372 



THE ANIMALS AND MAN 



the eye. It also contains large quantities of black pigment, 

 so that no light can enter the eyeball except through the 

 cornea. In the front of the eye, where it meets the cor- 

 nea, the choroid coat leaves the sclerotic and projects 

 toward the long axis of the eye, as a circular muscular 

 curtain with an opening in the middle. This is the iris (I) , 

 and is the part of the eye that we see as hazel, gray, or blue. 

 The pupil is the opening in the iris. 



Where the choroid 

 coat leaves the sclerotic 

 coat it is firmly joined 

 to it by small but strong 

 and important muscle 

 processes called ciliary 

 processes (Cp), which, 

 with the suspensory lig- 

 ament (SL) hold the lens 

 in place. 



Within the choroid 

 coat is the third and 

 innermost coat, called 



FIG. 185. Horizontal section of the eye- , . ~, 



ball. S, sclerotie coat; ch, choroid coat; the retina ( R ) he 



R, retina; O, optic nerve; Cp, ciliary retina is Composed of 



processes; Cm, ciliary muscles; L, lens; sensor y nerve endings 



C, cornea; I, iris; S. L, suspensory . . 



ligament. (After Brubaker.) that are Sensitive to 



light. The fibers from 



these nerve cells pass backward to the brain through the 

 optic nerve (O). 



Behind the iris lies the crystalline lens (fig. 185, L). Its 

 front surface is slightly flattened. The space between 

 the lens and the cornea is filled with a clear transparent 

 liquid, the aqueous humor. The cavity of the eyeball back 

 of the lens is also filled with a transparent semi-solid or 

 jelly-like substance called the vitreous humor. 



Functions of the eye. In general structure, the human 



