THE BRAIN IN MARSUPIAL ANIMALS. 
1)1 
lobe of the same hemisphere in the longitudinal direction. It also fulfils the other 
function of the fornix by sending down from the inferior surface two small nerve-like 
processes, which extend vertically, behind the anterior commissure, through the sub- 
stance of the optic thalami, near their mesial surfaces, to the corpus albicans, at the 
base of the brain. 
The superior view of the connexions of the hippocampal commissure of the Wom- 
bat is given at Plate VI. fig. 4. 
Returning to the Beaver’s brain, we raise the posterior thickened margin of the 
corpus callosum, and at the middle of its inferior surface we find it closely connected 
with the centre of a commissural band of fibres, arching over the anterior part of the 
optic thalami, and passing outwards and backwards along the floor of the lateral 
ventricles into the substance of the hippocampi, which are as largely developed as in 
the Wombat. The anterior part of the corpus callosum is bent downwards, and is 
attached along the middle line of its inferior surface by a uniting medium of medul- 
lary substance, representing the septum lucidum, to the hippocampal commissure or 
fornix. The teenise hippocampi, which form the lateral parts of this commissure, 
extend forwards, as in the Wombat, into the anterior lobes. 
The corpus callosum being removed, and the commissural fibres of the hippocampi 
being left behind (as shown on the left side at Plate VI. fig. 5.), the view of the 
Beaver’s brain now corresponds with that obtained in the previous dissection of the 
brain of the Wombat ; which we regard, therefore, as wanting the corpus callosum, 
septum lucidum, and consequently the fifth ventricle. The artery of the plexus cho- 
roides, in both the Beaver and Wombat, enters the lateral ventricle, where the hip- 
pocampus commences at the base of the hemisphere, and the plexus is continued along 
the under surface of the toenia hippocampi, and passes beneath the fornix, through the 
usual foramen, to communicate with its fellow in the third ventricle, immediately be- 
hind the anterior crura of the fornix, which are sent down in the Beaver, as in the 
Wombat, from the centre of the inferior surface of the hippocampal commissure. 
If we expose the lateral ventricle by removing its outer parietes in a marsupial 
and placental quadruped, as shown in Plate VII. figg. 4 and 5, in the Kangaroo and 
Ass, the hippocampus major ( n ), the tsenia hippocampi (o), the plexus choroides (p), 
and the foramen Monroianum (y) are brought into view. If a style be thrust trans- 
versely through the internal wall of the ventricle, immediately above the hippocam- 
pus, in the placental quadruped, it perforates the septum lucidum (q), and enters the 
opposite ventricle below the corpus callosum. If the same be done in the marsupial 
brain, the style passes into the opposite ventricle, but is immediately brought into 
view from above by divaricating the hemispheres, and is seen lying above the com- 
missure of the hippocampi. 
This commissure may nevertheless be regarded as representing, besides the fornix, 
he rudimental commencement of the corpus callosum ; but this determination does 
not invalidate the fact that the great commissure which unites the supraventricular 
n 2 
