OF THE CLASS MAMMALIA. 251 



in the scale, the direction becomes more and more lateral, 

 and the animal can no longer see directly before him : the 

 sphere of vision for each eye must therefore be distinct. 



401. The organs of hearing offer some modifications in 

 relation to the habits of the animal. In aquatic mammals 

 the external ear is small, or rudimentary, or absent ; in the 

 herbivora it assumes the form of an ear-trumpet, becoming 

 more and more detached from the head, and fitted thereby to 

 perform the functions of an acoustic tube. In the nocturnal, 

 the membrana tympani occupies more space, and is nearer 

 the surface. 



402. Nervous System. As regards the nervous system, 

 in mammals it differs only in respect of the more or less 

 development of its various parts. In all, the encephalic part 

 is very considerable, whether viewed proportionally as regards 

 the nerves or the bulk of the body of the animal; but all the 

 organs composing it do not contribute to this development : 

 thus the cerebral hemispheres are very large, whilst the 

 optic thalami may be very small, or even rudimentary ; the 

 reverse of what happens in birds, reptiles, and fishes. The 

 cerebellum is also generally large in mammals, and is com- 

 posed of a median portion (processus vermiformis superior)) 

 of two hemispheres, and of a commissure, surrounding the me- 

 dulla spinalis superiorly, the annular protuberance. More- 

 over, many differences exist among mammals in respect of 

 these organs, as well as regards the depressions and circum- 

 volutions on the surface of the brain. As we descend from 

 man to apes, from these to the carnivora, and from the car- 

 nivora to rodents, the brain generally becomes smaller and 

 smaller, and smoother. The face also is developed in an 

 inverse ratio to that of the brain, and the measure of the 

 intellectual faculties may be guessed at by observing this 

 increasing size of the face as compared with the brain. 



In the marsupialia and monotremes, the brain seems still 

 further degraded by the absence of, or at least the rudimen- 

 tary state of, the corpus callosum, which in all other mam- 

 mals unites the hemispheres of the brain to each other. 



403. Functions of Nutrition. The functions of nutri- 

 tion resemble each other throughout nearly the whole of the 

 class ; the digestive tube is the organ which shows the most 

 remarkable variety in its arrangements. 



Nearly all have teeth ( 53), which vary in number and 

 form, in accordance with the kind of food they live on. But 



