304 ANIMAL PHYSIOLOGY. [LESSON 95. 



which sends processes into its interior, forming cells in which the 

 humor is retained. A small artery may sometimes be traced through 

 the centre of the vitreous humor to the capsule of the lens. 



1381. The sclerotic coat is a tunic of protection, and the cornea 

 a medium for the transmission of light. The choroid supports the 

 vessels destined for the nourishment of the eye, and by its pigraentum 

 nigrum absorbs all loose and scattered rays that might confuse the 

 image impressed upon the retina. The iris, by means of its powers 

 of expansion and contraction, regulates the quantity of light admit- 

 ted through the pupil. 



1382. The transparent cornea, and the humors of the eye, have 

 for their office the refraction of the rays in such proportions as to 

 direct the image in the most favorable manner upon the retina. 



1383. Such, then, are the several parts, and such their uses in 

 the eyes of the higher animals, and it only now remains to point out 

 the peculiarities of structure which distinguish classes, or the indi- 

 vidual members of classes in the ascending scale of being. 



LESSON XCY. 



THE EYES IN REPTILES, BIRDS, AND MAMMALIA. 



1384. The eyes of Reptiles are more fitted to receive the rays 

 of light from the rare medium of the atmosphere than those of 

 fishes ; their cornea is generally more convex, their aqueous and 

 vitreous humors more abundant, and their lens less spherical in form ; 

 they also possess two movable eyelids, and a membrana nictitans (a 

 thin membrane, drawn rapidly across the front of the eye, by which 

 its surface is wiped, and obstructions removed ; the exercise of this 

 organ is said to simulate winking). 



1385. In many of the Crocodilian reptiles, and the Tortoises and 

 Turtles, the sclerotic, at its anterior part, supports a circle of osseous 

 plates, which surround the transparent cornea, as in birds (Fig. 

 431). These plates around the cornea existed in the Ichthyosaurus, 

 and are found abundantly in the fossilized condition. 



1386. In the eye of a Tortoise (Emys Europcea, Fig. 432) the 

 cornea (a) is pretty convex, from the abundance of aqueous humor 

 (b) in the anterior chamber, and the margin of the cornea is sup- 

 ported by ten osseous plates (Fig. 431), imbricated like those of 

 birds, and placed in the anterior part of the sclerotic (d, d), near to 

 the ciliary processes (/), and to the fixed margin of the iris (e). 



