BALL OF THE EYE. 409 



between the eyes of young and old persons, and which accounts 

 in part for the change which time effects in the power of vision. 

 The axis of the globe of the eye does not correspond with, the 

 axis of the orbit ; but is exactly parallel with that of the ball 

 of the opposite side, except in cases of obliquity or squinting. 

 The optic nerves running out in the direction of the axis of the 

 orbits, must therefore reach the balls of the eyes, on the inner 

 side of their axis. 



Fig. 188.* 



The sclerotic coat forms about five-sixths of the globe of the 

 eye. The cornea the remaining sixth. 



The sclerotica is lined by a thin and delicate membrane, the 

 Choroides, which is in close contact with it, but does not extend 



* 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 

 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, 

 containing the aqueous humor: the lining membrane by which the humor 

 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 humor enclosed in the hyaloid membrane, and in cells formed 

 in its interior by that membrane. 14. A tubular sheath of the hyaloid mem- 

 brane, which serves for the passage of the artery of the capsule of the lens. 

 15. The neurilema of the optic nerve. 16. The arteria centralis retinae, em- 

 bedded in its centre. 



VOL. ii. 35 



