THE BRAIN IN MARSUPIAL ANIMALS. 
89 
tudinal extent from behind forwards, and thus the corpora quadrigemina, pineal gland, 
and posterior part of the optic thalami are successively brought into view on divari- 
cating the cerebral hemispheres in the different Mammalia which exhibit this pro- 
gressive degradation of the great commissure. 
The researches of Tiedemann, as is well known, have shown that the anterior part, 
which is the most constant in the mammiferous series, is that from which the deve- 
lopment of the corpus callosum commences in the human brain. 
The aim of the present paper is not, however, to trace step by step the various 
modifications of the commissural apparatus of the hemispheres through the mammi- 
ferous class, but is limited to the description of a remarkable modification in that 
apparatus in the brains of the marsupial animals, to the detection of which I was led 
by observing that the commissural system presented the essential difference between 
the brains of the oviparous and mammiferous Vertebrata, and by associating the 
greater perfection of the brain, resulting from the development of the great commis- 
sure with the placental mode of development in the true Mammalia. 
The connexion subsisting between placentation and high cerebral organization 
may be one of simple coincidence, yet it is certain that of all the great organic 
systems, the cerebral or sentient organ is that which alone offers a marked improve- 
ment of gradational structure in the animals developed by a placenta. 
An attentive study of the manners of different Marsupiata in confinement, and an 
inspection of the exterior forms of the brain in some of the species, induced me to 
allude in a former paper to an inferiority of intelligence and a low development of 
the cerebral organ as being the circumstances in the habits and structure of these 
singular animals which were most constantly associated with the peculiarities of 
their generative economy*. I have since derived the most satisfactory confirmation 
of this coincidence from repeated dissections of the brains of Marsupiata belonging 
to different genera ; and although unable to explain how a brief intra-uterine exist- 
ence and the absence of a placental connexion between the mother and foetus can 
operate (if it be really effective) in arresting the development of the brain, yet it is a 
coincidence which has been so little suspected, and is so interesting in various points 
of view, that I believe the evidence of it will be acceptable both to the physiologist 
and the naturalist. 
In order to obtain satisfactory proof of the difference in the structure of the brain 
in the marsupial and placental quadruped, I dissected and compared together, 
step by step, the brains of a Wombat and Beaver. These animals, as is well known, 
are of nearly similar bulk, and manifest so many mutual affinities in their structure, 
that they have been, and still are, by some naturalists, classed in the same order of 
Mammalia. The Wombat is, in fact, in all its exterior characters, save the mar- 
supial 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 
* Philosophical Transactions, 1834, p. 358. 
MDCCCXXXVII. 
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