THE CENTRAL NERVOUS SYSTEM. 355 
6. The roof of the third ventricle is thin and united with 
the pia mater, as already 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, ¢) at the craniolateral borders of the thalami- 
The pineal body (corpus pineale) or epiphysis (Fig. 141, 
2; Fig. 143, v) 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, g). It is hollow, containing an exten- 
sion of the third ventricle. From its craniolateral angles two 
white strands, the habenule, extend into the stria medullares 
(Fig. 141, 2) 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, 2’) (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, 2). The optic tracts come from the thalami, as already 
described; they converge and unite to form the optic chiasma 
(Fig. 142, ¢), 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, 2) with the hypophysis (lig. 138, ¢); 
in cases where the two latter structures have been removed 
(Fig. 142) the tuber cinereum (%) bears a small longitudinal 
opening (2) for attachment of the infundibulum. The infundi- 
bulum (Fig. 138, @) is a hollow extension of the floor of the 
third ventricle, and is attached to the middle of the ventral 
