564 MANUAL OF PHYSIOLOGY. 



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 vessels 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 den- 

 sity of the two media and the angle at which the ray strikes the 

 surface 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 emitted by any luminous body become bent or refracted, 

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

 way as parallel rays of light from the sun may be focused on a 

 near object by means of an ordinary convex lens. 



Only so much light reaches the fundus of the eye as can pass 

 through the opening in the iris, so that a comparatively narrow 

 and varying beam is admitted to the chamber in which the 

 nerve endings are spread out for its reception. 



If we hold a biconvex lens at a certain distance from the eye 

 and look out of the window through it, we see an inverted image 



