336 ANATOMY AND PHYSIOLOGY 



and its convexity may be seen by looking across an eye from the 

 side. The junction of the cornea with the sclera resembles the 

 fitting of a watch-crystal in its case. 



The portion of the sclera which is visible when the eyelids are 

 separated, and also the cornea, are both covered by a thin mem- 

 brane called the conjunctiva; it is a modified mucous membrane, 

 bearing blood-vessels which can be seen, especially if a little 

 dilated. 



The choroid. The middle coat, next to tne sclerotic, is neither 

 dense nor tough, but is made up of fine tissue fibers bearing a 

 very delicate and close network of blood-vessels. It is the vascular 

 coat of the eye, and lines the sclera only, not the cornea. Many 

 pigment cells are contained in the choroid coat, giving to it a 

 deep brown color so that it makes a dark chamber of the eye. 



Sclera 



FIG. 215. THE CHOROID AND IRIS. (H olden.} 



The iris. There is no choroid behind the cornea. Its place 

 is supplied by the iris, which resembles in its shape a circular 

 curtain attached by its edge to the choroid, and having a round 

 aperture in the center called the pupil or the "star of the eye." 

 The iris contains a network of fine vessels and pigment cells, 

 varying in color according to the amount of pigment. (Blue eyes 

 have least, black eyes most.) It has muscular fibers arranged in 

 two sets circular, or ring fibers, and so-called radiating, or straight 

 fibers. The circular fibers surround the pupil. Thus, when they 

 contract, as in a bright light, they diminish its size. The straight 

 fibers run from the outer border of the iris toward the pupil, and 



