CRYSTALLINE LENS. 343 



diverge from the axis of vision. The light on leaving this 

 fluid falls on the next medium, L e. the crystalline lens, 

 (Fig. 98.) 



The lens is a beautiful double convex body, situated in 

 a depression in front of the vitreous humor, and behind 

 the pupil. It is more convex posteriorly than in front, and 

 has its axis on a line with that of the pupil. 



Its form varies: in the foetus it is more spherical; in the 

 adult the anterior and posterior convexities are greater, 

 while in old age these again diminish. This variation of 

 form is also seen in different individuals of the same age, 

 accounting for the different powers of vision in each. For 

 example, when the convexity of the lens is excessive, the 

 rays of light will be refracted too powerfully, and be 

 brought to a focus too soon, and in front of the retina, pro- 

 ducing that kind of defect of vision called myopia, or short 

 sightedness. The reverse of this occurs in old people, when 

 the lens is too flat, giving rise to that defect of sight called 

 presbyopia or far-sightedness. In this case the focus is 

 thrown behind the retina. The color of the lens varies as 

 much as its form. It has rather a pinkish tint in the 

 foetus, is perfectly clear in the adult, and of an amber color 

 in the old. Its density also varies, being much greater in 

 the centre than on the surface. The centre is firm and has 

 been compared to gum-arabic, and called the nucleus. 

 From this point to the surface, it becomes less and less 

 dense, so that the surface is semi-fluid, though every part 

 of the lens has greater density than either of the other hu- 

 mors. Berzelius gives the following analysis : water 58, 

 peculiar matter* 35.9, hydro-chlorates, lactates, and animal 



*The peculiar matter alluded to in Berzelius's analysis has since been shown 

 to be globulin, a substance which is also found in conjunction with faemoftn, for 

 the blood corpuscles. It is probably secreted by the lens from the blood fur- 

 nished it by the capsular artery, but how this substance is so perfectly separated 

 from the haematin, which every where else accompanies it, remains a pro- 

 found mystery. 



It exists not in the tissue of the lens, but is contained in the form of a very 

 concentrated solution in the cells of that organ. The object of it is, of course, 

 still further to correct chromatic aberration, by adding another highly refractive 



