CRYSTALLINE AND VITKEOUS IIUMOUK. 439 



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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 openings 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 options , through which the arteria centralis 

 retinae enters the eye. 



Fig. 136* 



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

 that forms 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 



* 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, alid 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. 6. 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 lens 

 (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 contain- 

 ing 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 

 enclosed 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 lens. 15. The neurilemma of the optic nerve. 16. The 

 arteria centralis retinas, embedded in its centre. 



