306 ANIMAL PHYSIOLOGY. [LESSON 95. 



1390. The ciliary processes are also well developed in this Tur- 

 tle (Fig. 434) ; they are, however, remarkable for their shortness 

 and thickness, as compared with the same organs in other animals. 

 The lower part of this preparation joins the upper part of 433. 



1391. In Birds the round pupil is capable of great and rapid 

 changes of dimension, aided by the remarkable mobility of the iris. 

 This highly movable and bright colored iris, with its black uvea (the 

 posterior coat of the iris), appears sometimes detached from the free 

 anterior margin of the choroid, and its colored surface presents ag- 

 gregations of minute globules, like those of the choroidal pigment. 



1392. Birds possess a membrana nictitans, very perfect upper 

 and lower eye-lids, which are provided with tarsal cartilages and 

 Meibomian glands, and even eye-lashes, in addition to the necessary 

 muscles for their elevation and depression. 



1393. The eyes are large in most of the Herbivorous mammalia, 

 especially the ruminantia, the rodentia, and most of the pachyder- 

 mata (thick-skinned animals), and also in most of the nocturnal 

 species. They are very small, even in the adult state, in the bur- 

 rowing animals, as Moles, Shrews, &c. 



1394. The same circumstances which modify the form of the eye, 

 and the proportions of its refractive parts in other classes and ani- 

 mals, affect the organ in this ; thus in the visual organ of swimming 

 mammalia, there are many affinities with the eyes of fishes ; those of 

 bats approach those of birds ; and intermediate forms are allied to 

 the eyes of reptiles. 



1395. In Cetaceous animals, which constantly reside in water, 

 and receive the rays of light through that dense refractive medium, 

 the eyes have little aqueous humor, the cornea is flat, the crystalline 

 lens is large, dense, and spherical, and the vitreous humor is less 

 abundant than in terrestrial quadrupeds ; and in order to preserve 

 this flatness of the forepart of the eye, the sclerotic coat, like that 

 of fishes, is thick, firm, and elastic, especially over the back and the 

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

 and the anterior parts of the eye in the Whale. 



1396. The large eye of the ruminantia, and of most other her- 

 bivorous quadrupeds, often presents a greater lateral than vertical di- 

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

 eye-ball, by which the lateral range of vision is extended in these 

 timid and watchful animals, during the inclined position of the 

 head. 



1397. The visual requirements of these animals are peculiar; 



