(30 THE POCKET ANATOMY 



THE CEREBRAL HEMISPHERES. 



The cerebral hemispheres form as a whole an oval mass, 

 with the larger end backwards, and separated into two equal 

 portions by the great longitudinal fissure. Each hemisphere 

 presents three surfaces : 



1. Outer : convex, occupying the vault of the cranium. 



2. Inner : flat, forming one side of the longitudinal fissure. 



3. Under: irregular, corresponding in shape to the anterior 

 and middle fossae, and to the upper surface of the tentorium. 

 It has a deep cleft, the fissure of Sylvius, between the portions 

 occupying the anterior and middle fossae. 



The surface of each hemisphere consists of grey matter, 

 and is divided into convolutions or gyri by intervening sulci. 



The great longitudinal fissure separates incompletely the 

 two hemispheres, dividing them completely in front and 

 behind ; but at the base near the middle they are united by 

 the corpus callosum. The falx cerebri occupies the fissure. 



The transverse fissure of the cerebrum is a cleft in the 

 posterior part of the brain, which is seen when the velum 

 interpositum and choroid plexuses of the lateral ventricles 

 are removed. It extends from the tip of the descending cornu 

 on one side, over the optic thalami, 3rd ventricle, and mesen- 

 cephalon to the tip of the descending cornu on the other side. 



The FISSURES of the Hemispheres, or Interlobar Sulci. 



1. The fissure of Sylvius commences below at the outer 

 side of the anterior perforated spot, passes upwards and out- 

 wards between the frontal and temporo-sphenoidal lobes, and 

 divides into two branches, an anterior or ascending, the shorter, 

 passing into the frontal lobe, and a posterior or horizontal, 

 passing behind the island of Reil, upwards and backwards to 

 about the middle of the outer surface of the hemisphere. 



2. The fissure of Rolando commences at the longitudinal 

 fissure near the vertex, passes downwards and forwards 

 nearly as far as the division of the Sylvian fissure. It 

 separates the parietal and frontal lobes. 



3. The parieto-occipital fissure consists of two portions, an 

 internal or perpendicular fissure, seen on the inner surface of the 

 hemisphere, and an external, extending on the outer surface, 

 between the parietal and occipital lobes, for about an inch. 



4. The calloso-marginal fissure commences in front near 

 the anterior perforated spot, and takes a course on the inner 

 surface of the hemisphere about midway between the corpus 

 callosum and the edge of the longitudinal fissure, ending a 

 little behind the upper end of the fissure of Rolando. 



5. The collateral fissure lies along the lower border of the 

 uncinate convolution ; it projects inwards and forms the 



