296 THE NERVOUS SYSTEM 



execution of skilled volitional movements of an associated kind. In the same 

 way, after a lesion in the posterior part of the left superior temporal gyrus, 

 the patient may hear the spoken word, but no longer comprehend its meaning. 

 This is sensory aphasia or word deafness. Word blindness, the inability to under- 

 stand the printed or written language, although there is no impairment of vision, 

 may result from lesions in the angular gyrus. These three areas are often spoken 

 of as speech centers and are closely united together by association fibers. In 

 fact, it is not altogether clear to what extent such defects as those mentioned 

 above are dependent upon the destruction of these association tracts which lie 

 subjacent to the speech centers. 



THE MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 



The medullary center of the cerebral hemisphere underlies the cortex and 

 separates it from the lateral ventricle and corpus striatum. It varies greatly 

 in thickness, from that of the thin lamina separating the insula and the claus- 

 trum (Fig. 191) to that of the massive centrum semiovale (Fig. 174). The 

 myelinated nerve-fibers of which it is composed are of three kinds, namely, as- 

 sociation fibers, projection fibers, and commissural fibers. 



Commissural Fibers. As was stated in Chapter XV, there are three com- 

 missures joining together the cerebral hemispheres. Of these, the corpus callo- 

 sum is by far the largest and its radiation contributes largely to the bulk of the 

 centrum semiovale (Fig. 174). The fibers which compose it arise in the various 

 parts of the neopallium of each hemisphere; they are assembled into a broad 

 compact plate as they cross the median plane, and then spread out again to 

 terminate in the neopallium of the opposite side. As they spread through the 

 centrum semiovale they form the radiation of the corpus callosum. Some cor- 

 tical areas are better supplied with these fibers than others, few, if any, being 

 associated with the visual cortex about the calcarine fissure (Van Valkenburg, 

 1913). The majority of the callosal fibers do not connect together symmetric 

 portions of the cortex; but, after crossing the median plane, the fibers from a 

 given point in one hemisphere spread out to many parts of the opposite side. 

 The anterior and hippocampal commissures connect portions of the rhinencephalon 

 in one hemisphere, with similar parts on the opposite side. The anterior com- 

 missure connects together by its rostral part the two olfactory bulbs and by its 

 caudal part the two pyriform areas (Figs. 187, 194, 195). The hippocampal 

 commissure is composed of fibers which join together the two hippocampi by 

 way of the fimbriae and the psalterium. 



