194 



GENERAL ZOOLOGY 



beneath which in turn lies the retina. The retina is composed 

 of an inner nervous layer and a thin pigmented layer in close 

 contact with the choroid. Neither the retina nor the choroid 

 layer extends in front beyond the line of junction of the cornea 

 and sclera. The lens is a doubly convex disk, composed of 

 transparent cells, enclosed in a thin tough capsule, held in place 

 by suspensory ligaments attached to the sclera along the line of 

 the forward edge of the choroid coat. The iris is a pigmented 

 muscular diaphragm with an opening, the pupil, in its center. 



Fig. 127. — Diagram of a vertical section of human eye and eyelids, a, eye- 

 lash; b, lid; c, conjunctiva, a thin membrane continuous with the lining of the 

 eyelid; d, cornea; e, anterior chamber filled with aqueous humor; /, iris; g, lens; 

 h, muscles to ligament suspending lens; i, suspensory ligament of lens; k, chamber 

 filled with vitreous humor; I, retina; m, blind spot; n, fovea centralis; o, optic 

 nerve; p, choroid coat; q, sclera; r, superior rectus muscle of eyeball; s, inferior 

 rectus muscle. 



It lies in front of the lens and is attached at its outer edge to the 

 ciliary process, which is a circle of plain musculature inserted in 

 the sclera between the corneosclerotic juncture and the choroid 

 layer. The lens divides the eye into an anterior region con- 

 taining a liquid, the aqueous humor, and a posterior region filled 

 with a jellylike vitreous humor. These two fluids keep the eye 

 distended. The iris divides the anterior region into anterior and 

 posterior chambers which are connected by the pupillary opening. 

 Retina. — The arrangement of the nervous elements of the 

 retina is highly complicated and only the barest outlines of their 

 histology will be considered. The light receptors of the retina 

 are the rods and cones, which are cells lying in the outer zone of 

 the sensory part of the retina and pointing toward the pigment 

 layer of the retina. Under certain conditions processes from the 



