284 



FUNCTIONS OF THE NERVOUS SYSTEM. 



361. This is still more evident in BIRDS, in whose Encephalon the Tuber- 

 cula Quadrigemina or Optic Ganglia, and the Thalami with their included 

 ventricle, are obviously very distinct parts. The Cerebral Hemispheres 

 attain a great increase of development, and arch backwards, so as partly to 

 cover the Optic ganglia ; and these are separated from one another, and thrown 

 to either side. The Cerebellum also is much increased in size, proportion- 

 ably to the Medulla Oblongata and its ganglia ; and it is sometimes marked 

 with transverse lines, which indicate the intermixture of grey and white mat- 

 ter in its substance ; there is as yet, however, no appearance of a division 

 into hemispheres. On drawing apart the hemispheres of the Cerebrum, the 

 Corpora Striata, Optic Thalami, and Tubercula Quadrigemina or Optic Gan- 

 glia, are seen beneath them ; the size of the last still bears a considerable pro- 

 portion to that of the whole Encephalon. The Optic Ganglia are still hollow, 

 as they are in the embryo condition of Man. Indeed the Brain of the Human 

 foetus about the twelfth week will bear comparison, in many respects, with 

 that of the Bird. The Cerebral hemispheres, much increased in size, and 

 arching back over the Thalami and Optic ganglia, but destitute of convolutions, 

 and imperfectly connected by commissures, the large cavity still existing in 



Fig. 147. 



Fig. 148. 



cs.. 



t/ivl 



Brain of Human Embryo at twelfth week. A, seen from behind ; B, side view; c, sectional view; a, 

 corpora quadrigemina; bb, hemispheres; d, cerebellum; e, medulla oblongata; ./, optic thalamus; g-, 

 floor of third ventricle; I, olfactory nerve. 



the Optic ganglia, and freely communicating with the third ventricle, and 

 the imperfect evolution of the Cerebellum, make the correspondence in the 

 general condition of the two very considerable. 



362. The Brain of the lowest MAMMALIA presents but a slight advance 



upon that of Birds, in regard both to the rela- 

 tive proportions of its parts, and to their degree 

 of development. Thus, in the Marsupialia, the 

 Cerebral hemispheres exhibit no convolutions ; 

 and the great transverse commissure. the 

 Corpus Callosum, is deficient. There is gra- 

 dually to be noticed, however, in ascending the 

 scale, a backward prolongation of the Cerebral 

 hemispheres ; so that first the Optic ganglia, 

 and then the Cerebellum, are covered by them. 

 The latter partly shows itself, however, in all 

 but the Quadrumana, when we look at the 

 brain from above downwards; in the Rabbit, 

 which is in this respect among the lowest of 

 the true Viviparous Mammalia, nearly the 

 whole of the Cerebellum is uncovered. In 

 proportion to the increase of the Cerebral 

 hemispheres, there is a diminution in the size 

 of the ganglia immediately connected with the 

 organs of sense ; and this in comparison, not 

 only with the rest of the Encephalon, but even 



Brain of Squirrel, laid open ; the 

 hemispheres, B, being drawn to either 

 side to show the subjacent parts; c, 

 the optic lobes ; D, cerebellum ; thai, 

 thalamus opticus; cs, corpus striatum. 



