OF DENISON UNIVERSITY. 
41 
gathers fibres from the cephalad as well as occipital regions at least of 
the ventral portions of the hemispheres. 3d, the commissure of the 
hippocampus is quite distinct from the previously mentioned, and is 
quite a narrow and slender band of considerable length, as the hippo- 
campi, posteriorly, are oval or quadrate in section and at the point of 
origin of the fornix or hippocampal commissure are widely separate. 
4th. A rudiment of what may be called corpus callosum, although we 
are unwilling to homologize it definitely with that body. The last 
mentioned is a small fibre-tract which lies anterior to but adherent upon 
the hippocampal commissure and soon loses itself in the median walls 
of the hemispheres corresponding to the septum pellucidum. Being 
a tract of the cortex this band has as great claim to be homologized 
with the corpus callosum as the relatively larger commissure of the 
alligator. 
Exactly the same relations between the hippocampal commissure 
and the callosum are found in the insectivora, as may be- seen by com- 
paring Plate X, Fio. 7, except that the latter possesses a well-defined 
corpus callosum above the ammon’s horn. ‘ 
Comparing with these a section through a corresponding region 
of the kitten (Plate XII, Fig. 6), the relations appear very different, 
but the difference arises chiefly from the different proportions and the 
relative compactness of the brain whereby the thalmus and corpora 
striata are crowded upon each other. The wide interval between the 
anterior commissure and the callosum may be regarded as an incident 
of this consolidation. Finally, the examination of Plate X, Fig. 5, 
reveals the fact that the hippocampus has a very similar relation in the 
rabbit. This section, although nearly horizontal, inclines toward the 
median line from the cortex of the right hemisphere and embraces a 
segment from the middle region simply. 
Before passing to a description of our own investigations, it may 
be well to present a summary of the results of Stieda’s work upon the 
rabbit and rat. This summary will serve as an introduction to the 
subsequent paper as well. 
After removing the tuber cinereum, which is but a small protuber- 
ant portion of the thalamus below, the latter appears divided by the 
thin cleft representing the third ventricle into two symmetrical halves. 
The ventricle is divided into a lower and upper portion by the adhe- 
sion of the walls, often called commissura mollis.^ which, however, is 
not a true commissure. The ventricle descends to the tuber cinereum. 
