638 
ME. W. H. FLO WEE ON THE CEEEBEAL COMMISSIJEES 
above, with a thickened posterior end (E) turned somewhat downwards, and a distinct 
genu (C) and rostrum (D) in front. The latter has a smaller proportional development 
than in the human brain. On the other hand, the slightly projecting posterior fold 
observed in the human corpus callosum is prolonged forwards as a thin layer of transverse 
fibres (N) arching across the under surface of the longitudinal fibres of the fornix, and 
ending in no abrupt edge in front. The difference in the form and extent of this part 
of the great transverse commissure may be clearly seen to depend upon the difference in 
the form, and more extensive proportions of the parts that have to be brought into rela- 
tion to each other by it, viz. those forming the inner wall of the descending cornu of the 
lateral ventricle. At a considerable distance below the anterior part of the corpus cal- 
losum the small anterior commissure (F) is seen, with the wide septal area (G) in front 
of and above it. The portion of this part to which the term “ septum lucidum ” can 
be applied, is reduced to a small strip beneath the anterior third of the corpus callosum, 
exactly defined below and in front by the extent of the rostrum of that body. The 
greater part of the septum is formed by a thick layer, consisting of a great development 
of the precommissural fibres of the fornix, associated with much grey matter. The small 
white column (L) of the fornix is seen passing down behind the anterior commissure. 
The ventricular aperture is less regularly curved than in man, being bent almost at a 
right angle. Above and behind it is seen a broad corpus fimbriatum (M), behind which 
the abrupt termination of the cortical substance of the hemisphere in the fascia dentata 
(P) is very distinctly seen. The regularly curved hippocampal sulcus (Q) ends beneath 
the hinder end of the corpus callosum, the grey matter of the fascia dentata being con- 
tinued superficially round its extremity into that of the next succeeding gyrus. 
In the transverse section (Plate XXXVII. fig. 2), at the bottom of the deep longitu- 
dinal fissure, is seen the corpus callosum (B), a transverse white band of moderate thick- 
ness, and slightly arched upwards externally, where its fibres radiate out in the medullary 
substance of the hemisphere. The anterior commissure (F) is readily recognized near 
the lower part of the section. The cavities of the lateral ventricles are somewhat tri- 
angular in form and bounded above by the under surface of the corpus callosum, 
towards the middle line by the septum, and externally by the corpora striata. The 
septum obviously consists of two halves, one belonging to each hemisphere, but more or 
less joined together in the middle line. The upper part (septum lucidum) is extremely 
thin, and here the absence of union between the two halves allows the existence of a 
minute cavity, the fifth ventricle. The lower and larger part is very thick, with rounded 
outer surface. It contains much grey matter, with white longitudinal fibres externally. 
Within it, near the middle line, on each side, can be seen two bundles of white fibres, 
standing nearly perpendicularly and slightly diverging from each other below ; they are 
the upper part of the columns of the fornix. 
The most essential deviations in the commissures of this brain from those of Man con- 
sist in the reduction of the rostrum of the corpus callosum and the septum lucidum, and 
the augmentation of the inferior thick part of the septal area and of the psalterial fibres. 
