TUNICA ALBUGINEA. 495 



Anteriorly it presents a bevelled edge which receives the cornea in 

 the same way that a watch-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 conjunc- 

 tiva ; 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 opticus, through which the arteria centralis retinae 

 enters the eyeball. 



Fig. 153.* 



* 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 connected with it by means of a bevelled edge. 3- The cho- 

 roid, connected anteriorly with (4) the ciliary ligament, and (5) the ciliary pro- 

 cesses. 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. 1. 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, containing 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 retinse, embedded in 

 the centre of the optic nerve. 



