664 SPECIAL SENSES 



posterior surface of the iris and the anterior surface of the lens are in 

 contact, except, perhaps, near their periphery or when the iris is much 

 dilated. The liquid filling the chambers of the eye is rapidly reproduced 

 after it has been evacuated, as occurs in many surgical operations. 



The aqueous humor is colorless and transparent, faintly alkaline, of 

 a specific gravity of about 1005, and with nearly the same index of 

 refraction as that of the cornea and the vitreous humor. It contains a 

 small quantity of an albuminous matter, but it is not rendered turbid by 

 heat or other agents that coagulate albumin. Various inorganic salts 

 (the chlorides, sulphates, phosphates and carbonates) exist in small pro- 

 portions in this liquid. It contains also traces of urea and glucose. 



The anterior and the posterior chambers of the eye are regarded as 

 lymph-spaces communicating with the lymphatics of the conjunctiva, 

 cornea, iris and ciliary processes. In addition a lymph-space is described 

 as existing between the choroid and the sclerotic. This space is sup- 

 posed to communicate with a perivascular-canal system around the vasa 

 vorticosa, and through these vessels, with the space between the capsule 

 of Tenon and the sclerotic (Schwalbe). The latter is connected with 

 lymph-channels that surround the optic nerve (Key and Retzius). 



Vitreous Humor. The vitreous humor is a clear glassy substance, 

 occupying about the posterior two-thirds of the globe. It is enveloped 

 in a delicate structureless capsule, called the hyaloid membrane, which 

 is about -Q-QQ-Q of an inch (4 //,) in thickness. This membrane adheres 

 rather strongly to the limitary membrane of the retina. In front, at the 

 ora serrata, the hyaloid membrane is thickened and becomes continuous 

 with the suspensory ligament of the lens. 



The vitreous humor itself is gelatinous, of feeble consistence and 

 slightly alkaline in its reaction, with a specific gravity of about 1005. 

 On section there oozes from it a watery and slightly mucilaginous liquid. 

 This humor is not affected by heat or alcohol but is coagulated by cer- 

 tain mineral salts, especially lead acetate. When thus solidified it pre- 

 sents regular layers, like the white of an egg boiled in its shell ; but 

 these are artificial. In the embryo the vitreous humor is divided into 

 a number of little cavities and contains cells and leucocytes. It is also 

 penetrated by a branch from the central artery of the retina, which 

 passes through its centre to ramify on the posterior surface of the lens. 

 This structure, however, is not found in the adult, the vitreous humor 

 being then without bloodvessels. The vitreous humor is divided into 

 compartments formed by delicate membranes radiating from the point 

 of penetration of the optic nerve to the anterior boundary where the 

 hyaloid membrane is in contact with the capsule of the lens. In this 

 way the humor is divided up, something like the half of an orange, 



