72 PHYSIOLOGY. 



terior surface backward, joined by a small quantity of cement sub- 

 stance. The outer fibers have oval nuclei, whilst in the center of 

 tho lens no nuclei are found. The capsule of the lens is thicker on 

 the anterior surface than on the posterior. It is a clear, refractive 

 membrane, nonvascular. Between the anterior surface of the lens 



Fig. 332. Transverse Section of Lens Fibers. (J. ARNOLD.) 



and the capsule there is a single layer of cubical, nucleated cells. 

 The radius of curvature of the anterior surface of the lens varies 

 with the accommodation for distant vision. It is about 10 milli- 

 meters to 6 millimeters in near point of distinct vision. 



Cataract. Normally the lens is transparent. When it becomes 

 opaque for any reason, then there results the condition known as 

 cataract. This condition is artificially produced in frogs by the 

 injection of grape-sugar. Cataract in diabetes is from the same 

 cause. 



Fig. 333. Anterior Surface of the Lens of an Adult. (J. ARNOLD.) 



THE EETINA. The retina contains the terminations of the 

 optic-nerve fibers. It ends at the pupillary border of the iris. The 

 optical part of the retina ends in the ora serrata, a zigzag line in the 

 vicinity of the ciliary body. 



Rods and Cones. Each rod consists of a rod and a rod-fiber, 

 the fiber containing the nucleus. The rods are cylindrical and 



