202 _ RALPH EDWARD SHELDON 
whole region, although he shows both in his figures and descrip- 
tions that it contains different groups of cells with different char- 
acteristics. Kappers (’06) pointed out that the center previously 
described as nucleus rotundus is really made up of several char- 
acteristic eroups of cells. That 
situated most dorsally, proximally and laterally, is the nucleus praero- 
tundus. This group . . . .. gradually passes backward into a 
much larger group situated under and lateral to the level of the nucleus 
rotundus and ending where the real nucleus rotundus has its largest 
size. This latter group, which belongs entirely to the lobi inferiores, 
I shall distinguish as the nucleus subrotundus from the nucleus rotundus 
proprius, as it extends in part under the real nucleus rotundus so that 
the com. horizontalis, before it enters the lower border of the latter, lies 
for some distance over it and between it and the nucleus rotundus pro- 
prius. 
This separation of the nucleus rotundus of the earlier authors 
into three different components is a matter of considerable mor- 
phological importance, as will be brought out later. Kappers’ 
description applies in a general way to the’relations in the carp, 
with some important modifications. 
At the level of the rostral margin of the lateral lobes, the nucleus 
prerotundus appears ventro-laterally immediately ventro-lateral 
to the commissura transversa (fig. 78). It consists here of a 
fairly compact mass of irregularly shaped cells of medium size. 
A short distance further caudally this nucleus lies wedged in 
between the lateral lobe and the commissura transversa. Dorso- 
laterally it forms a small protuberance on the lateral surface of 
the brain (fig. 81). From this point the nucleus prerotundus 
extends caudo-mesially to the region of the nucleus posterior 
tuberis. It may be compared in shape to the caudate nucleus in 
the human brain, with a large and conspicuous head rostrally, 
gradually diminishing in size caudo-mesally (figs. 84, 89, 103, 106). 
The nucleus rotundus proprius is by far the largest and most 
conspicuous nucleus of the thalamic region. It appears rostrally 
at about the rostral margin of the commissura posterior and ex- 
tends caudo-mesally, lateral to the nucleus prerotundus, almost’ 
to the commissura ansulata, meeting the corpus mammillare 
ventro-mesially (figs. 84, 89, 103, 106, 117). 
