670 



THE NERVOUS SYSTEM. 



the cerebral hemispheres which have fused together to form the corpus callosum 

 and the fornix, and is separated from the latter by the two layers of pia mater 

 which form the velum interpositum. Inferiorly it reaches the base of the brain, 

 forming the structures contained in the interpeduncular space. 



The third ventricle is the cavity of the inter-brain (Fig. 359). It is a narrow 

 median crevice between the two optic thalami, which constitute the side walls of the 

 inter-brain. Its roof is formed by the velum interpositum, from which are sus- 

 pended the choroid plexuses of the third ventricle. Its floor, somewhat oblique in 

 its direction, is formed, from before backward, by the tuber cinereum, with its 

 infundibulum and pituitary body ; the corpora albicantia ; the posterior perforated 

 space; and the tegmenta of the crura cerebri. Its sides are formed by the optic 

 thalami, and are limited above by a delicate band of white fibres, the stria pinealis, 



FIG. 359. The third and fourth ventricles. An arrow has been placed in the position of the foramen of Monro. 



which runs along the junction of the mesial and upper surfaces of the optic thala- 

 mus to join the anterior pillars of the fornix. Its sides are somewhat convex, so 

 that in the middle of the ventricle the two lateral walls are almost in contact, and 

 are here united across the middle line by a band of gray nervous matter, the middle, 

 gray, or soft commissure. The ventricle is bounded in front by the anterior pillars 

 of the fornix and the lamina cinerea ; behind by the pineal gland, the posterior 

 commissure, and the upper end of the iter a tertio ad quartum ventriculum. The 

 cavity is much deeper in front than behind, and presents a recess at its anterior 

 part, which lies over the optic commissure and is therefore termed the optic recess. 

 Behind and below this is the conical depression of the infundibulum, passing down- 

 ward and forward to the pituitary body. At its posterior extremity the cavity 



