THE NERVOUS SYSTEM 



hearing, smell, and the general sensations from the surface of the body and the 

 deeper tissues. The latter region, known as the common sensory or somesthetic 

 area, is located in the posterior central gyrus (Areas 1, 2, and 3 of Brodmann). 

 It receives fibers belonging to the thalamic radiation from the lateral nucleus of 

 the thalamus and representing neurons of the third order in the afferent paths 

 from the skin, muscles, joints, and tendons. 



The most conclusive evidence of the sensory function of the posterior central 

 gyrus is furnished by Cushing's (1909) observations on the electric excitability 

 of the human cerebral cortex. These tests were made on unanesthetized patients 

 in the course of operations for brain tumors. Stimulation of the cortex within 

 the posterior central gyrus caused the patients to experience cutaneous sensa- 

 tions, which seemed to come from the skin of the hand, but did not elicit any 

 motor responses; while in these same cases stimulation of the anterior central 



Calcarine fissure- 



Transition between striate 

 and peristriate areas 



Cuneus - 



Tangential fibers 



^ - - -Stria of Gennari 



-White center 



Fig. 222. Section through the most rostral part of the cuneus. Pal-Weigert method. 



gyrus gave rise to no sensations, but did call forth muscular contractions. On 

 the other hand, Head (1918), in a recent study of "Sensation and the Cerebral 

 Cortex," would include in the somesthetic area the anterior as well as the posterior 

 central convolution, and also the anterior part of the superior parietal lobule 

 and the angular gyrus. This study shows, perhaps better than any other work, 

 how intricate and difficult the problem of cortical localization really is and how 

 far we are from an ultimate solution. 



The visual receptive center is located in the cortex forming the walls of the 

 calcarine fissure and in the adjacent portions of the cuneus and the lingual 

 gyrus (Figs. 217, 221). Rostral to the point where the calcarine is joined by the 

 parieto-occipital fissure the visual cortex is located only along the ventral side 

 of the former. Sometimes the center may extend around the occipital pole on 

 to the lateral surface of the brain (Fig. 216, Area 17). The structural peculiar- 

 ities of the visual cortex are very evident. It is not more than one-half as thick 



