center 



Somesthetic area 



Visual receptive center 



Fig. 220. Diagram of the cortical pro- 

 jection centers on the lateral aspect of the 

 cerebral hemisphere. 



Visual re- 

 Olfactory center ce P the center 



Fig. 221. Diagram of the cortical pro- 

 jection centers on the medial aspect of the 

 cerebral hemisphere. 



The motor cortex of the chimpanzee corresponds in its arrangement with 

 that of man; and by the electric excitation of its different portions muscular 

 contractions can be excited in the corresponding parts of the opposite side of 

 the body (Griinbaum and Sherrington, 1903). In addition, there is an area 

 farther forward in the frontal lobe the stimulation of which produces conjugate 

 movements of the eyes. A similar center for the conjugate deviation of the 

 head and eyes is situated in the posterior part of the middle frontal gyrus in 

 man. It is probable, however, that this motor center is of a different kind 

 from those found in the anterior central gyrus, from which all of the fibers of 

 the pyramidal system are believed to take their origin (Fig. 236). 



The sensory projection centers are the areas within which terminate the 

 sensory projection fibers. We have learned to locate such centers for vision, 



