446 STRUCTURE OF THE CORNEA. 



glass is received by the groove in its case. Its anterior surface is covered 

 by a thin tendinous layer, the tunica albuginea, derived from the expansion 

 of the tendons of the four recti muscles. By its posterior surface it gives 

 attachment to the two oblique muscles. The tunica albuginea is covered, 

 for a part of its extent, by the mucous membrane of the front of the eye, 

 the conjunctiva ; and, by reason of the brilliancy of its whiteness, gives 

 occasion to the common expression, "the white of the eye." 



At the entrance of the optic nerve, the sclerotic forms a thin cribriform 

 lamella (lamina cribrosa), which is pierced by a number of minute open- 

 ings for the passage of the nervous filaments. One of these openings, larger 

 than the rest, and situated in the centre of the lamella, is the porus opticus 

 through which the arteria centralis retinae enters the eyeball. 



Fig. 199* 



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 cir- 

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

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

 rotic, 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'te about one-sixteenth shorter than the trans- 

 verse, 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 lamella? connected together 

 by an extremely fine areolar tissue ; of the cornea elastica, a " fine, elastic, 

 and exquisitely transparent membrane, exactly applied to the inner surface 



* A longitudinal section of the globe of the eye. 1. The sclerotic, thicker behind than 

 in front. 2. The cornea, received within the anterior margin of the sclerotic, and con- 

 nected with it by means of a bevelled edge. 3. The choroid, connected anteriorly 

 with (4) the ciliary ligament, and (5) the ciliary processes. G. The iris. 7. The pupil. 

 8. The third layer of the eye, the retina, terminating anteriorly by an abrupt border at 

 the commencement of the ciliary processes. 9. The canal of Petit, which encircles the 

 icns (12) ; the thin layer in front of this canal is the zonula ciliaris, a prolongation of 

 the vascular layer of the retina to the lens. 10. The anterior chamber of the eye, con- 

 taining the aqueous humour: the lining membrane by which the humour is secreted is 

 represented in the diagram. 11. The posterior chamber. 12. The lens, more convex 

 behind than before, and enclosed in its proper capsule. 13. The vitreous humour en- 

 closed in the hyaloid membrane, and in cells formed in its interior by that membrane. 

 14. A tubular sheath of the hyaloid membrane, which serves for the passage of the artery 

 of the capsule of the lent. 15. The neurilemma of the optic nerve. 1C. The arteria 

 fientralie retinse, embedded in the centre of the optic nerve. 



