ORGANS OF THE SENSES IN MAMMALIA. Ill 



nocturnal Mammalia, than in those which seek their food by day ; 

 and, among the former, the pupil, when contracting under the 

 influence of light, instead of preserving its rounded form, usually 

 becomes a narrow slit. Amongst those which are condemned, 

 by their subterranean life, to complete darkness, (as the Moles, 

 for example), the eyes become extremely small, and sometimes 

 only exist in a rudimentary state ; and in those Mammalia which 

 live in the water, the crystalline lens is more spherical than 

 among those that live in the air. This arrangement is necessary, 

 in order to increase the refractive power of the eye ; which, every 

 thing else being equal, requires the power of refracting the rays 

 of light with greater power, in proportion to the density of the 

 medium in which it is placed. It is remarked, also, that among 

 many of these animals there is, on the choroid at the bottom of 

 the eye, a brilliantly-coloured spot, which is called the tapetum ; 

 but of its use we are ignorant. There are several, also, which 

 have a third well-developed eye-lid, placed vertically at the in- 

 ternal angle of the two others. The direction of the eyes varies 

 much. In Man, they are placed nearly straight forwards ; but, 

 as we descend in the series of Mammalia, towards those whose 

 faculties are less developed, we see these organs becoming more 

 and more lateral in their direction, until, among several, the 

 sphere of vision is extremely different for each eye, so that the 

 animal can scarcely see directly before him. This is particularly 

 the case in such as are most liable to be pursued by others, and 

 is an obvious adaptation to their timorous habits, giving them 

 nearly as complete a survey of the country behind as of that in 

 front of them. 



95. The apparatus of Hearing also, presents, amongst the 

 Mammalia, some modifications which appear to be connected with 

 the habits of the animals. Among those which live in the water 

 or beneath the ground, the external ear is generally very small, 

 or even altogether rudimentary ; and, as we descend from Man 

 to the Herbivorous Mammals, we find this part of the ear more 

 and more trumpet-shaped, becoming more and more detached 

 from the head, and also more movable. Among the Nocturnal 

 Quadrupeds, the membrane of the tympanum generally occupies 



