THE NBRVOVS SYStEM 1415 



it lies in the roof of the deep extremity of the descending comu of 

 the lateral ventricle. The putamen of the nucleus lenticularis hes 

 above it; anteriorly it is continuous \vith the cerebral cortex; 

 inferiorly it receives the narrow part or tail of the nucleus caudatus; 

 and the taenia semicircularis arises from it. 



The taenia semicircularis is a narrow white band of fibres which, 

 as just stated, arises from the amygdaloid nucleus. The band 

 passes backwards in the roof of the descending comu of the lateral 

 ventricle, and then it sweeps upwards and forwards into the body 

 of the lateral ventricle, lying between the nucleus caudatus and the 

 optic thalamus. Anteriorly, in the region of the foramen of Monro, 

 it dips downwards towards the anterior pillar of the fornix and the 

 anterior white commissure. In this situation its fibres are variously 

 disposed. Some pass into the anterior pillar of the fornix ; others 

 pass in front of the anterior commissure, and enter the grey matter 

 between the head of the nucleus caudatus and the septum lucidum ; 

 whilst a few are regarded as entering the nucleus caudatus. The 

 vein of the corpus striatum hes superficial to the taenia semicircularis. 



Relation of Structures in the Region of the Corpus Striatum. — 

 When a coronal section has been made the relation of structures, 

 from within outwards, is as follows : 



1. Nucleus caudatus. 4. External capsule. 



2. Internal capsule. 5. Claustnun. 



3. Nucleus lenticularis. 6. Island of Reil. 



THALAMENCEPHALON OR DIENCEPHALON. 

 Tiiird Ventricle. 



The third ventricle is the cleft-like interval which is situated, in 

 the median line, between the two optic thalami. It extends from 

 the pineal body posteriorly to the anterior pillars of the fornix in 

 front, is very narrow from side to side, and is deeper in front than 

 behind. The cavity presents a roof, a floor, two lateral walls, an 

 anterior botmdary, and a posterior boundary. 



The roof is formed by a delicate layer of epitheliiun which extends 

 across between the upper margins of the lateral walls, and is con- 

 tmuous with the ependymal lining of the ventricle. Superficial 

 to this epithelial roof, and intimately connected with it, is the 

 velum interpositimi, from the under surface of which the two 

 choroid plexuses of the ventricle project downwards, one on either 

 side of the middle line, each invaginating the epithehum of 

 the roof. The epithelium of the roof is so intimately connected 

 with the velum interpositum that, when the latter is removed, the 

 epithelium comes away with it, and the cavity of the ventricle 

 is exposed. Superficial to the velum interpositum is the body of the 

 fornix, and, above this again, is the body of the corpus callosum. 



Summary of the Roof. — To expose the ventricle from above the following 

 structures must be removed, in the order named : (i) the bodv of the corpus 

 callosum ; (2) the body of the fornix ; and (3) the velum interpositum, along 

 \vith the epithelium of the roof. 



