DISEASES OF THE EYE. 339 



The refracting media of tlie e\e are tliree in number, namely (1) 

 the aqueous humor, a watery fluid inclosed in a chamber behind the 

 cornea; (2) the crystalline lens and its capsule, a transparent, soft 

 solid of a biconvex form, and placed behind the iris; (3) the vitreous 

 humor, a transparent material with a consistence like thin jelly, and 

 occupying as much of the interior of the eye as is subjacent to the 

 choroid. 



The sclerotic is a strong, opaque, fibrous membrane, which, in a 

 great measure, maintains the form of the eyeball and protects the 

 more delicate structures within it. Its interior portion, which is 

 covered by the ocular conjunctiva, is commonly known as the "white 

 of the eye."' In form it is bell-shaped, and the optic ner\'e pierces it 

 behind like a handle, the perforation being a little to its inner side. 

 In front, the rim of the bell becomes continuous with the cornea. 

 The outer surface of the membrane receives the insertion of the mus- 

 cles of the eyeball. The coat is thickest over the posterior part of 

 the eyeball, and is thinnest a little behind its junction with the 

 cornea. 



The cornea is the anterior transparent portion of the outer coat of 

 the eyeball. It may be viewed as a part of the sclerotic specially 

 modified to permit the passage of light into the interior of the eye. 

 Its outline is elliptical, nearly circular, and its greatest diameter is 

 trans veree. At its periphery it joins the sclerotic by continuity of 

 tissue, and as the edge of the cornea is slightly beveled and has the 

 fibrous sclerotic carried for a little distance forward on its outward 

 surface, the cornea is generally said to be fitted into the sclerotic like 

 a watch glass into its rim. The venous canal of Schlemm runs cir- 

 cularly around the eyeball at the line of junction of the sclerotic and 

 cornea. The anterior surface of the cornea is exquisitely smooth, 

 and is kept moist by the lacrimal secretions. Its posterior surface 

 forms the anterior boundary of the chamber in which the aqueous 

 humor is contained. The cornea is of uniform thickness and is of 

 a dense, almost horny, consistence. Save a few capillary loops of 

 blood vessels at its margin the cornea is without vessels. Its struct- 

 ure is comprised of five distinct layers. 



The aqueous humor occupies a chamber which is bounded in front 

 by the posterior surface of the cornea and behind by the capsule and 

 suspensory ligament of the lens and by the ends of the ciliary proc- 

 esses. It is across this chamber that the iris extends. The aqueous 

 humor is composed of water, with a small proportion of common salt 

 in solution. 



The iris is a muscular, pigmented curtain extending across the inte- 

 rior of the eye and having about its center an aperture termed the 

 pupil. By variations in the size of this aperture the amount of light 

 transmitted to the retina is regulated. It varies somewhat in color, 



