THIRD VENTRICLE. 



527 



they are composed of white fibres intermixed with gray matter. Each thalamus 

 rests upon its corresponding crus cerebri, which it embraces. Externally, it is 

 bounded by the corpus striatum, and tcenia semicircularis ; and is continuous with 

 the hemisphere. Internally, it forms the lateral boundary of the third ventricle ; 

 and running along its upper border is seen the peduncle of the pineal gland. Its 

 upper surface is free, being partly seen in the lateral ventricle ; it is partly covered 

 by the fornix, and marked in front by an eminence, the anterior tubercle. Its 

 under surface forms the roof of the descending cornu of the lateral ventricle ; into 

 it the crus cerebri passes. Its posterior and inferior part, which projects into the 

 descending horn of the lateral ventricle, presents two small round eminences, the 

 internal and external geniculate bodies. Its anterior extremity, which is narrow, 

 forms the posterior boundary of the foramen of Monro. 



Fig. 269. The Third and Fourth Ventricles. 



The third ventricle is the narrow oblong fissure placed between the thalami 

 optici, and extending to the base of the brain. It is bounded, above, by the under 

 surface of the velum interpositum, from which are suspended the choroid plexuses 

 of the third ventricle ; and, laterally, by two white tracts, one on either side, the 

 peduncles of the pineal gland. Its floor, somewhat oblique in its direction, is 

 formed, from before backwards, by the parts which close the interpeduncular space, 

 viz., the lamina cinerea, the tuber cinereum and infundibulum, the corpora albicantia, 

 and the locus perforatus ; its sides, by the optic thalami ; in front, by the anterior 



