MARSUPIALIA. 



293 



From the fact that the cerebral organ is that 

 which exhibits the most marked degradation of 

 structure in the class of wurm-blooded Verte- 

 brate animals which are characterized by an 

 oviparous generation, I was induced to sus- 

 pect, after having ascertained how closely the 

 Marsupialia 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 lamina, termed the 

 septum lucidum. 



The corpus callosum is the principal bond of 

 union between the opposite hemispheres; it 

 extends, 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 



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-titerine existence and the absence of a 

 placental connexion between the mother and 

 fetus 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. 



In 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 eacli 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 bigemina] bodies 

 (B,fig-. 117,) and pineal gland (u,Jlg. 117,) 



* Phil. Trans. 1334, p. 358. 



