354 DISEASES OF CATTLE. 



aqueous humor, a watery fluid iuclosed 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 deli- 

 cate 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 han- 

 dle, 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 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 modi- 

 fied to permit the passage of light into the interior of the eye. Its out- 

 line is elliptical, approaching the circular, and its greatest diameter is 

 transverse. At its periphery it joins the sclerotic by continuity of tis- 

 sue, 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 rini. The venous canal of Schlemm runs circu- 

 larly 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 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 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 sus- 

 pensory ligament of the lens, and by the ends of the ciliary processes. 

 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 trans- 

 mitted 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 surface is iu 

 contact with the capsule of the lens and glides on it during the move- 

 ments of the curtain. The circumferential border is attached within 

 the j unction of the sclerotic and cornea. The inner border circumscribes 



