MEMBRANES OF THE EYE. 597 



The whole labyrinth (internal ear), consisting of the cochlea and 

 semicircular canals, laid open, is shown by Fig. 424. Its relations 

 to the external and middle ear are seen in Fig. 425. 







Y. THE EYE. 



Some of the structural elements of the eye have already been 

 spoken of (pp. 449-53) ; and the rest will be described here. 



I. Membranes of the Eye. 



A section of the eyeball is shown by Fig. 426. The three mem- 

 branes of its posterior seven-eighths, or more, are the tunica scle- 

 rotica, the choroid, and the retina; while the cornea is seen project- 

 ing in front, and the iris is represented by 6, in the figure. 



1. The sclerotic coat is composed of white fibrous tissue and a 

 few elastic fibres (p. 279); and is shown, together with the choroid, 

 by Fig. 427. 



2. The choroid coat is continuous in front with the iris; a narrow 

 ring of white fibrous tissue the ciliary ligament connecting them 



Fig. 426. 



A longitudinal section of the globe of the eye. 1. The sclerotic, thicker behind than in front. 2. 

 The cornea apparently received within the anterior margin of the sclerotic, and connected with it by 

 means of a bevelled edge, though really continuous with it. 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 (ora ^errata) at the commencement of the 

 ciliary processes. 9. The canal pf Petit, which encircles the lens (12) ; the thin layer in front of 

 this canal is the zonula ciliaris, a vascular prolongation 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. 11. Posterior chamber. 12. The lens more convex behind than before, and 

 inclosed in its proper capsule. 13. The vitreous humor inclosed in the hyaloid membrane, and its 

 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. Perineurium of the optic 

 nerve. 16. The arteria centralis retina;, imbedded in its centre. 



