440 STRUCTUKE OF THE CORNEA. 



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, 1, of the conjunctiva; 2, 

 of the cornea proper, which consists of several vthin lamellae con- 

 nected together by an extremely fine cellular tissue ; 3, of the cornea 

 elastica, a " fine, elastic, and exquisitely transparent membrane, 

 exactly applied to the inner surface of the cornea proper ;" and 4, 

 of the lining membrane of the anterior chamber of the eyeball. The 

 cornea elastica is remarkable for its perfect transparency, even 

 when submitted for many days to the action of water or alcohol ; 

 while the cornea proper is rendered perfectly opaque by the same 

 immersion. To expose this membrane, 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 with the cap- 

 sule 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, 

 results from the infiltration of fluid into the cellular tissue connect- 

 ing 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 per- 

 formed, the only connexions 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 circumference 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 

 choroid, to be distributed to the iris. 



2. Second tunic. The second tunic of the eyeball is formed by 

 the choroid, ciliary ligament and iris, the ciliary processes being an 

 appendage developed from its inner surface. 



The c/ioroid* is a vascular membrane of a rich chocolate-brown 



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

 originally applied to the membrane of the foetus called chorion from the Greek word 

 ^6/j/tv, domicilium, that membrane being, as it were, the abode or receptacle of the 

 foetus. Xep/ov comes from %upiui, to take or receive. Now it so happens that the cho- 

 rion in the ovum is a vascular membrane of a peculiar structure. Hence the term 



