THIRD VENTRICLE. 597 



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 interpositurn, from which are suspended the choroid 

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



Fig. 333. The Third and Fourth Ventricles. 



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 infun- 

 dibulum, the corpora albicantia, and the locus perforatus posticus ; its sides, by 

 the optic thalami ; it is bounded, in front, by the anterior crura of the fornix, 

 and part of the anterior commissure; behind, by the posterior commissure, and 

 the iter a tertio ad quartum ventriculum. 



The cavity of the third ventricle is crossed by three commissures, named, 

 from their position, anterior, middle, and posterior. 



The anterior commissure is a rounded cord of white fibres, placed in front of 

 the anterior crura of the fornix. It perforates the corpus striatum on either 

 side, and spreads out into the substance of the hemispheres, over the roof of 

 the descending horn of each lateral ventricle. 



The middle or soft commissure consists almost entirely of gray matter. It 

 connects together the thalami optici, and is continuous with the gray matter 

 lining the anterior part of the third ventricle. It is frequently broken in 

 examining the brain, and might then be supposed to have been wanting. 



