100 THE STRUCTURE OF THE FOWL 



the interior of the eyeball from the region of 

 the entrance of the optic nerve. The pecten is 

 folded in a fan-like manner, and is pigmented 

 and vascular like the choroid, of which it may- 

 be regarded as an appendage. 



Continuous with the choroid is the iris, a 

 pigmented diaphragm pierced by a round 

 pupil. The yellow colour of the iris of the 

 fowl is apparently dependent upon the presence 

 of fat-globules contained within its cells. The 

 contraction and dilatation of the pupil is 

 brought about by the action of sphincter and 

 dilator muscle-fibres embedded in the substance 

 of the iris. 



Within the choroid is the thin retina, con- 

 sisting of nerve-cells and fibres directly or in- 

 directly continuous with the optic nerve and 

 arranged in layers as in the mammalian eye. 

 The crystalline lens is a colourless, transparent, 

 biconvex object with a posterior surface some- 

 what more convex than the anterior. The lens 

 is connected with the ciliary body, and, being 

 elastic, its convexity can be modified through 

 the agency of the ciliary muscle. 



The eyeball in front of the lens contains a 

 watery fluid ; while behind the lens the cavity 

 is occupied by a clear, jelly-like vitreous body. 



