CEREBRUM. 183 



The cerebellum may possibly exert some influence over the sexual functions, 

 but physiologic and pathologic facts are opposed to the idea of its being the 

 seat of the sexual instinct. It appears to be simply a center for the coordina- 

 tion and equilibration of muscular movements. 



THE CEREBRUM. 



The cerebrum is the largest portion of the encephalic mass, constituting 

 about four fifths of its weight; the average weight of the adult male brain is 

 from forty-eight to fifty ounces, or about three pounds while that of the adul t 

 female is about five ounces less. After the age of forty the weight of the cere- 

 brum gradually diminishes at the rate of one ounce every ten years. In 

 idiots the brain weight is often below the normal, at times not amounting to 

 more than twenty ounces. 



The cerebrum is connected with the pons Varolii and medulla oblongata 

 through the crura cerebri, and with the cerebellum through the superior 

 peduncles. It is divided into two lateral halves, or hemispheres, by the 

 longitudinal fissure running from before backward in the median line; each 

 hemisphere is composed of both white and gray matter, the former being 

 internal, the latter external; it covers the surfaces of the hemisphere which are 

 infolded, forming fissures and convolutions. 



Fissures. 



1. The fissure of Sylvius is one of the most important; it is the first to appear 

 in the development of the fetal brain, being visible at about the third month; 

 in the adult it is quite deep and well marked, running from the under sur- 

 face of the brain upward, outward, and backward, and forms a boundary 

 between the frontal and temporosphenoid lobes. 



2. The fissure of Rolando is second in importance, and runs from a point on 

 the convexity near the median line transversely outward and downward 

 toward the fissure of Sylvius, but does not enter it. It separates the frontal 

 from the parietal lobe. 



3. The parietal fissure, arising a short distance behind the fissure of Rolando, 

 upon the convexity of the hemisphere, runs downward and backward to its 

 posterior extremity. 



4. The parieto-occipital fissure separates the occipital from the parietal lobe. 

 Beginning upon the outer surface of the cerebrum, it is continued on the 

 mesial aspect downward and forward until it terminates in the calcarine 

 fissure. 



5. The callosomarginal fissure lies upon the mesial surface, where it runs 

 parallel with the corpus callosum. 



