304 VERTEBRATE ZOOLOGY 



ture the anterior surface of which is flattened. Its outer covering 

 is the sclerotic coat, a thick, tough membrane of connective tissue, 

 the anterior portion of which is the transparent cornea. The 

 boundary between the cornea and the sclera is marked by a 

 prominent white ridge. 



With sharp scissors cut two large openings in the side of the 

 eyeball, opposite to each other, and study the interior of the eye. 

 Remove the other eye from the orbit and cut it in two by an 

 equatorial incision. Note behind the pupil the large transparent 

 crystalline lens; do not disturb it. The layer just within the 

 sclerotic is the choroid coat; the anterior portion of this coat, 

 lying behind the cornea, is the iris. The choroid coat contains the 

 blood vessels of the eye. The boundary between the iris and the 

 remainder of the choroid is thickened by the presence of a large 

 number of radiating ridges which form the ciliary body. This 

 structure forms the broad, black ridge around the inner surface of 

 the eye. Extending inward from this ridge is a delicate membrane 

 called the suspensory ligament, which is continuous with the trans- 

 parent capsule in which the lens is suspended. In the ciliary body 

 are delicate muscle fibers by which the position and shape of the 

 lens are changed when the focus of the eye is adjusted. 



The inner coat of the eye is the retina. It is a soft, delicate mem- 

 brane which will often be found separated from the choroid ; it is 

 the sensitive portion of the eye, and is in direct connection with the 

 optic nerve, extending forward in the eye to the iris. Just behind 

 the ciliary body it forms a serrated ridge called the ora serrata. 



The crystalline lens is a large biconvex, transparent body situ- 

 ated immediately behind the iris, its convexity being greater on 

 the anterior than on the posterior side. It is suspended in a deli- 

 cate capsule which is attached to the ciliary body by the suspen- 

 sory ligament. 



The interior of the eye is divided into two principal chambers 

 by the lens and the iris. The posterior cavity is filled with a jelly- 

 like mass called the vitreous humor, and the anterior cavity with a 

 fluid mass called the aqueous humor. 



Exercise 29. Draw a diagram showing the structure of the eye. 

 Carefully label all the parts. 



