DISSECTION OF THE EYEBALL. 



267 



We 

 ilijis.ted 

 kt' up: 



j tissiine jand as the edge of the cornea is slightly bevelled, and has the 

 Ifijjriy elfcclerotic carried for a little distance forward on its outer surface, 

 jth^e nasjiea is generally said to be fitted into the sclerotic like a watch- 

 to its rim. The venous canal of Schlemm runs circularly around 

 ' 3all at the line of junction of the sclerotic and cornea. The 

 auttle sf surface of the cornea is exquisitely smooth, and is kept moist 

 bpendenjlachrymal secretion. Its posterior surface forms the anterior 

 bsf the Vj of the chamber in which the aqueous humour is contained. 

 T :'vcfUinea. is of uniform thickness ; and, as will afterwards be proved, 

 in'rjemoi ving it, it is very difficult to cut, being of a dense, almost horny 

 oonfeistp fence. When its normal convexity is disturbed the cornea becomes 

 opa^qucia 



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

 withoutic vessels. Itsstructure comprises the following layers, which are 

 enuineri ^ ited in order from the anterior to the posterior surface : — 



1. Tl;eie Anterior Epithelium is a stratiiied, pavement epithelium, 

 continui ous at the margin of the cornea with the conjunctival epithelium. 



2. T'^^e Anterior Elastic Lamina, (Bowman's membrane). This is a 

 structi^i Lreless, elastic layer. It is extremely thin in the eye of the 

 lower £pf inimals, but is better developed in the human eye. 



3. T's he Substantia Propria. This, which forms the main thickness of 

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

 parallel'^ ^, to the surfaces of the cornea. Between adjacent lamellae there 

 is left a 4 network of spaces and branching canals, in which are found the 

 branche''-' d corneal corpuscles. 



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

 structui' -eless, elastic layer. 



5. TH"! le Posterior Epithelium is a single layer of polygonal cells. 



Diret <^ tiona. — A strong pin should now be passed through the optic 

 ,1 



nerve, 

 wide a 

 section'"* 



md used to fasten the eye beneath the surface of water in a 

 id shallow vessel, as already directed in the case of the frozen 

 While one hand steadies the eye beneath the water, an incision 

 is to br*^ ■ 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 

 aqueou> fS humour will escape into the water, and may possibly be recog- 

 nised '(by a slight inky discoloration, which is due to a postmortem 

 disinte'' ^gration of the pigmented epithelium lining the cavity in which 

 the hu*fmour is contained. Still keeping the eye under water, one blade 

 of-a pj3 jiir of small scissors should be introduced within the incision, and 

 the co'^ rnea should be excised immediately in front of its junction with 

 the sc^ferotic. The iris will by this means be exposed, and the next step 

 must *°P^ *° remove a portion of the sclerotic so as to expose the sub- 

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

 wards'^'] towards the optic nerve, snipping it bit by bit with the point of 



