CEREBRUM. 123 



CEREBRUM. 



The Cerebrum is the largest portion of the encephalic mass, constitut- 

 ing about four-fifths of its weight; the average weight in the adult male 

 is from 48 to 50 ozs., or about three pounds, while in the adult female it is 

 about five ozs. less. After the age of forty the weight of the cerebrum 

 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 Blood Supply to the cerebrum is unusually large, considering its 

 comparative bulk; nearly one-fifth of the entire volume of blood being 

 distributed to it by the carotid and vertebral arteries. These vessels anas- 

 tomose so freely, and are so arranged within the cavity of the cranium, that 

 an obstruction in one vessel will not interfere with the regular supply of 

 blood to the parts to which its branches are distributed. A diminished 

 amount, or complete cessation, of the supply of blood is at once followed 

 by a suspension of its functional activity. 



The cerebrum is connected with the pons Varolii and medulla oblon- 

 gata through the crura cerebri, and with the cerebellum, through the supe- 

 rior 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 convolutions, for economy of space. 



Fissures. 



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

 appear in the development of the foetal 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, outward and backward, and forms 

 a boundary between the frontal and temporo-sphenoidal 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 sepa- 

 rates the frontal from the parietal lobe. 



3. The Parietal 'fissure ', arising a short distance behind the fissure of Ro- 

 lando, upon the convexity of the hemisphere, runs downward and back- 

 ward to its posterior extremity. 



Secondary fissures of importance are found in different lobes of the 

 cerebrum, separating the various convolutions. In the anterior lobe are 

 found the pre-central, superior frontal and inferior frontal fissures ; 



