STETJCTUKE OF THE EYEBALL 457 



is a second layer, called the choroid, composed of loose con- 

 nective tissue, and colored black by pigment particles 

 similar to those in the skin. At the front Avhere the cornea 

 joins the sclerotic this middle layer is thrown into plaits 

 (ciliary processes) beyond ^vhich it is continued as a 

 muscular curtain (the ins). This iris is composed of cir- 

 cular and radial unstriped muscle fibers, is colored by 

 pigment, and has a circular apertiu-e in the center (the 

 pupil). At its outer edge it is firmly united with the 

 sclerotic layer by a ligament called the ciliary ligament, 

 and to the ciliary process of the choroid. The contraction 

 of its circular fibers decreases the size of the pupil, while 

 the contraction of the radial fibers reverses this action. 

 By means of this variation in the pupil, the amount of 

 light which enters the eye is kept under control. 



The third layer of the eye is an extension of the optic 

 nerve called the retina. It consists of the endings of the 

 optic nerve fibers and a supporting layer of connective 

 tissue. The entire layer is very thin and lines the whole 

 interior of the eyebaU with the exception of the front, 

 where it stops at the edge of the ciliary process. This 

 retina is the sensitive layer of the eye and the one in which 

 the visual impulses originate. 



Just behind the iris is a biconvex, transparent body, 

 called the crystalline lens. The front surface of this lens 

 is slightly flatter than the back and is in contact with the 

 inner surface of the iris. Between it and the cornea is a 

 space which is filled with transparent watery liquid called 

 the aqueous humor. The entire cavity of the eyeball back 

 of this lens is .filled \\-ith a glassy serai-solid mass called the 

 vitreous humor. The lens itself is surrounded by a thin 

 membranous capsule, and is kept in place by a circular 



