774 



THE NERVOUS SYSTEM. 



Structure of the Convolutions. The outer surface of each convolution, as well 

 as the sides and bottom of the sulci between them, are composed of gray matter, 

 which is here called the cortical substance. The interior of each convolution is 

 composed of white matter, medullary centre, the white fibres of which blend with 

 the gray matter not only on the surface of the gyrus, but at the sides and bottom 

 of the sulci as well. By this arrangement the convolutions are adapted to 

 increase the amount of gray matter without occupying much additional space, 

 and to afford a greater extent of surface for the termination of the white 

 fibres. 



EXTERNAL LOBES AND FISSURES OF THE HEMISPHERE. Each hemisphere of 



Splenivm 



Lyra 



Ant. pillar 

 of fornix 



Ant. 



Post, pillar 

 ofjornix 



Hipp. 



gyrus 



Fascia 

 dent. 



FIG. 465. Mesial or inner aspect of part of left hemisphere. Tbe large central concavity shows place of 

 removal of optic thalamus. (Heule.) 



the brain on its external surface is divided into five lobes, the division being made 

 by the main fissures and by imaginary lines drawn to connect them (Fig. 466). 



The fissures dividing the five lobes on the external surface of the hemispheres 

 are three in number, and are named fissure of Sylvius, fissure of Rolando, and 

 parieto-occipital fissure. 



The fissure of Sylvius separates the frontal froir the temporal lobe, and 

 lodges the middle cerebral artery. It begins, at the base of the brain, at the 

 outer side of a depression at the bottom of which is the anterior perforated space. 

 This depression is called the vallecula Sylvii. It then passes outward to the exter- 

 nal surface of the hemisphere, and gives off a short anterior limb, which passes 

 forward, and another, ascending limb, which passes upAvard into the inferior frontal 

 convolution. It is then continued backward as the horizontal limb, and terminates 

 in the parietal lobe after curving upward for a short distance. It separates the 

 frontal and parietal lobes from the temporal, and occupies about the middle third 

 of the lateral surface of the hemisphere. 



The fissure of Rolando is situated about the middle of the outer surface of the 

 hemisphere. It commences at or near the longitudinal fissure, and runs downward 

 and forward to terminate a little above the beginning of the horizontal limb of the 

 fissure of Sylvius, and about half an inch behind the ascending limb of the same 

 fissure. It separates the frontal from the parietal lobe. 



The parieto-occipital fissure is only seen to a slight extent on the outer surface 

 of the hemisphere, and is not so distinctly marked as the others. The portion on 

 the outer surface of the hemisphere is sometimes called the external /><>/ !< to-occip- 

 ital fissure, to distinguish it from the continuation of the sulcus on the internal 

 surface of the hemisphere, which would then be termed the internal parieto-occip- 

 ital fissure. It commences about midway between the posterior extremity of the 



