The iris is elliptical in shape like the cornea; it is a thin, 

 perforated, contractile curtain, suspended behind the cornea 

 in the aqueous humor, in front of the crystalline lens, form- 

 ing the anterior portion of the middle coat of the eyeball; 

 it is composed of radiating and circular muscular fibres and 

 a fibrous frame work. Its anterior surface is covered by a 

 layer of polyhedral cells on a fine basement membrane; its 

 posterior face is opposite the lens and ciliary processes, and 

 is covered by a thick layer of pigment called the uvea; loose 

 predunculated portions of this pigmented layer may project 

 through the pupillary aperture ; they most frequently project 

 from the superior border of the pupil and extend into the an- 

 terior part of the aqueous chamber, where they are known as 

 "soot balls" or corpora nigra. These black, spongy masses 

 may obstruct the passing of the light into .the eye, but if 

 they are small, little harm is done by them. These "soot 

 balls" are brovvuish black aud arelargar wheu along the upper 

 border of the pupil than when at the lower border. 



The color of the eye depends upon the quantity of pigment 

 in the uveal layer of the iris. In man, the color of the iris 

 varies with the different individuals; while in the horse it is 

 generally of a brownish yellow hue; sometimes, however, it 

 is nearly white or bright gray— the animal possessing such 

 eyes is said to hd '^vall eye 1." The aperture or elliptical 

 central opening in the iris forms the pupil, which is expanded 

 Of enlarged when the radial muscular fibres of the iris con- 

 tract ; and it is contracted or decreased in size when the cir- 

 cular muscular fibres contract. Strong sunlight produces 

 contraction of the pupil ; while weak light or darkness causes 

 the pupil to expand. 



The internal coat of the eyeball or shell is the retina. It 

 lies on the inner surface of the choroid to which it loosely 

 adheres. 



This most essential, delicate, grayish, transparent, nervous 

 membrane is thicker behind than in front, aud extends as far 



