294 
but the optic thalami (¢, ¢, fig. 117) are im- 
mediately brought into view, and instead of a 
Fig. 117. 
Phascolomys fusca. 
broad corpus callosum, we perceive, situated 
deeply at the bottom of the longitudinal 
fissure, a small commissural medullary band, 
(m, fig. 117) passing in an arched form over 
the anterior part of the thalami, and extending 
beneath the overlapping internal or mesial sur- 
faces of the hemispheres, (9, fig. 117,) which 
are thus, as in the Bird, disconnected with 
each other. 
On gently raising the hemispheres from 
above the commissure and pressing them out- 
wards with the handle of a scalpel, the instru- 
ment passes into the fissure upon which the 
hippocampus (n, fig. 117) is folded; and, on. 
continuing the pressure, the hippocampus is 
torn through, and the lateral ventricle is ex- 
posed, The mesial wall of the hemisphere 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 a detached 
layer of the septum lucidum. In the Kan- 
garoo the mesial parietes of the lateral ventricles 
are stronger, being about two lines in thickness. 
The posterior transverse fibres of the com- 
missure are continued outwards and backwards 
beneath the more longitudinal fibres, which 
overlap them as they pass from the tenie 
hippocampi (0, fig. 117) 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. Thus the commissure, which is brought 
into view on divaricating the cerebral hemi- 
spheres in the Wombat, is seen to be partly 
the bond of union of the two hippocampi 
majores in the transverse direction, and eahie 
of the hippocampus and anterior lobe of the 
same hemisphere in the longitudinal direction, 
Tt also fulfils the other function of the fornix 
MARSUPIALIA. 
by sending down from the inferior surface two — 
small nerve-like processes, which extend ver 
cally, hehind the anterior commissure, thre 
the substance of the optic thalami, near t 
mesial surfaces, to the corpus albicans, at t 
base of the brain. 
The superior view of the connexions of the 
hippocampal commissure of the Wombat is” 
given at m, n, 0, fig. 117. lp 
If in the Beaver's brain the posterior thie 
ened margin of the corpus callosum be raist 
it is observed at the middle of its inferia 
face to be closely connected with the cent 
a commissural band of fibres, arching over the” 
anterior part of the optic thalami, and passing” 
outwards and backwards along the floor of t 
Jateral ventricles into the substance of the hip- 
pocampi, which are as largely developed as ii 
the Wombat. The anterior part of the corp 
callosum is bent downwards, and is attache 
along the middle line of its inferior surface by 
a uniting medium of medullary substance, r 
presenting the septum lucidum, to the hippo- 
campal commissure or fornix ; the teniz hip- 
pocampi extend forwards, as in the Wombat 
into the anterior lobes. “Ss 
The corpus callosum being removed, and t 
commissural fibres of the hippocampi bei 
left behind, the view of the Beaver’s brain now 
corresponds with that obtained in the previot 
dissection of the brain of the Wombat, 
we regard, therefore, as wanting the principal 
mass of the corpus callosum, theseptum lucidum, 
and consequently the fifth ventricle. The ar- 
tery of the plexus choroides, (p, fig. 117,) im ~ 
both the Beaver and Wombat, enters the lateral 
ventricle, where the hippocampus commences 
at the base of the hemisphere, and the plext 
is continued along the under surface of the — 
tenia hippocampi, and passes beneath the 
fornix, through the usual foramen, to gommu- 
nicate with its fellow in the third ventricle im- 
mediately behind the anterior crura of the 
fornix, which are sent down in the Beaver, as 
in the Wombat, from the centre of the inferio 
surface of the hippocampal commissure. 
If we expose the lateral ventricle by removit 
its outer parietes in a marsupial an pee: ta 
quadruped, as, for example, in the garoo 
and Ass, the hippocampus major, the teni 
hippocampi, the plexus choroides, and th 
foramen Monroianum are brought into view. | 
a style be thrust transversely through the inter 
nal wall of the ventricle, immediately aboy 
the hippocampus, in the placental quadrape 
it perforates the septum lucidum and enter 
the opposite ventricle below the corpus callo=— 
sum. If the same be done in the marsupia 
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 fornix or commissure of — 
the hippocampi. 
is commissure may, nevertheless, be re- 
garded as representing, besides the fornix, the — 
rudimental commencement of the corpus callo- 
sum ; but this determination does not invalid 
the fact that the great commissure which unites 
the supra-ventricular masses in the Beaver, 
the Bat, and all other placentally develo 
y 
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