THE CEREBRUM. 203 



After the age of forty the weight of the cerebrum gradually dimin- 

 ishes 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 back- 

 ward 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 surface of the brain upward, out- 

 ward, 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. 



Secondary fissures of importance are found in different lobes of 

 the cerebrum, separating the various convolutions. In the anterior 

 lobe are found the precentral, superior frontal, and inferior frontal 

 fissures ; in the temporosphenoid lobes are found the first and second 

 temporosphenoid fissure; in the occipital lobe, the calcarine and 

 hippocampal fissures. 



