THE DIENCEPHALIC FLOOR eos 
conspicuous in carnivores. In primates it is not always well 
marked, yet in all the apes examined it was present. In man, 
especially in later adult life, considerable care may be required 
to detect it, although in many instances it is quite as evident 
as in the carnivores; this is particularly true in the brain of the 
infant and child. The eminence appears in ungulates; at least 
it was observed in several varieties of artiodactyla (sheep, 
mountain goat, mule deer, aoudad and Virginia deer). It is 
prominent in the proboscidea (Indian elephant) and also occurs 
in rodents and marsupials. The post-infundibular eminence 
appears as a transverse ridge extending across the ventricular 
‘floor. It is most prominent at and near the median line; it 
presents no sagittal division into bilateral halves and laterally 
merges with the general plane of the basal region. Its shape 
is somewhat variable; often it is elongated cephalo-caudad and 
it may be asymmetrical. Its ventral surface may arise sharply 
to the floor of the ventricle or it may blend gradually with this 
area. Its caudal surface usually rises abruptly to the ven- 
tricular floor. 
The developmental history of the post-chiasmatic eminence 
shows that it is partially constricted off from the basal portion 
of the interbrain by the growth ofthe pars tuberalis of the pitui- 
tary gland. Two basal regions thus lie above the eminence, 
symmetrically placed, one on either side of the median line. 
In shape they are roughly triangular having their bases turned 
mesad and their apices projecting laterally. The base of each 
triangle extends from the post-infundibular eminence almost 
as far forward as the chiasm. As they are followed laterad 
each presents a protuberance which is most pronounced near 
the apex of the triangle. These protuberances and the basal 
areas with which they are in continuation constitute the lateral 
eminences (22). 
The median sagittal view of the model reconstructed from the 
diencephalic floor in the adult cat is shown in figure 3. This 
view gives the ventricular recesses many of whose surface ex- 
pressions have already been discussed. Cephalad to the chiasm 
_ (4) the ventricular cavity extends forward and slightly down- 
