336 be external Coats of the Eye. [Book IX. 



removal. Affections of the mind allb fomctimes occa- 

 fion an increafed flow of tears j the efficient caufe of 

 this connection we cannot trace; but the final caufe 

 feems to be to excite fympathy, and urge the unfeeling 

 heart to acts of mercy and benevolence. 



The ball of the eye is a cafe of a globular form. 

 It confifls of three coats, an external one called the 

 fcierotica, which is white and gliftening like the ten- 

 don of a mufcle; an intermediate one, abounding with 

 blood-veflels, called the choroides; and an internal 

 coat, called the retina, which is an extremely tender 

 film or network, formed by the expanfion of the 

 optic nerve. This defcription, however, applies only 

 to the poftcrior and lateral parts of the eye, for at the 

 lore part of the eye, inftead of the opake tunica 

 fclerotica, we obferve a projecting tranfparent circu- 

 lar part, continued from the fclerotica, which from its 

 fubftance being tranfparent like horn, is galled the 

 cornea. This portion is fomewhat more convex than 

 the - fclerotica, and reprefents the fegment of a fmall 

 fphere added to the fegment of a greater, or, to ex- 

 prefs the fame idea in more familiar language, it may 

 be .confidered as refembling a convex watch-glafs, 

 .fixed on the lefs convex furface of a watch cafe. 



The tunica choroides extends from the back part of 

 the' eye as far as the termination of t^e- fclerotica, 

 where it is firmly connected by means of a white ring 

 projecting inwards, and called the ciliary circle or liga- 

 ment. From this edge proceeds a very fine weblikc 

 membrane or curtain, called the iris. Its difference of 

 colour in different perfons is a matter of common ob- 

 fervation. In the middle of the iris is an. opening 

 which always appears black, and which is rendered nar- 

 rower or wider by the contractile powers of the iris. 

 This opening is called the pupil, through which the 



ravs 



