140 ELEMENTS OF COMPARATIVE ZOOLOGY. 



watery fluid (aqueous humor), while behind it and between 

 it and the retina is the somewhat denser vitreous humor. 



The optic nerve enters the eye from behind, passing 

 through sclerotic, choroid, and retina, and is then dis- 

 tributed over the inner surface of the latter layer. 



The eyeball is moved by six muscles, which are essentially 



FIG. 65. Diagram of vertebrate eye. c, choroid ; , iris ; Z, lens ; n, optic 

 nerve ; r, retina ; s, sclerotic. 



alike in all vertebrates. Four of these are straight or rectus 

 muscles, two are oblique. These muscles are controlled by 

 the three eye-muscle nerves (p. 136). 



The alimentary canal runs through the body from mouth 

 to vent. In it several parts can be distinguished. 



The mouth, at or near the anterior end, is without fleshy 

 lips, except in the mammals. The mouth is frequently 

 armed with teeth, and even in those groups, like the turtles 

 and the birds, where they are absent the germs occur in the 

 young, a fact which points to the descent of these from 

 toothed ancestors. 



The tongue is formed as a fold of the floor of the mouth, 

 and is usually supported by a skeleton (hyoid, p. 132) derived 



