172 



THE EYE AND ITS APPENDAGES. 



The sclerotic coat^ is a dense, fibrous, white membrane, thinner 

 in front, where it receives the cornea, than it is behind, where it is 

 perforated by the optic nerve. The circular edge in contact with 

 the cornea is Ixivelled. This external surface is anteriorly covered 

 by the conjunctiva^ and jiosteriorly by fat. Its internal surface is 

 connected with the choroid coat. It is perforated by numerous 

 small foramina transmittini^ arteries and nerves. 



The cornea^ fills up the opening in the anterior part of the sclero- 

 tica, and forms about one-sixth of the whole eyeball ; it is more 

 convex than the sclerotica, and therefore forms a projection in front. 

 It is transparent and apparently homogeneous, but is in fact composed 

 of numerous laminii3, between which there is a slight quantity of 

 fluid. It is covered anteriorly by the conjunctiva, and posteriorly 

 by the epithelium of the aqueous humour. 



The canal of Schlemm is a venous sinus, near the border of the 

 cornea. 



Fig. 149. 



The choroid coal? is a thin, vascular tunic of the same extent as 

 the sclerotica, with which it is in contact externally. It is filled 

 with a black colouring matter called pig?7ientu7)i nigrimt, and con- 

 sists principally of arteries and veins. It is traversed by several 

 nerves. Its internal layer is composed principally of arteries, and 

 called memhrana RuyscJiiana ; externally it is composed principally 

 of veins, call(;d vasa vorticosa. 



The ciliary ligaincnt* is a flat, gray, delicate ring, at the junc- 

 tion of the cornea and sclerotica, which unites the external l)order 

 of the iris and the ciliary body. 



The ciliary body^ is formed by th(5 anterior portion of the 

 choroid coat, in contact with the ciliary ligament, and from it pro- 

 ceed seventy or eighty sliort folds, called ciliary processes, which 

 converge and are smaller behind than they are in front ; they sur- 

 round the lens. 



