THE BRAIN 



295 



528. The cerebrum. The main nerve tract, after pass- 

 ing through the spinal cord, medulla, and optic tubercles, 

 spreads out to form a mass called the 



cerebrum. While in frogs and fishes it 

 is no larger than the medulla or optic 

 tubercles, in man it forms more than 

 four fifths of the whole brain and over- 

 hangs all the other parts. 



It consists of a central mass of nerve Cellg from the gray 

 threads covered with a layer of gray matter of the cerebrum, 

 matter one eighth of an inch in thick- 

 ness, containing numerous large cells. Each cell gives off 

 numerous fine fibers. Most of these fibers form an intri- 

 cate network among the cells, but one from each cell takes 

 a covering and becomes a nerve thread of the white mat- 

 ter, and finally reaches other cells of the brain or even of 

 the spinal cord. 



529. Fissures of the brain. The cerebrum is divided 

 nearly into two parts, called hemispheres, by a deep furrow 



running forward and 

 backward upon the mid- 

 dle of its upper surface. 

 Another furrow, called 

 the Sylvian fissure, starts 

 near the bottom of the 

 fore part of the side of 

 the cerebrum and runs 

 backward and upward. 

 Many other furrows and 

 fissures from one quarter 

 to one half inch in 



depth, run in waving lines between its main furrows, throw- 

 ing its surface into folds called convolutions. The convo- 



