NERVOUS SYSTEM IN THE VERTEBRATA. 53 



MAMMALIA. 



In this class the increased development of the nervous system is 

 marked by the size and length of the spinal chord ; the magnitude 

 of the cerebrum and cerebellum, and the number of their gray 

 deposits and commissures, as well as by the number and arrange- 

 ment of the ganglions, together with the extent and systematic dis- 

 tribution of the great sympathetic. 



The spinal chord is larger in proportion to the size of the body, 

 but smaller when compared with the brain, in this than in any of 

 the preceding classes; its internal canal has almost ceased to exist, 

 and its lateral portions are more intimately united. It is shortest 

 in man, quadrumana, and the tailless cheiroptera, and longest in 

 the cetacea, where, as in apodal fishes, tadpoles, serpents, and the 

 human embryo, it presents no posterior enlargements. In the long- 

 tailed quadrupeds it extends to the sacrum; the posterior groove is 

 generally shallow, though sometimes of considerable depth, as in 

 cheiroptera and rodentia. The medulla oblongata is small : the 

 corpora pyramidalia decussate very distinctly: the olivary bodies 

 are small, and generally contain a corpus dentatum in their interior, 

 and the transverse fibres of the pons or great cerebellic commissure 

 are well seen. 



Brain. — In the class mammalia, the cineritious matter bears a 

 small proportion to the white substance. The convolutions are 

 very superficial in the cetacea, edentata, ruminantia, and pachyder- 

 mata, and wholly absent in the rodentia, and monotremata, as in 

 birds; whilst they are deep in man, monkeys, and carnivorous ani- 

 mals. The optic lobes, without cavities, are smallest in man, 

 quadrumana, and carnivora, larger in the herbivora, and largest of 

 all in rodentia and edentata. A contrary ratio obtains respecting 

 the development of the cerebral hemispheres, and the olfactory 

 tubercles. The anterior and inferior cornua of the lateral ventri- 

 cles, as well as the several commissures, are always present in this 

 class. The posterior lobe of the brain, and the posterior cornua of 

 the lateral ventricle, first appear in the quadrumana ; the former 

 has no convolutions. 



The vast superiority of man over all other animals in mental 

 faculties, led physiologists at a very early period to seek for corres- 

 ponding differences in the brains of man and animals. They 

 compared the weight of the brain with that of the body, and their 

 researches led them to conclude that man had the largest brain in 

 proportion to his body. Since the time of Aristotle till within a 

 late period this opinion has been received : but more modern inves- 

 tigations have proved that the proportion of the brain to the body 

 in some birds exceeds that of man, and that several of the quadru- 

 mana and some rodentia equal him in this respect. The illustrious 

 Soemmering proposed another mode of comparison, that of the 

 ratio which the mass of the brain bears to that of the nerves arising 

 from it, and in this point of view man is decidedly pre-eminent. 



