782 SPECIAL SENSES. 



separate at a certain distance from the lens, one passing to the anterior and the other 

 to the posterior portion of the capsule, there remains a triangular canal, about -^ of an 

 inch wide, surrounding the border of the lens, called the canal of Petit. Under natural 

 conditions, the walls of this canal are nearly in apposition and it contains a very small 

 quantity of clear liquid. 



As we have already remarked in describing the retina, this membrane is closely con- 

 nected, at the ora serrata, by a mutual interlacement of fibres, with the suspensory liga- 

 ment. It is important to appreciate clearly the relations of the suspensory ligament, in 

 order to understand the mechanism of accommodation of the lens to vision at different 

 distances. The ciliary muscle being in repose, during what is termed the indolent 

 condition of the eye, when it is adapted to vision at long distances, the tension of the 

 parts flattens the lens ; but, in the effort of accommodation for near objects, the ciliary 

 muscle contracts, compresses the contents of the globe, relaxes the suspensory ligament, 

 and the inherent elasticity of the lens renders it more convex. It is by a delicate use of 

 this muscle, that the proper adaptation of the curvatures of the lens is obtained. 



The membrane forming the suspensory ligament is composed of pale longitudinal and 

 transverse fibres of rather a peculiar appearance, which are much less affected by acetic 

 acid than the ordinary fibres of connective tissue. 



Aqueous Humor. The space bounded in front by the cornea, posteriorly by the crys- 

 talline lens and the anterior face of its suspensory ligament, and, at its circumference, by 

 the tips of the ciliary processes, is known as the aqueous chamber. This contains a clear 

 liquid, called the aqueous humor. The iris separates this space into two divisions, which 

 communicate with each other through the pupil ; viz., the anterior chamber, situated be- 

 tween the anterior face of the iris and the cornea, and the posterior chamber, between the 

 posterior face of the iris and the crystalline. It is evident, from the position of the iris, 

 that the anterior chamber is much the larger ; and, indeed, the posterior surface of the 

 iris and the anterior surface of the lens are in contact, except, perhaps, near their periph- 

 ery or when the iris is very much dilated. The liquid filling the chambers of the eye is 

 said to be secreted by the blood-vessels of the ciliary processes; at all events, it is rapidly 

 reproduced after it has been evacuated, as occurs in many surgical operations upon the eye. 



The aqueous humor is colorless and transparent, faintly alkaline, of a specific gravity 

 of about 1005, and possesses the same index of refraction as the cornea and the vitreous 

 humor. It contains a small quantity of an albuminoid matter, but it is not rendered 

 turbid by heat or other agents which coagulate albumen. Various inorganic salts (the 

 chlorides, sulphates, phosphates, and carbonates) exist in this liquid, in small proportion. 

 It contains also traces of urea and glucose. 



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

 the posterior two-thirds of the globe. It is enveloped in an exceedingly delicate, struct- 

 ureless capsule, called the hyaloid membrane, which is about ^^ of an inch in thick- 

 ness. This membrane adheres pretty strongly to the limitary membrane of the retina. 

 In front, at the ora serrata, as we have already seen, the hyaloid membrane is thickened 

 and becomes continuous with the suspensory ligament of the lens. 



The vitreous humor itself is gelatinous, of feeble consistence, slightly alkaline in its 

 reaction, with a specific gravity of about 1005. Upon section, there oozes from it a 

 watery fluid of a slightly mucilaginous consistence. This humor is not affected by heat 

 or alcohol, but it is coagulated by certain mineral salts, especially the acetate of lead. 

 When thus solidified, it presents regular layers, like the white of an egg boiled in its 

 shell ; but these are artificial. In the embryon, the vitreous humor is divided into 

 many 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 upon 

 the posterior surface of the crystalline lens. This structure, however, is not found in the 



