36 INNERVATION. [CHAP. XVII. 



The increasing density of the lens towards its centre is at- 

 tended with an increase of the refracting power, designed to aug- 

 ment the convergence of the central rays of the transmitted pencils 

 in their course through the lens, and thus to bring them to the same 

 focus with the circumferential rays. Sir D. Brewster states the 

 refracting power of the lens at its surface to be 1*3767, and at the 

 centre 1-3990. 



The lens, during its development, has a very copious distribution 

 of blood to the outer surface of its capsule, from two sources. The 

 central artery of the retina sends a vessel through the vitreous 

 humor to the centre of its posterior surface, which branches into a 

 radiating series of capillaries investing it as far as the border, where 

 they anastomose with vessels derived from the ciliary processes, 

 which proceed some way over the front of the capsule, and return 

 in loops. None of these vessels continue after the lens has attained 

 to maturity. 



The lens consists chiefly of albumen, and becomes hard and 

 opaque by boiling. The central parts evidently contain a smaller 

 proportion of water than the outer layers, which merely become 

 flocculent by the action of heat. The fibrous and lamellar structure 

 is more easily seen when thus rendered opaque, and it then separates 

 more easily along the triple or multiple planes already indicated. 

 Berzelius states the precise chemical constitution of the lens to be 

 as follows : 



Water . . . . . . . 58'0 



Albumen . . :., .' . . 35'9 



Alcoholic extract, with salts . . . 2'4 



Watery extract . , . 1-3 



Membrane . I . . . .2-4 



100-0 



The aqueous humor, as its name imports, is very nearly pure 

 water, containing, according to Berzelius, less than a fiftieth of its 

 weight of other matters, of which more than half is chloride of 

 sodium, and the rest extractive, soluble either in water or alcohol. 



It fills up the space between the cornea and lens a space divided 

 into two cavities by the membrana pupillaris in the fcetus, and still 

 partially divided by the iris into an anterior and posterior chamber, 

 continuous through the pupil. The anterior, though small, is much 

 larger than the posterior, and is bounded by the cornea in front, 

 the iris behind, and a portion of the ciliary ligament at its circum- 

 ference. The posterior is bounded by the iris in front, the lens and 



