THE NEETOUS SYSTEM AND OEGAlfS Or SENSE. 221 



the cornea. Within this are two unequallyrsized chambers 

 separated by a vertical curtain, the iris, with a central aperture, 

 and by a dense transparent body enclosed in a very delicate mem- 

 brane or capsule. This dense, transparent body is the crystalline 

 Z«ns, which iills the aperture — the pupil — left in the middle of the 

 iris. Around the lens are small processes termed ciliary processes. 

 Bound the iris is a fibrous band called the ciliary liffamentjWlaih 



Fig. 169. 



Side View of Membranous Labykintii of Pigeon : greatly enlarged. 



ca, cp,. and ce, anterior, posterior, and external Semicircular canals ; 

 CO, cochlea. 



on the outer surface of the choroid is a similar band of contractile 

 fibres — the ciliary muscle. The chamber in front of the lens is 

 filled with the aqueous humour, while the vitredvs Tiumour fills 

 that behind it. Lining this posterior chamber is the essential 

 organ of sight, the expanded optic nerve or retina. External to 

 this is a dark, highly vascular membrane termed the choroid. 



