NERVOUS SYSTEM IN THE VERTEBRATA. 55 



the monk ape and the dog, 1 to 8 ; in the magot, papio, and wild 

 boar, 1 to 7 ; in the sai and hare, 1 to 6 ; in the mole, 1 to 4 ; in 

 the rat and beaver, 1 to 3 ; and in the mouse, 1 to 2. 



Nerves. — The olfactory nerves are largest in the ruminantia, 

 pachydermata, and carnivora, smaller in the cheiroptera and qua- 

 drumana, and discoverable with difficulty in many of the cetacea. 

 In the squirrel, rabbit, hare, and other large-eyed nocturnal quadru- 

 peds, the optic nerves are very large; they are small in rats, mice, 

 bats, hedge-hogs, and subterranean moles, and in the sorex arraneus, 

 mus typhlus, muscapensis. and others, they are said to be altogether 

 wanting. They unite before the infundibulum, and form a partial 

 decussation of their fibres. The third, fourth, and sixth nerves are 

 distributed as in man, and are very small in subterranean animals. 

 Of all the cerebral nerves, none reaches so great a degree of deve- 

 lopment as the fifth pair, in the inferior classes of animals and in 

 the foetal state of the human subject: it is also of enormous size in 

 most aquatic birds. Its branches are freely and extensively distri- 

 buted in those animals with proboscis, long muzzles, large lips, and 

 broad bills, as the cetacea, ruminantia. pachydermata, carnivora, 

 and ornithorhynchi, and also in those possessing horns, spines, 

 bristles, and whiskers. This nerve is supposed to preside over the 

 peculiar instinctive actions so remarkable in those grades of ani- 

 mals which indicate an inferior degree of mental endowment, and 

 this opinion receives strength from the fact of its great size in the 

 very early periods of human existence, when we know the actions 

 are purely instinctive. The remaining cerebral, the spinal, and the 

 sympathetic nerves are distributed so much after the human type 

 as to merit no particular remarks. 



In most mammalia the arteries of the brain form a complicated 

 net-work around the petuitary body at the base of the cranium 

 named rete mirabile, obviously designed to impede the flow of 

 blood to the brain, in those animals with pendent heads. The 

 veins occasionally run in osseous canals in order to avoid pressure; 

 this is well seen in the cribriform plate of the mole's skull, and in 

 the bony falx cerebri of the porpoise. A bony falx cerebri is also 

 found in the ornithorhynehus, an animal which abounds in in- 

 stances of anomalous structure. Animals which possess a bony 

 tentorium are of far more common occurrence: it is well developed 

 in most species of the cat and bear kind; it is not so well marked 

 in the dog, seal, horse, and wombat, and it is merely rudimentary 

 in the pig, the rabbit, and the mouse. 



It has been generally supposed that these structures exist in such 

 animals only as jump far, and that they served the purpose of pro- 

 tecting the respective portions of the cerebrum and cerebellum from 

 undue pressure during these active movements, but this opinion is 

 rendered quite untenable from the fact of their absence in many 

 animals notable for jumping, as the wild goat, (fee, and their pre- 

 sence in those animals alike remarkable for their slow and easy 

 movements, as the bear. It is more probable they exist for the 



