THE DIENCEPHALIC FLOOR Zoi 
forms a well defined ridge crossing the outer surface of the 
floor, but immediately caudal to it is the largest protuberance of 
this region, the post-chiasmatic eminence or bulbus infundibuli 
(32). It is difficult to demonstrate this eminence on the actual 
brain, for the reason that it is almost entirely invested by a 
portion of the pituitary gland, the pars tuberalis. As a rule, 
this protuberance is torn away with the hypophysis when the 
attempt is made to study the structures in the floor of the third 
ventricle and such removal produces an artificial slit-like open- 
ing into the ventricle which has been called the lura. The 
post-chiasmatic eminence presents a long ventral surface which 
slants caudad and ventrad from the optic chiasm; it leaves the 
general plane of the floor at this level and proceeding for a con- 
siderable distance in the direction of the mammillary bodies 
reaches its greatest prominence about midway between these 
bodies and the chiasm. The ventral surface presents a shallow 
furrow whose long axis is in the median plane. This is the 
median post-chiasmatic groove. In this region the neural tissue 
forming the floor of the eminence is thin. Laterad in both 
directions the neural tissue rapidly increases in thickness; its 
ectal surface becoming convex forms two lateral processes of the 
post-chiasmatic eminence, cone on either side of the median 
post-chiasmatic groove and each projecting free of the adjacent 
basal surface. The dorsal surface of the post-chiasmatic emi- 
nence is, in the main, parallel with its ventral surface but caudally 
it turns sharply upward to meet the plane of the floor. Two 
lateral borders bound the eminence, becoming more prominent 
as they are traced caudad; for about three-quarters of their 
distance they are divergent; they then become convergent 
caudad and as they approach each other form with the dorsal 
and ventral surfaces of the eminence a constricted stem-like 
prolongation, the infundibular stem (1/7) which projects caudad 
to become continuous with the expanded infundibular process 
(13). The infundibular stem and the infundibular process are 
invested by the pars infundibularis of the pituitary gland. 
In all the other mammalian forms examined the post-chiasmatic 
eminence is a prominent feature of the diencephalon; it maintains 
