222 ELEMENTS OF ORNITHOLOGY, 
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 

DiaGraM or A VerticAL ANTHRO-POSTERIOR SECTION OF THE 
Eyz or A Birp (enlarged). 
1. Optic nerve. 2. Sclerotic. 3. Choroid. 4. Retina, 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 bifureates 
anteriorly and goes to the capsule of the lens. 10. Iris, 11. Ante- 
rior chamber of the eye containing the aqueous humour. 12. Capsule of 
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 Reptiles number amongst the species 
which compose their class, forms which are naturally blind, but 
* See ante, p. 182. t See ante, p. 142. 
