222 ELEMENTS OT OENIXHOLOGT. 



Directly enclosing the vitreous humour is the hyaloid membrane, 

 which splits around the capsule enclosing the lens, leaving a 

 space termed the canal of Petit. The optic nerve passes from the 

 brain to the eyeball through the optic foramen, as already* 

 mentioned. This eyeball is moved in its socket by straight 

 and oblique muscles implanted into the outer surface of the 



Fig. 170. 



DiAGEAM OJ? A Vertical ANTBBO-POSTEaioR Section of the 

 Eye of a Bird (enlarged). 



Optic nerve. 2. Sclerotic. 3. Choroid. 4. Eetina. 5. Marsupium. 

 6. Cornea. 7, 7. Bony plates enclosed in sclerotic. 8. Corrugations 

 of choroid forming the ciliary processes. 9. Canal of Petit — formed by 

 a bifurcation of a most delicate membrane, the hyaloid membrane, 

 which immediately surrounds the vitreous humour, and which bifurcates 

 anteriorly and goes to the capsule of the lens. 10. Iris. 11. Ante- 

 rior chamber of the eye containing the aqueous humour. 12. Oapsuleof 

 the lens. 13. Lens. 14. Posterior chamber of the eye containing the 

 vitreous humour. 15. Ciliary muscle. 



sclerotic, and it is protected in front by an upper and lower 

 eyelid and also by a third eyelid or nictitating membrane f. 



Both Mammals and Eeptiles number amongst the species 

 which compose their class, forms which are naturally blind, but 



See ante, p. 182. 



t See ante, p. 142. 



