OF THE CLASS MAMMALIA. 237 



J399. The organs of the other senses are also analogous 

 1 the mammals to those of man. In those remarkable for 

 the fineness of their powers of smell, the dog, for example, 

 the nasal fossse and frontal sinuses are much enlarged, and 

 the turbinated bones of the nostrils are greatly developed, 

 thus extending the surface of the pituitary membrane, on 

 which the nerves of smell are distributed. 



400. In nocturnal animals, the eyes are generally larger 

 than in the diurnal ; and in the former the pupil of the eye, 

 when contracting, assumes the form of a fissure, losing its 

 circular character. In moles, whose residence is underground, 

 the eyes become extremely small, and seem in some to be 

 mere vestiges or rudimentary organs ; in the aquatic mam- 

 mals, the lens is, for an obvious reason, spherical. In many 

 mammals there exists a coloured part of the choroid tunic of the 

 eye, called tapetum, the uses of which are unknown.* Many 

 have a third eyelid placed vertically at the inner angle of the 

 horizontal eyelids. Finally, the direction of the eyes varies 

 much : in man they are directed forwards ; but, as we descend 

 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 vermiform is superior), 



* The glaring of the eyes in the dark depends on the presence of this 

 tapetum; its colour varies" in different animals. R. K. 



