SENSE OF SIGHT. 195 



coat, and is composed of six concentric plates ; but its blood- 

 vessels are so small, that they exclude the red particles alto- 

 gether, and admit nothing but serum. Under the first plate, 

 little glands are found, perceptible only by aid of the micro- 

 scope, which are supposed to secrete an oil which gives the 

 eye its bright, sparkling appearance. In the last stage of 

 life, we find this fluid forming a thin pellicle over the cornea?. 

 I What is called the ciliary ligament, is the groove or circle, 

 where the cornea is inserted in the sclerotic coat^ 



9. \We find the tunics just described in the lower grades 

 of animals, as well as in man, differing of course in form, 

 density, &c., but always fitted to the circumstances of the 

 animal. Thus in soft, molluscous animals, the defensive coats 

 of the eye are soft also ; but then their eyes are placed on ten- 

 tacula, or feelers, so that they can retract them within their 

 shells.! In the articulata, as insects, the coverings of the eye 

 are firmer, and of a horny consistence. In the Crustacea, such 

 as crabs, lobsters, &c., the eyes are set in short bony cylinders, 

 and the cornea is often covered merely by a portion of skin, 

 which passes over it. In fishes, we see the cornea flat, on 

 account of the absence of the aqueous humour. In birds, 

 the cornea is conical, corresponding with the quantity of 

 aqueous humour ; and it is more prominent in rapacious 

 birds, such as the hawk, eagle, and kite, than in aquatic 

 birds. It is remarkable that the sclerotica in birds consists 

 of three different layers, the middle one of which is bone, 

 and composed of several plates, overlapping each other ; 

 designed doubtless to protect their eyes against each other's 

 sharp-pointed bills, which they have to encounter in their 

 attacks. In the higher animals, the coverings of the eye are 

 similar to those of man. 



10. tThe Iris, is a circular membrane, which hangs sus- 

 pended like a curtain in the aqueous humour, and is attach- 

 ed to one of the coats of the eye at its circumference. It 

 derives its name iris, or rainbow, from the various colours it 

 has in different individuals, such as dark-brown, or black, 



