AQUEOUS HUMOR. 445 



No blood-vessels are to be seen in any of the humors of 

 the eye, or their capsules, excepting those of the foetus. In 

 the fcetus a branch can be traced from the central artery of the 

 retina, through the vitreous humor, to the posterior surface of 

 the crystalline capsule, on which it ramifies. In similar sub- 

 jects vessels have been seen passing from the ciliary processes 

 to the anterior surface of the capsule ; and it is also asserted by 

 very respectable anatomists that they have seen vessels passing 

 from the capsule into the substance of the lens. 



The use of the crystalline lens is to concentrate the rays of 

 light, so as to form a distinct image at the bottom of the eye. 



The Aqueous Humor 



Occupies the space which is between the crystalline lens and 

 central extremities of the ciliary processes and the cornea. 

 This space is divided by the iris into two chambers, very dis- 

 similar in figure, which communicate with each other by means 

 of the pupil. The posterior chamber is much smaller than the 

 anterior, and its existence has been doubted ; but it is easily 

 proved by freezing the eye, when it is found filled with a part 

 of the aqueous humor in a state of congelation. 



The aqueous humor, in a natural state, is perfectly trans- 

 parent ; but in the fcetus, and for a short time after birth, it is 

 reddish and turbid. It consists of water impregnated with 

 albumen, gelatine, and muriate of soda ; and of course resem- 

 bles somewhat the fluid of the vitreous humor. 



It is probable that this fluid has also a capsule appropriated 

 to it ; for, after boiling an eye, a delicate membrane can be 

 found lining the internal surface of the cornea, and extending 

 from it over the anterior surface of the iris. It has not yet 

 been traced as far as the pupil, but it is probable that it 

 extends through the pupil, and lines the posterior chamber 

 also.* 



* This capsule was discovered by Messrs. Demours and Descemet, who dis- 

 puted each other's claim in 1767. It is thought to be useful in preventing the 

 cornea from being penetrated by the aqueous humors, and the pigmentum 

 nigrum from being washed off the iris, and making this humor turbid. M. J. 



VOL. II. 38 



