DISEASES OF THE EYE AND ITS APPENDAGES. 353 



The refracting media of the eye are three 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 anterior 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 nerve 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 muscles 

 of the eyeball. The coat is thickest over the posterior part of the eye- 

 ball, 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, approaching the circular, and its greatest 

 diameter is transverse. At its periphery it joins the sclerotic by con- 

 tinuity 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 c^inal of 

 Schlemm runs circularly 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 secretion. Its 

 posterior surface forais 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 

 v^essels. It structure 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 

 hmnor 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, 

 but is most fi-equently of a yellowisli brown tint. Its anterior face is 

 16923°— 12 23 



