CHAP. XVII.] THE CRYSTALLINE LENS. 33 



would act rather on the vitreous body around the lens, than on the 

 lens itself. Its existence may be easily shown by filling it with 

 mercury or air, through an artificial orifice in its anterior wall. 

 The injected fluid fills out the plai tings of the ciliary processes. 



The refracting index of the vitreous body is about 1'339, that of 

 water being 1*336; so that the difference between them is very 

 trifling, and may be referred in part to the transparent fibrous 

 tissue. Its chemical constitution, according to Berzelius, is as 

 follows : 



Water ..... 



Chloride of Sodium, with extractive matter 

 Albumen ..... 



100-00 



In early life, the vitreous body gives passage by a canal to a 

 branch of the central artery of the retina to the back of the 

 lens; and in large animals, though not in man, this appears to 

 supply some branches to the vitreous body itself even in the adult 

 state. 



The crystalline, as already mentioned, is a double convex lens; 

 but its surfaces are of unequal curvature, the posterior being the 

 more convex. In the adult the difference is nearly as 4 to 3, but it is 

 liable to some variety in different subjects. Chossat has observed that 

 the curvatures of the lens in the ox are ellipsoids of revolution round 

 the lesser axis, but whether they are so in man is not determined : 

 the subject is one very difficult Fig. 121. 



of investigation. The lens al- -, 



ters its shape with age; being ^. 



in the foetus more spherical, ffll (II 

 more flattened in childhood, 



anrl <afill rnnrp n in nrlvanpprl Human Lens: a. At birth. 6. At six years old. 

 more SO in advanced Adult A Hardened in spirit, and partially separated 

 life. In infancy it projects into into segments. After Scemmerring. 



the aqueous humor, so as to touch the iris; but in old age there 

 in a space intervening. The lens also varies in consistence with 

 age; being very soft at an early period, very firm in declining 

 years. At no epoch of life, however, is it of uniform consistence 

 throughout; being always denser and firmer from without inwards. 

 In the adult its diameter is from ^ to -| of an inch, and its antero- 

 posterior axis about ^ to ^ of an inch. It weighs from three to four 

 grains. 



The lens is divided into capsule and body. The manner in which 

 it is encased and fixed by the capsule in the vitreous body, has been 



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