90 
MR. OWEN ON THE STRUCTURE OF 
brain of the Beaver was also preferred for this comparison of internal organization, 
because on an outward inspection it would be pronounced to be the less highly 
organized of the two ; the hemispheres in the Wombat presenting a few convolutions 
(Plate V. fig. 3.), whilst in the Beaver they are perfectly smooth (Plate V. fig. 1.). 
In the Beaver, however, the cerebrum is extended further backward, though still 
leaving the cerebellum quite uncovered ; while in the Wombat a portion of the optic 
lobes (corpora quadrigemina) is also exposed. 
On divaricating the hemispheres of the brain in the Beaver, we bring into view, 
about three lines below the surface, the corpus callosum ; and on removing the ce- 
rebral substance to a level with this body, its fibres are observed to diverge into the 
substance of each hemisphere, in the usual manner, some bending upwards, but a 
greater proportion arching downwards, and embracing the cerebral nuclei ; the an- 
terior fibres radiating into the anterior, the posterior fibres into the posterior extre- 
mities of the hemispheres. (Plate VI. fig. 3.) 
The portions of the brain which are removed in thus tracing the extent of the 
corpus callosum, bring into view the corpora bigemina and the pineal gland ; but the 
optic thalami are concealed by the great commissure above described. 
On separating the hemispheres of the brain of the Wombat, not only the bigeminal 
bodies and pineal gland, but the optic thalami are immediately brought into view, 
and instead of a broad corpus callosum, we perceive, situated deeply at the bottom 
of the hemispheric fissure, a small commissural medullary band, m, (Plate VII. fig. 4.) 
passing in an arched form over the anterior part of the thalami, and extending be- 
neath the overlapping internal or mesial surfaces of the hemispheres, which thus 
appear, as in the Bird, to be wholly disunited. 
On gently raising the hemispheres from above the commissure, and pressing them 
outwards with the handle of a scalpel, the instrument passes into the fissure upon 
which the hippocampus is folded ; and on continuing the pressure the hippocampus 
is torn through, and the lateral ventricle is exposed. The mesial wall of the hemi- 
sphere is continued from the superior and internal border of the hippocampus, and 
is composed in the Wombat, as in the Bird, of a thin lamina of medullary substance 
analogous to the septum lucidum. In the Kangaroo, the mesial parietes of the lateral 
ventricles are stronger, being about two lines in thickness. 
The posterior transverse fibres of the commissure are continued outwards and 
backwards, beneath the more longitudinal fibres, which overlap them as they pass 
from the taenise hippocampi forwards to the anterior cerebral lobes. All the fibres 
of the commissure pass along the floor of the lateral ventricles into the substance 
of the hippocampi majores, which are of proportionally very large size. (See Plate VI. 
and VII. fig. 4, n .) 
Thus the commissure which is brought into view on divaricating the cerebral 
hemispheres in the Wombat is seen to be partly the bond of union of the two hippo- 
campi majores in the transverse direction, and partly of the hippocampus and anterior 
