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VIII. On the Structure of the Brain in Marsupial Animals. By Richard Owen, Esq. 
F.R.S. Hunterian Professor of Anatomy to the Royal College of Surgeons. 
Received October 31, 1836, — Read January 26, 1837. 
THE brain in Mammalia is essentially characterized by the complexity and mag- 
nitude of the apparatus by which its different masses are brought into communica- 
tion with one another. With respect to size, the cerebral hemispheres are in many 
species proportionally inferior to those of Birds ; and in most Insectivorous and Ro- 
dent Mammalia they present an equally smooth and uniform external surface ; but 
notwithstanding the absence of convolutions and diminished size of the cerebral he- 
mispheres in such Mammalia, a large apparatus of medullary fibres is present, which 
connect together either the opposite hemispheres, or the distant parts of the same 
hemisphere ; and this apparatus, or great commissure, is superadded to the anterior, 
posterior, and soft commissures, which, with the exception of a very slight rudiment 
of the fornix, are alone developed in birds for the purpose of uniting the opposite 
hemispheres. In the higher Mammalia, in which the cerebral hemispheres acquire 
superior size and increased extent of surface by means of convolutions, the super- 
added commissural apparatus presents a corresponding development and a highly 
complicated structure ; its several parts being distinguished as the corpus callosum, 
fornix, and their intercommunicating laminae, termed the septum lucidum. The 
fornix, by means of its posterior crura and the intermediate medullary tract termed 
the lyra, brings the hippocampi majores into communication with each other, and 
with the posterior folds of the corpus callosum*'; by means of its anterior crura it 
establishes a communication between the hippocampi and the optic thalami ; and by 
means of the septum lucidum its connexion with the corpus callosum is continued to 
the anterior fold of that body-f. 
In the Human brain the fornix, though of complex structure and developed as a very 
distinct part, is of small size as compared with the corpus callosum ; while the delicate 
* “ The fasciculi from the fornix form in part the covering of the hippocampus, and in part its loose fold, the 
taenia hippocampi.” — Reil in Mayo’s Anatomical Commentaries, p. 116. 
“ L’envelope medullaire de la corne d’ Ammon se continue avec la partie posterieure du corps calleux, et en 
partie aussi avec le pilier posterieur de la voute : c’est dans ce dernier que va se jeter le corps frange tout 
entier.” — Meckel, Anatomie Descript, tom. ii. p. 679. 
f “ Ainsi la voute represente une chaine trbs complexe qui unit les deux hemispheres l’un avec l’autre sur 
plusieurs points, et qui, de plus, etablit une communication entre la partie anterieure et la partie posterieure 
de chaque hemisphere.” — Meckel, Anatomie Descript, tom. ii. p. 658. 
