260 THE ANATOMY OF THE HORSE. 



fibrous sclerotic carried for a little distance forward on its outer 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 circularly around 

 the eyeball at the line of junction of the sclerotic and cornea. The 

 anterior surface of the coniea 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 humour is contained. 

 The cornea is of unifoi-m thickness ; and, as will afterwards be proved 

 in removing it, it is very difficult to cut, being of a dense, almost 

 homy consistence. When its normal convexity is disturbed, the cornea 

 becomes opaque. 



Strtidvre. — Save a few capillary loops at its margin, the cornea is 

 without vessels. Its structure comprises the following layers, which are 

 enumerated in order from the anterior to the posterior surface : — 



1. The Anterior Epithelmm is a stratified, pavement epithelium, 

 continuous at the margin of the cornea with the conjunctival epithelium. 



2. The Anterior Elastic Lamina (Bowman's membrane). This is a 

 structureless, elastic layer. It is extremely thin in the eye of the 

 lower animals, but is better developed in the human eye. 



3. The Substantia Propria. This, which forms the main thickness of 

 the cornea, is composed of fibrous connective-tissue arranged in lamellse 

 parallel to the surfaces of the cornea. Between adjacent lamelhe there 

 is left a network of spaces and branching canals, in which are found the 

 branched corneal corptiscles. 



4. The Posterior Elastic Lamina (Descemet's membrane) is a thick, 

 structureless, elastic layer. 



5. The Posterior Epithelium is a single layer of polygonal cells. 

 Directions. — A sti'ong pin should now be passed through the optic 



nerve, and used to fasten the eye beneath the surface of water in a 

 wide and shallow vessel, as already directed in the case of the frozen 

 section. While one hand steadies the eye beneath the -svatei*, an incision 

 is to be made with the other through the cornea, using for the purpose 

 a very sharp scalpel. As soon as the incision is made, some of the 

 aqueous humour Avill escape into the Avater, and may possibly be recog- 

 nised by a slight inky discoloration, which is due to a 2^ost-mortem 

 disintegration of the pigmented epithelium lining the cavity in which 

 the humour is contained. Still keeping the eye under water, one blade 

 of a pair of small scissors should be introduced Avithin the incision, and 

 the coniea should be excised immediately in front of its junction Avith 

 the sclerotic. The iris will by this means be exposed, and the next step 

 must be to remoA-e a portion of the sclerotic so as to expose the sub- 

 jacent choroid. Beginning at its anterior edge, it may be incised back- 

 Avards tOAvards the optic nerve, snipping it bit by bit Avith the point of 

 the scissors. Another incision may then be made parallel to the first. 



