342 AQUEOUS HUMOR. 



are supposed to be a peculiar modification of fibro-cellular 

 tissue. 



The cornea elastica is posterior to the cornea proper, and 

 is characterized by the very singular fact that its trans- 

 parency is not in the least effected by any of the agencies 

 which destroy that of the true cornea. It^is a strong, 

 though thin, elastic cartilaginous layer, in close connec- 

 tion with the cornea. Its function is to support and pre- 

 serve the proper curvature of the cornea. 



The lining membrane of the anterior aqueous chamber 

 forms the fourth and last layer of the cornea. It is of such 

 extreme delicacy, that its existence is rather inferred than 

 proved by dissection. 



The cornea, in its healthy state, has no red vessels, 

 though when inflamed, is highly vascular, and though no 

 nerves have been satisfactorily traced into it, its surface is 

 quite sensible. Thus constituted, the cornea presents the 

 most perfectly transparent lens, and is the first of the 

 media through which the light has to pass before reaching 

 the retina. From its great convexity and density, it pow- 

 erfully refracts and converges the light in its passage 

 through it. 



Aqueous humor. This forms the second refracting me- 

 dium in order, and is immediately behind the cornea, oc- 

 cupying the cavity between this latter and the crystalline 

 lens. It consists of a perfectly transparent and colorless 

 fluid, secreted by the lining membrane of this cavity. It is 

 called aqueous from its resemblance to water, its specific 

 gravity differing very little from that liquid. Analysis 

 makes it to consist, in 100 parts, of water 98, and the re- 

 maining two parts, of chloride of sodium and albumen. 



The aqueous humor fills both the anterior and posterior 

 chambers of the eye, having free and easy communication 

 through the pupil. 



This humor being fluid, and consequently less dense 

 than the cornea, will not, according to the laws of the re- 

 fraction of light, so strongly converge the rays in their pas- 

 sage through itj but on the contrary, cause them rather to 



