INTKODUCTION. XIX 



long immersed in water ; some arteries appear to be intended as 

 reservoirs of arterial blood, and the dilatations of some of the veins 

 serve to prevent too great distension while the respiration is stopped. 



The brain is larger than in any other class of animals, owing chiefly 

 to the increased size of the cerebral hemispheres, which are united in 

 most by a fibrous band called the corpus callosum. The cerebellum 

 too has lateral lobes, which are united by the pons varolii, and the 

 hemispheres are usually convoluted externally. The Marsupial 

 animals, however, do not possess the great commissures, and the 

 cerebrum is smooth externally, thus more resembling Birds. The 

 olfactory tubercles, and the optic lobes, are greatly reduced in size. 



The organs of the senses are highly specialized, and in a great state 

 of perfection in Mammals. The sense of touch is very delicate, and 

 is usually concentrated in various parts, e.g., in the whiskers of the 

 Seals, of some Insectivora and Carnivora, in the lips of the Horse, the 

 trunk of the Elephant ; and this sense is greatly developed and 

 extended among Bats in every part of the body, but especially in the 

 naked wings, the ears, and facial appendages. 



The sense of smell is very acute in most animals, and many have 

 a mobility of the outer nostrils to aid it, never found in the other 

 Vertebrata. The olfactory tubercles are so diminished in size as 

 barely to be recognized. We are all familiar with the power of the 

 scent in the Dog, as well as in most of the Carnivora, which enables 

 them to discover and hunt down their prey. Antelope, Deer, and 

 other timid animals have likewise very acute sense of smell, to enable 

 them to avoid approaching danger. The nasal plates of the ethmoid 

 bone, and the convolutions of the turbinated bones, are coated 

 Avith a delicate membrane, which forms a large surface, amply sup- 

 plied with minute branches of the olfactory nerves, and the air 

 inhaled passing to the lungs over this membrane, imparts the im- 

 pression. 



The structure of the eye is in all Mammals almost identical with 

 that in Man. In some the pupil is round, in others oblong or linear. 



In Bats, and some .nocturnal Insectivora, the external eye is very 

 minute, but the sense of sight appears to be compensated for by an 

 increase o-f that of touch. Many of the Ruminants have the eye 



