588 



NERVOUS SYSTEM. 



The fissure of Sylvius separates the anterior and middle lobes, and lodges the 

 middle cerebral artery. At its commencement is seen a point of medullary 

 substance, corresponding to a subjacent band of white fibres, connecting the 

 anterior and middle lobes, and called the fasciculus unciformis; on following 

 this fissure outwards, it divides into two branches, which inclose a triangular- 

 shaped prominent cluster of isolated convolutions, the island of Reil. These 

 convolutions, from being covered in by the sides of the fissure, are called the 

 gyri operti. 



Fig. 329. Base of the Brain. 



The anterior perforated space is situated at the inner side of the fissure of 

 Sylvius. It is of a triangular shape, bounded in front by the convolution of 

 the anterior lobe and the roots of the olfactory nerve ; behind, by the optic 

 tract; externally, by the middle lobe and commencement of the fissure of 

 Sylvius; internally, it is continuous with the lamina cinerea, and crossed by the 

 peduncle of the corpus callosum. It is of a grayish color, and corresponds to 

 the under surface of the corpus striatum, a large mass of gray matter, situated 

 in the interior of the brain ; it has received its name from being perforated by 

 numerous minute apertures for the transmission of small straight vessels into 

 the substance of the corpus striatum. 



The optic commissure is situated in the middle line, immediately behind the 

 lamina cinerea. It is the point of junction between the two optic nerves. 



Immediately behind the diverging optic tracts, and between them and the 

 peduncles of the cerebrum (crura cerebri], is a lozenge-shaped interval, the inter- 



