18 INTRODUCTION. 



not only seen in the diminution of the cerebral hemispheres ; in the 

 smoothness of their surface ; in the more or less complete removal of the 

 cerebellum from the cerebrum, so as frequently to bring the corpora qua- 

 drigemina (or optic lobes) into view, as in the Opossum ; and in the mag- 

 nitude of the olfactory ganglia ; but, in the absence of that union, by 

 means of the great commissure (corpus callosum), which exists in ordinary 

 Mammalia, and of which the extent is in proportion to the development 

 of the cerebral hemispheres.* In birds the corpus callosum is entirely 

 wanting ; the union between the cerebral hemispheres consisting only in 

 a rudimentary fornix, and in the anterior, posterior, and soft commissures, 

 which are also present in ordinary Mammalia (see fig. 3, p. 11). Now, 

 in the Marsupialia, as has been shewn by Professor Owen, a small com- 

 missural medullary band represents the corpus callosum, but does not 

 unite the masses of the two hemispheres above the ventricles, as in other 

 mammals (see fig. 3, p. 11, c, c); and is, in fact, the fornix, or commis- 

 sure of the hippocampi, into which its fibres, running along the floor of the 

 ventricles, pass. Here, then, is a cerebral condition approximating to that 

 of birds, between which class and the ordinary Mammalia, as it respects 

 the development of the brain, the marsupials are intermediate. The 

 anterior commissure is remarkable for its comparative magnitude, being 

 in proportion to the size of the ganglion, or protuberance forming the root 

 of the olfactory nerve. 



The annexed sketches represent the upper surface of the brain of the 

 Chimpanzee (fig. 4) ; of the Tigress (fig. 5) ; of the Kangaroo (fig. 6) ; 

 and of the Wombat (fig. 7). 



The Chimpanzee is the most anthropomorphous, or man-like, of the 

 Simiae, and, accordingly, we find an approach to Man in the general form of 

 the brain (fig. 4), and the proportions of its respective parts. Its general 

 form, viewed from above, is a short oval, and the convolutions of the 

 cerebral hemispheres (a, a,) are well marked. Nevertheless, the cerebellum 

 (b, &,) is not completely covered by the cerebrum, but projects beyond the 

 posterior line of the latter, so as to be visible, which, on reference to 

 the brain of the human subject (fig. 1, page 11), will be found not to be the 

 case in Man. In the Orang, the posterior projection of the cerebellum 

 is carried somewhat farther. In the feline animals, of which the Tiger is 

 an example, the brain (fig. 5) presents us with a still greater departure, in 

 form and proportions, from that of Man : the cerebellum (b, b,) is com- 

 pletely posterior to the cerebral hemispheres (a, a), the comparative mag- 

 nitude of which is diminished; notwithstanding, the convolutions are 

 strongly marked. The medulla oblongata (c,) has now acquired a greater 



* It is found that extensive and intimate communication, by means of this great commissure, between 

 the cerebral hemispheres, has more influence upon, or accordance with, intellectual superiority, than 

 has the mere size of the hemispheres. 



