OP THE MARSTJPIALIA AND MONOTREMATA. 
645 
transverse fibres (N), almost equal in size to the upper portion of the commissure, 
is intimately united. Beneath the anterior part of this, close to the middle line, a 
distinct white cylindrical band of fibres is seen to pass down, behind and in close con- 
tact with the anterior commissure, at first directed somewhat backwards and afterwards 
downwards until it loses itself in the thalamus opticus. This evidently answers to one of 
the columns of the fornix, its position being somewhat disturbed by the immense deve- 
lopment of the anterior commissure. Between the superior transverse commissure (by 
which name I propose for the present to call the part marked B) and the anterior com- 
missure are some fibres continued forwards from above the anterior end of the ventri- 
cular aperture, and mixed in this region with much grey matter, forming the greatly 
reduced septal area (G). They curve forwards and downwards, encircling the anterior 
half of the anterior commissure, and represent, doubtless, those designated as “ precom- 
missural ” fibres in the higher mammals. The ventricular aperture is seen to occupy 
its ordinary position. Its upper margin is formed by the edge of a broad white band, 
corpus fimbriatum (M). On tracing this band forwards, it is found to be continuous 
with the hinder edge of the whole of the upper transverse commissure. The superficial 
grey layer (P) external to the corpus fimbriatum is readily recognized as the fascia den- 
tata. This is bounded on the outer side by the hippocampal sulcus ; but in respect to 
this sulcus a great peculiarity presents itself. On tracing it forwards, instead of stop- 
ping short beneath the projecting posterior rounded end of the corpus callosum, as in 
most, if not all placental mammals *, it is continued on, passing over the top in close 
contact with the upper transverse commissure, and is not lost until it reaches the inner 
surface of the anterior lobe, considerably in advance of both the upper and anterior 
commissures. The remarkable disposition of this sulcus must be particularly noted in 
reference to the nature of the commissure in close relation with it. 
In the transverse section (Plate XXXVIII. fig. 2) the immense size of the anterior 
commissure (F) is as conspicuously seen as in the longitudinal section. It occupies 
one-fourth of the whole height of the brain in the middle line. Its fibres spread them- 
selves outwards, the lower ones sweeping first slightly downwards, then curving up into 
the white medullary substance of the middle of the hemisphere. The higher fibres, 
taking a course more directly upwards, penetrate the grey matter of the corpora striata 
(R R), which they here divide into two distinct masses, and finally reach the medullary 
substance of the upper part of the hemisphere. Lying immediately upon the anterior 
commissure, close to the median line, are two bodies, which, taken together, present a 
surface broad from side to side, slightly concave above, nearly flat below, and rounded 
off at the outer inferior angles. These consist mostly of grey substance, with some white 
fibres, especially collected into two bands close to the median line (the roots of the 
columns of the fornix). These bodies are the two lateral halves of the very much 
thickened and depressed ventricular septum. Below they are in contact with the anterior 
commissure, on each side with the cavity of the lateral ventricle, above with a white 
* A partial exception was shown in the Two-toed Slotln 
