THE CENTRAL NERVOUS SYSTEM. 355 



h. The roof of the third ventricle is thin and united with 

 the pia mater, as aheady stated. The pia mater bears many- 

 blood-vessels, and the two are folded into the groove between 

 the optic thalami, forming the lamina chorioidea epithelialis, 

 or choroid plexus of the third ventricle (Fig. 143, w). The 

 roof is attached to the dorsomedial borders of the thalami and 

 becomes continuous with the choroid plexus of the lateral ven- 

 tricles (Fig. 148, r) at the craniolateral borders of the thalami. 



The pineal body (corpus pineale) or epiphysis (Fig. 141, 

 2; Fig. 143, y) is a small conical body which is formed as an 

 outgrowth of the caudal part of the roof of the third ventricle. 

 It lies on the roof of the brain between the two cranial corpora 

 quadrigemina (Fig. 141, q). It is hollow, containing an exten- 

 sion of the third ventricle. From its craniolateral angles two 

 white strands, the habenulae, extend into the striae medullares 

 (Fig. 141, 7() of the thalami. 



Just ventrad of the pineal body is a transverse band of 

 white fibres, lying in the caudal part of the roof of the third 

 ventricle. This interconnects the two thalami, and forms the 

 posterior commissure (Fig. 143, z') (commissura posterior). 

 From this commissure a thin sheet of tissue extends to the 

 pineal body. 



c. The floor of the third ventricle appears in a ventral view 

 of the brain (Fig. 138, Fig. 142) as a somewhat diamond- 

 shaped space craniad of the pedunculi cerebri (Fig. 142, a) and 

 bounded along its cranial margin by the optic tracts (Fig. 

 142, d). The optic tracts come from the thalami, as already 

 described ; they converge and unite to form the optic chiasma 

 (Fig. 142, c), from which the optic nerves (/) diverge. 



Immediately caudad of the optic chiasma lies a considerable 

 rounded gray elevation, the tuber cinereum (Fig. 142, //). 

 This bears on its ventral surface in the natural condition the 

 infundibulum (Fig. 138, d) with the hypophysis (Fig. 138, e)\ 

 in cases where the two latter structures have been removed 

 (Fig. 142) the tuber cinereum (//) bears a small longitudinal 

 opening {i) for attachment of the infundibulum. The infundi- 

 bulum (Fig. 138, d) is a hollow extension of the floor of the 

 third ventricle, and is attached to the middle of the ventral 



