658 



THE NERVOUS SYSTEM. 



The fissure of Sylvius at the base of the brain separates the frontal from the 

 temporal lobe, and lodges the middle cerebral artery. It has also been described 

 (page 645). 



The optic tracts are well-marked flattened bands of fibres, which run obliquely 

 across the crus cerebri on either side, and unite anteriorly to form the optic com- 

 missure. They will be described in connection with the cranial nerves. 



The crura cerebri (peduncles of the cerebrum^) are two thick cylindrical bundles 

 of white matter, which appear in front of the anteriqr border of the pons, and 

 diverge as they pass forward and outward to enter the under surface of each hemi- 

 sphere. Each crus is about three-quarters of an inch in length, and is about the 

 same in breadth anteriorly, but somewhat less posteriorly. They are marked upon 

 their surface with longitudinal striae, and each is crossed, just before entering 

 the hemisphere, by the fourth nerve and the optic tract, the latter of which is 

 adherent by its upper surface to the peduncle. 



FIG. 352. Section of the brain. Made on a level with the corpus callosum. 



The Temporal Lobe. The under surface of the temporal lobe is visible at the 

 base of the brain, on either side of the crura and the structures contained in the 

 interpeduncular space. It is separated anteriorly from the frontal lobe by 

 the fissure of Sylvius, and behind is limited by the anterior border of the lateral 

 hemispheres of the cerebellum. The fissures and lobes on its surface have already 

 been described (page 652). 



The hemispheres of the cerebellum are situated on either side of the middle 

 line, and cover the occipital lobes of the cerebrum, when viewed from the base. 

 The cerebellum differs much in appearance from the rest of the encephalon, being 



