THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 293 



as the motor cortex, and the outer line of Baillarger is greatly increased in thick- 

 ness and known as the line of Gennari (Fig. 218, C). Because of the prominence 

 of this line the region is known as the area striata. It is surrounded by cortex 

 of quite different structure; and nowhere can the differences in adjacent cortical 

 areas be better illustrated than at its border, where the prominent line of Gennari 

 is seen to terminate abruptly (Fig. 222). The fibers of the optic radiation from 

 the pulvinar and lateral geniculate body terminate in the visual projection center. 

 These fibers carry impulses from the temporal side of the corresponding retina 

 and the nasal side of the opposite one. The visual cortex of one hemisphere, 

 therefore, receives impressions from the objects on the opposite side of the line 

 of vision (Figs. 162, 163). 



The auditory receptive center is located in the anterior transverse temporal 

 gyrus, which lies buried in the floor of the lateral sulcus. The area comes to 

 the surface near the middle of the dorsal border of the superior temporal gyrus 

 (Fig. 220). It receives the auditory radiation from the medial geniculate body. 

 The olfactory receptive center is located in the uncus and adjacent portions 

 of the hippocampal gyrus (principal olfactory area of Cajal). Within it ter- 

 minate the fibers of the lateral olfactory stria. They form a rather thick layer 

 of tangential fibers on its surface, which increases the thickness of the plexiform 

 layer. 



Association Centers. It will be seen that the sensory and motor projection 

 centers occupy only a small part of the entire area of the cortex. The remaining 

 parts are connected with these centers by association fibers and are known as 

 association centers. Each area of sensory projection is surrounded by a zone 

 closely linked up with it by such fibers, and therefore probably under the dom- 

 inating influence of the particular sensory impulses reaching that projection 

 center. Their positions are indicated by lighter shading in Figs. 220 and 221. 

 Campbell (1905) has applied to them the designations "audito-psychic" and 

 "visuo-psychic fields" (Figs. 223, 224). The same author has designated 

 the portion of the frontal cortex immediately rostral to the motor projection 

 center the "intermediate precentral area," and is of the opinion it is especially 

 concerned with the "execution of complex movements of an associated kind, 

 of skilled movements, and of movements in which consciousness or volition takes 

 an active part." There still remains more than half of the cortical area, in- 

 dicated in white in Figs. 220 and 221, which is probably less intimately related 

 to any particular projection center. The fact that the increased size of the 

 human cerebral hemisphere over that of the higher apes is due to the much 



