644 
ME. W. H. FLOWEE ON THE CEEEBEAL COMMISSURES 
length of the hemisphere. It is situated deeply in the great longitudinal fissure, is 
thickened and most elevated posteriorly, where the margin, slightly and evenly concave, 
crosses the cavity of the third ventricle (S), the peduncles of the pineal gland (T), and 
the optic thalami (U). The anterior margin is also concave, but extremely narrow, the 
white substance being continued on each side of a longitudinal median cleft for some 
distance towards the front of the cerebral hemisphere, as if in this anterior part the two 
lateral halves of the commissure had not been joined together in the middle line. On 
close examination it is seen to be composed of fibres of which the general direction is 
transverse, but on its upper surface can be distinguished a longitudinal median raphe, 
and on each side of this a few longitudinal white fibres, corresponding to the “striee late- 
rales” of other mammals. 
On either side, the transverse fibres are lost beneath the overlapping grey matter 
constituting the margin of the convolution of the corpus callosum, the “labia cerebri” 
of some authors. To follow them further, the last named parts must be carefully 
removed with the handle of a scalpel or some similar instrument, when a delicate broad 
lamina formed by the lateral expansion of the narrow transverse band will come into 
view, passing at first horizontally outwards and then curving upwards above the precom- 
missural fibres of the fornix (I), the cavity of the lateral ventricle, and the corpus stri- 
atum (R), and finally losing themselves in the medullary substance of the upper part of 
the cerebral hemispheres. The fibres radiate extensively forwards and backwards but 
forming a continuous lamina, posteriorly conterminous with those on the surface of the 
hippocampus major, anteriorly becoming much more delicate, so much so, indeed, that 
it is not easy to make a complete dissection of them without causing some rents, like 
that on the left side shown in the figure, through which the cavity of the ventricle below 
is exposed. This expansion of the transverse commissure in the hemisphere, though 
described by Pappenheim in the Opossum, appears not to have been observed by Owen 
in any of his dissections. 
Plate XXXVIII. fig. 1 is a view of the inner surface of the right hemisphere of the 
Great Kangaroo. The hemisphere is short, and deep from above downwards, obtusely 
pointed in front and flattened or abruptly truncated behind. The temporal lobe is 
largely developed. Several well-marked sulci are seen upon the surface of the hemi- 
sphere. One of the most striking characteristics presented by this section is the 
great development of the anterior commissure (F), far exceeding that seen in any 
placental mammal. The form of its section is oval, with the long diameter nearly 
vertical, or inclining slightly forwards at the upper end. It consists of firm, white, 
transverse fibres, distinctly defined from the surrounding part, and forms a good 
landmark to the adjoining structures, as about its homologies there can be no ques- 
tion. At a very short distance above this is seen the section of the median part of 
that transverse band before described (B). This is oval, elongated from before back- 
wards, slightly arched on its upper border. Its anterior and posterior extremities are 
rounded, the former is the narrowest. To the under surface of the latter, a body of 
