MARSUPIALIA. 
Mammalia, and which ‘exists in addition to the 
hippocampal commissure, is wanting in the 
brain of the Wombat; and as the same defi- 
ciency exists in the brain of the Great and Bush 
Kangaroos, the Vulpine Phalanger, the Pera- 
meles lagotis, the Ursine and Mauge’s Dasy- 
ures, and the Virginian Opossum, it is most 
probably the cerebral characteristic of the mar- 
as ip division of Mammalia. 
n the modification of the commissural ap- 
paratus above described, the Marsupialia pre- 
sent a structure of brain which is intermediate 
between that of the placental Mammalia and 
Birds; as in the latterclass the great commissure 
is wholly wanting, and the hemispheres, though 
comparatively larger than in many of the Mam- 
malia, are brought into communication only by 
means of the anterior, posterior, and soft com- 
missures, and by a slight trace of the fornix or 
pacereepel commissure. 
_ Of the other peculiarities of the marsupial 
brain, the relatively large size of the anterior 
commissure (c, fig. 118) is most worthy of 
notice; its development corresponds with 
the large size of the cerebral ganglion, which 
a 
Didelphys Virginiuna, 
forms the chief origin of the olfactory nerve, 
and some of the anterior fibres of this com- 
missure arch forwards, and are directly con- 
tinued into those nerves. 
In the position, superficial transverse fissure, 
and solidity of the bigeminal bodies, the mar- 
supial brain adheres to the Mammiferous type, 
as also in the exterior transverse fibres of the 
commissure of the cerebellum, forming the 
pons Varolii, the presence of which relates to 
the development of the lateral lobes of the 
cerebellum. 
In one of the latest published treatises on 
comparative anatomy, the Lehrbuch der Ver- 
gleichenden Zootomie, zweite auflage, 1834, 
of Carus, the first and chief structural charac- 
teristic of the brain in Mammalia is stated, as 
in the Legons d’Anatomie Comparée of Cuvier, 
to be the presence of the corpus callosum.* 
The brain of the Rodentia is cited as the 
example of the transitional condition of this 
organ from Mammalia to Birds.¢ Besides tlie 
* <Theils und vorziizglich werden sie in den 
durch eine neue grosse Commissur vereinigten,’ 
122, B.i. p. 77. ‘* Die einzelnen Hirnmassen 
etreffend, so aiissert sich, wie schon bemerkt, das 
Eigenthiimliche der ersten, der Hemispharen, vor- 
ziiglich durch die Erscheinung des Balkens (corpus 
callosum) und des Gewolbes (fornix).” 
+ ‘Im allgemeinen bildet zur Hirnform dieser 
Klasse von der der vorigen (der Vogel) dice Gehirn- 
bildung, wie sie in den Nagethieren (Rodentia) 
ae wird, dendeutlichsten Uecbergang.” Ibid. 
| 
295 
Rodentia, Carus afterwards states that the 
Monotremata, Marsupialia, and Insectivora 
(shrews, moles, and bats) also present the more 
simple form of brain among the Mammalia, 
“ the hemispheres being of an ovate form con- 
tracted anteriorly, and their surface perfectly 
smooth, as in Birds, not extending over the 
cerebellum, and sometimes not over the corpora 
quadrigemina. Internally the great commissure 
is generally very short (in the Bats and Kan- 
garoo hardly so long as the corpora quadrige- 
mina, a structure which reminds one of that in 
Birds). The fold of the corpus callosum and 
the cornua ammonis (which Carus terms the 
processes of the corpus callosum in the ven- 
tricles) are commonly broad and large.”’* In 
illustration of this simple structure of the 
mammiferous brain Carus gives a figure of the 
brain of a Rodent, and from the whole descrip- 
tion the reader is led to infer that the Rodent 
and Insectivorous placental Mammalia partici- 
pate with the marsupial Mammalia in all the 
characters of the cerebral organization which 
approximate to those in Birds. Rudolph 
Wagner also describes the brain in the Roden- 
tia, Cheiroptera, Edentata, and Marsupialia as 
being characterised by the small size of the 
corpus callosum,t which does not extend far 
back. It must not be supposed that the pre- 
ceding quotations are adduced to detract from 
the merit of works whose reputation deservedly 
stands high. The position of the accomplished 
and indefatigable authors in a central town of 
Germany is unfavourable to the acquisition of 
specimens of animals which, like the Marsu- 
pialia, are almost confined to one of our most 
distant colonies; but it would be unpardonable 
in an English Comparative Anatomist, pos- 
sessing the requisite opportunities of consult- 
ing nature, to content himself with copying 
the generalizations of foreign systematic writers, 
which, as regards the marsupial Mammalia, 
are liable to repose on so limited and imperfect 
an induction. 
The Spinal Chord.—The spinal chord mani- 
fests all the usual Mammalian characters; the 
brachial and pelvic enlargements correspond 
with the relative size and muscularity of the ex- 
tremities to which they furnish the nerves, the 
lumbar or pelvic enlargement is consequently 
most marked in the Kangaroo (fig. 119) and 
Potoroos, but does not exhibit the rhomboidal 
sinus which characterises this part of the 
chord in Birds. The disposition of the layer of 
* <« Die aussere gestalt der hemispharen ist in 
den Nagera, so wie in den Schnabel-und Beutel- 
thieren, Spitzmausen, Maulwiirfen und Fleder- 
miusen ein vorwarts sich verschma‘erndes Eirund, 
und ihre Oberflache, wie in Vogel volikommen 
glatt; hinterwarts werden dadurch weder kleines 
Hirn, ja oft nicht einmal die Vierhugel bedeckt ; 
innerlich ist die grosse Commissur (corpus callosum ) 
gewohnlich noch sehr kurz (bei den Fiedermausen 
und dem Kinguruh kaum so lang als die Vierhiigel, 
eine Bildung, so an die Vogel erinnert) der Ums- 
chlag des Balkens und Forsetzung desselben in die 
Seitenhohlen (cornua Ammonis) vorziiglich breit 
und gros (f. v.e. g.)” Ibid. § 124, p. 79. 
j Der Balken ist noch schmal bei den Nagern, 
Fledermausen, Edentaten und Bentelthieren und 
geht nicht weit nach hinten.—LZehrbwch der Ver- 
. gleichenden Anatomie, p. G09, 1835, 
