DISEASES OF THE EYE, ETC. 341 



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

 a transparent material with a consistence like thin jelly, and occupy- 

 ing 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 cov- 

 ered 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 scle- 

 rotic like a watch glass into its rim. The venous canal of Schlemm 

 runs circularly around the eyeball at the line of junction of the scle- 

 rotic and cornea. The anterior surface of the cornea is exquisitely 

 smooth, and is kept moist by the lachrymal secretion. 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 struc- 

 ture 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, 

 but is most frequently of a yellowish brown tint. Its anterior face is 

 bathed by the aqueous humor. The greater part of the posterior sur- 

 face is- in contact with the capsule of the lens and glides on it during 

 the movements of the curtain. The circumferential border is attached 

 within the junction of the sclerotic and cornea. The inner border 

 circumscribes the pupil, which varies in outline according to its size. 



