ORGANS OF THE SENSES. 27 1 



in this : thus in the visual organ of swimming mammalia 

 we observe many affinities with the eyes of fishes, those 

 of bats approach to those of birds, and intermediate forms 

 are allied to the eyes of reptiles. In cetaceous animals, 

 which constantly reside in the water and receive the rays 

 of light through that dense refractive medium, the eyes 

 have little aqueous humour, the cornea is flat, the crystalline 

 lens is large, dense and spherical, and the vitreous humour 

 is less abundant than in terrestial quadrupeds ; and in order 

 to preserve this flatness of the fore-part of the eye, the 

 sclerotic coat, like that of fishes, is here thick, firm, and 

 elastic, especially over the back and the anterior parts of 

 the eye. The sclerotic is an inch thick at the back part of 

 the eye in the whale. The superior oblique muscle of the 

 eye appears still destitute of a pully for its tendon in the 

 cetacea, these lowest aquatic mammalia, and they are almost 

 in the condition of fishes in the imperfect development 

 of their eye-lids and even of their lachrymal apparatus ; but 

 the smallness of their visual organs compared with those 

 of fishes, accords with the higher development of their in- 

 ternal organs of perception, and their whole nervous system. 

 Many intermediate forms of the organs of vision between 

 those of cetacea and those of land quadrupeds are seen 

 in species which have only semi-aquatic habits, as in walruses, 

 otariae, seals, otters and beavers, where the eye gradually 

 becomes more spherical in form, its coasts around the middle 

 of the eye more thin and membranous, the cornea more 

 convex, the crystalline lens more compressed from before, 

 backwards, and of a softer texture, and the glandular and pro- 

 tecting apparatus of the eye more complicated and more 

 perfect in structure. The large eye of the ruminatia and 

 most other herbivorous quadrupeds accord with the imper- 

 fect development of their intellectual organs, and they 

 often present a greater lateral than vertical extension 

 of the transparent cornea, the pupil, and even of the 

 whole eye-ball, by which the lateral range of vision is 

 extended in these timid and watchful animals during the 

 inclined position of the head. The prominent cornea of 

 carnivorous quadrupeds covers a pupil most frequently ex- 

 tended in a vertical direction which is that most suited 

 to their leaping and predaceous habits, and in the back part 



