388 



ANATOMY FOR NURSES 



[Chap. XIX 



the medulla and the cerebrum. It is also a port of exit for the 

 fifth, sixth, seventh, and eit^hth cranial nerves. 



Cerebrum. — The cerebrum is by far the largest part of the 

 brain. It is egfj-shaped, or ovoidal, and fills the whole of the 

 upper portion of the skull. The entire surface, both upper and 

 under, is composed of layers of gray matter, and is called the cortex 

 because, like the bark of a tree, it is on the outside. The bulk of the 

 white matter in the interior of the cerebrum consists of very small 

 fibres running in three principal directions : (1) from above down- 



JNTCRNAL JUCUUIfl 



P'iG. 184. — Falx Cerebri and Tentorium, Left Lateral View. (Gerrish.) 



ward, (2) from the front backward, and {'.]) from side to side. 

 The fibres link the dift'erent parts of the brain together, and con- 

 nect the brain with the spinal cord. 



Fissures and convolutions. — In early life the cortex of the 

 cerebrum is comparatively smooth, but as time passes and the 

 brain develops, the surface becomes covered with depressions 

 which vary in depth. The deeper depressions are called fissures, 

 the more shallow ones sulci, and the ridges between the sulci are 

 called convolutions. The fissures and sulci are infoldings of 

 gray matter, consequently the more numerous and deeper they 

 are, the greater is the amount of gray matter, and the greater 

 is the extent of surface for che termination of fibres. The 

 number and depth of these fissures and sulci is thought to bear a 



