MARSUPIALIA. 
_ From the. fact that the cerebral organ is that 
which exhibits the most marked degradation of 
structure in the class of warm-blooded Verte- 
brate animals which are characterized by an 
oviparous generation, I was induced to sus- 
t, after having ascertained how closely the 
pialia approached Birds in their mode 
of generation, that the brain might present in 
them some corresponding inferiority of structure, 
as compared with the Placental Mammalia. 
The brain in the placental Mammalia is es- 
sentially characterized by the complexity and 
magnitude of the apparatus by which the he- 
mispheres are brought into communication 
with one another. With respect to size, the 
cerebrum is in many species proportionally 
inferior to that of Birds; and in most Insecti- 
vorous and Rodent Mammalia it presents an 
equally smooth and uniform external surface ; 
but notwithstanding the absence of convolu- 
tions and its diminished size, a large appa- 
ratus of medullary fibres is present, which 
connect together the opposite hemispheres, as 
well as the distant parts of the same hemi- 
sphere; and this apparatus, or great commis- 
sure, is superadded to the anterior, posterior, 
and soft commissures, which, with the excep- 
tion of a very slight rudiment of the fornix, 
are alone developed in Birds for the purpose 
of uniting the opposite sides of the brain. In 
the higher Mammalia, in which the cerebral 
hemispheres acquire superior size and increased 
extent of surface by means of convolutions, the 
superadded commissural apparatus presents a 
corresponding development and a highly com- 
plicated structure; its several parts being dis- 
tinguished as the corpus callosum, fornix, and 
their intercommunicating lamine, termed the 
septum lucidum. 
The corpus callosum is the principal bond of 
union between the opposite hemispheres; it 
extend$, as is well known, horizontally above 
the ventricles, its middle fibres passing trans- 
versely, while those of its extremities, which 
are more or less bent beneath its body, radiate, 
and all intermix, in apposition with the as- 
cending and diverging fibres of the peduncles 
of the cerebral hemispheres. It has hitherto 
been considered as the great characteristic of 
the brain in the Mammalia, and, taking the 
human brain as the term of comparison, to be 
developed in the ratio of the magnitude of the 
cerebral hemispheres. 
In the placental Mammalia this is a pretty 
accurate expression of the relations of the 
corpus callosum; and as the posterior lobes of 
the hemispheres are the first to disappear in 
_ the descending comparison, so the corpus cal- 
losum diminishes in longitudinal extent from 
behind forwards, and thus the corpora quadrige- 
mina, pineal gland, and posterior part of the optic 
thalami are successively brought into view on 
divaricating the cerebral hemispheres in the 
different Mammalia, as the Rodentia, Chei- 
roptera, and Edentata, which exhibit this pro- 
gressive degradation of the great commissure. 
An attentive study of the manners of dif- 
ferent Marsupials in confinement, and an in- 
spection of the exterior forms of the brain in 
some of the species, induced me to allude, in 
293 
my paper on the generation of the Kangaroo,* 
to an inferiority of intelligence and a low de- 
velopment of the cerebral organ, as being the 
circumstances in the habits and structure of 
these singular animals, which were most con- 
stantly associated with the peculiarities of their 
generative economy. I have since derived the 
most satisfactory confirmation of this coinci- 
dence from repeated dissections of the brains 
of Marsupials belonging to different genera ; 
and although unable to explain how a brief 
intra-uterine existence and the absence of a 
placental connexion between the mother and 
foetus can operate (if it be really effective and 
any thing more than a relation of simple co- 
existence) in arresting the development of the 
brain, yet it is a coincidence which has not 
been suspected, and is, in various points of 
view, perhaps the most interesting of the ana- 
tomical peculiarities of the quadrupeds here 
treated of. 
In order to obtain satisfactory proof of the 
difference in the structure of the brain in the 
marsupial and placental quadruped, I have 
dissected and compared together, step by step, 
the brains of a Wombat and Beaver. These 
animals are of nearly equal bulk, and manifest 
so many mutual affinities in their structure 
that they have been classed in the same order 
of Mammalia. The Wombat is, in fact, in all 
its exterior characters, save the marsupial and 
scrotal pouch, a Rodent; and in its internal 
anatomy, especially its digestive organs, more 
nearly resembles the Beaver than do many of 
the true Rodent animals. The brain of the 
Beaver was also preferred for this comparison 
of internal organization, because, on an out- 
ward inspection, it would be pronounced to 
be the less highly organized of the two; the 
hemispheres in the Wombat presenting a few 
convolutions, as before described, whilst in the 
Beaver they are perfectly smooth. 
Tn the Beaver, however, the cerebrum is ex- 
tended further backward, although it still leaves 
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 cerebral 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 anterior 
fibres radiating into the anterior, the posterior 
fibres into the posterior extremities of the 
hemispheres. : 
The portions of the brain which are removed 
in thus tracing the extent of the corpus callo- 
sum, 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 
(B, fig. 117,) and pineal gland (u, Jig. 117,) 
* Phil. Trans, 1334, p, 358. 
