562 MANUAL OF PHYSIOLOGY. 



wards of the epithelium, which meets the optic cup, and after a 

 time is cut off from the parent tissue. The stages of its devel- 

 opment may be followed in the accompanying woodcuts (v. Fig. 

 218). 



The lens is composed of a number of peculiar band-like cells, 

 derived from the epithelium. These are cemented together in 

 parallel rows, eccentrically arranged in layers. These bands are 

 hexagonal in transverse section, and in the younger periods of 

 life may be seen to contain nuclei. 



In the living state the lens is perfectly transparent, but after 

 death it becomes slightly opaque. The nutriment for the adult 

 lens is derived from the yessels of the choroid, which, however, 

 do not come into direct communication with its texture. On 

 this account the nutrition of the lens is not so perfect as that 

 of many other tissues, and it is but imperfectly repaired after 

 injury, which always leaves more or less opacity. Even without 

 injury, opacity, giving rise to cataract, sometimes occurs during 

 life. 



Chemically the lens is made up of globulin, and furnishes a 

 ready source for obtaining this form of albumin for examination. 



THE DIOPTRICS OF THE EYE. 



Light travels through any even transparent body, such as the 

 atmosphere, in a straight line. But when it meets any change in 

 density, particularly when it has to pass obliquely into a denser 

 medium, the ray is bent so as to run in a direction more perpen- 

 dicular to the surface of the denser body. The degree of bending 

 or refraction of the rays depends chiefly on the difference in density 

 of the two media and the angle at which the ray strikes the sur- 

 face of the more dense. 



On its way to the sensitive retina, the light has to pass through 

 the various transparent media just named, viz., the cornea, the 

 aqueous humor, the crystalline lens, and the vitreous humor. On 

 entering these media, which have different densities, the rays of 

 light reflected from any luminous body become bent or refracted, 

 so that they are brought to a focus on the retina, just in the same 



