304 ANATOMY OF THE DOMESTIC FOWL 



the choroid coat bearing the ciliary processes, consist of numerous 

 folds. The ciliary muscles are arranged obliquely. Each consists 

 of three digitations. 



The iris is covered on the posterior side with pigment, the color 

 of which determines the color of the eye. The yellow pigment of 

 the iris has been said to be due to carotin and zanthophyll and the 

 black pigment to melanin. The enlarging and the contracting of 

 the pupil is brought about by muscles. The reduction of the 

 pupillary caliber is due to the sphincter pupillary muscle. It is 

 said by some anatomists that the muscles controlling the caliber of 

 the pupil in the bird furnish voluntary motion and that the capability 

 of accommodation of the eyes is greater in birds than in mammals. 



The retina contains no blood-vessels; otherwise the structure is 

 similar to that of mammals. The crystalline lens is flattened on the 

 corneal side and is convex posteriorly. The lens epithelium de- 

 velops into fibers in the parts close to the equator, and are almost 

 perpendicular to the eye axis. The corneal portion is relatively 

 small. 



The sclerotic coat is dense and white. It is divided into three 

 layers. It is thin and flexible, and somewhat elastic posteriorly. 

 It has an internal layer of hyaline cartilage. Anteriorly its form 

 is maintained by the circle of osseous plates mentioned above. 

 These plates, interposed between the exterior and the middle layer, 

 are located immediately behind the cornea. The scales are thin 

 and of oblong quadrate shape, being elongated from before 

 backward. 



The choroid coat is a membrane loosely cellular and highly 

 vascular. It is impregnated by a black pigment. Opposite the 

 bony circle the choroid separates into two layers. The external 

 layer is the thinner and adheres at first firmly to the sclerotic; it 

 passes forward to become continuous with the iris. The inner layer 

 is thicker than the external. The two layers are made up of radiat- 

 ing fibers which terminate anteriorly in the ciliary processes, the 

 ends of which are adherent to the capsule of the crystalline lens. 



The iris is delicate in structure. It is composed of a fine network 

 of interlacing fibers. 



The ciliary nerves and blood-vessels, run in the form of single 

 trunks between the choroid and the sclerotic, and terminate ante- 

 riorly in a ring-shaped plexus for the supply of the iris and the 

 muscular circle of the cornea. As stated before, the pupil in the 



