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, Fi^. 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, Fi^. 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 an'l .in^r .< ,< 1 

 segment from the middle region simply. 



Before passing to a description of our own investigation^, 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 smiU 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 tuln-r , in.-r.Mim. 



