496 STRUCTURE OF THE CORNEA. 



The Cornea (corneus, horny) is the transparent projecting layer that 

 constitutes the anterior fifth of the globe of the eye. In its form it is 

 circular, concavo-convex, and resembles a watch-glass. It is received 

 by its edge, which is sharp and thin, within the bevelled border of the 

 sclerotic, to which it is very firmly attached, and it is somewhat 

 thicker than the anterior portion of that tunic. When examined from 

 the exterior, its vertical diameter is seen to be about one sixteenth 

 shorter than the transverse, in consequence of the overlapping above 

 and below, of the margin of the sclerotica ; on the interior, however, 

 its outline-is perfectly circular. 



The cornea is composed of four layers, namely, of the conjunctiva ; 

 of the cornea proper, which consists of several thin lamellae connected 

 together by an extremely fine areolar tissue ; of the cornea elastica, 

 a " fine, elastic, and exquisitely transparent membrane, exactly ap- 

 plied to the inner surface of the cornea proper ;" and of the lining 

 membrane of the anterior chamber of the eyeball. The cornea elastica 

 is remarkable for its perfect transparency, even when submitte6T~for 

 many days to the action of water or alcohol ; while the cornea proper 

 is rendered opaque by the same immersion. To expose this mem- 

 brane, Dr. Jacob suggests that the eye should be placed in water 

 for six or eight days, and then that all the opaque cornea should be 

 removed layer after layer. Another character of the cornea elastica is 

 its great elasticity, which causes it to roll up when divided or torn, 

 in the same manner as the capsule of the lens. The use of this layer, 

 according to Dr. Jacob, is to " preserve the requisite permanent correct 

 curvature of the flaccid cornea proper." 



The opacity of the cornea, produced by pressure on the globe, re- 

 sults from the infiltration of fluid into the areolar tissue connecting its 

 layers. This appearance cannot be produced in a sound living eye. 



Dissection. The sclerotic and cornea are now to be dissected away 

 from the second tunic ; this, with care, may be easily performed, the 

 only connections subsisting between them being at the circumference 

 of the iris, the entrance of the optic nerve, and the perforation of the 

 ciliary nerves and arteries. Pinch up a fold of the sclerotic near its 

 anterior circumference, and make a small opening into it, then raise 

 the edge of the tunic, and with a pair of fine scissors, having a probe 

 point, divide the entire circumference of the sclerotic, and cut it away 

 bit by bit. Then separate it from its attachment around the circum- 

 ference of the iris by a gentle pressure with the edge of the knife. 

 The dissection of the eye must be conducted under water. 



In the course of this dissection the ciliary nerves and long ciliary 

 /\ arteries will be seen passing forwards between the sclerotic and cho- 

 roid, to be distributed to the iris. 



SECOND TUNIC. The second tunic of the eyeball is formed by the 

 chqfpid, ciliary ligament and im, the ciliary processes being an ap- 

 pendage developed from its inner surface. 



The Choroid * is a vascular membrane of a rich chocolate- brown 



* The word choroid has been very much abused in anatomical language ; it 



