THE REFRACTING MEDIA OF THE EYE. 491 



30, and a posterior, 31 (Fig. 124). Chemically, the aque- 

 ous humor consists of water holding in solution a small 

 amount of solid matters, mainly common salt. 



The crystalline lens (28, 26, 27) is colorless, transparent, 

 and biconvex, with its anterior surface less curved than 

 the posterior. It is surrounded by a capsule, and tliefinner 

 edge of the iris lies in contact with it in front. In consist- 

 ence it is soft, but its central layers are rather more dense 

 than the outer. 



The vitreous humor is a soft jelly, enveloped in a thin 

 capsule, the hyaloid membrane. In front, this membrane 

 splits into two layers, one of which, 22, passes on to be 

 fixed to the lens a little in front of its edge. This layer is 

 known as the suspensory ligament of the lens; its line of at- 

 tachment around that organ is not straight but sinuous 

 as represented by the curved line between 28 and 26 in Fig 

 124. The space between the two layers into which the 

 hyaloid splits is the canal of Petit. The vitreous humor 

 consists mainly of water and contains some salts, a little 

 albumin, and some mucin. It is divided up, by delicate 

 membranes,, into compartments in which its more liquid 

 portions are imprisoned. 



The Ciliary Muscle. Running around the eyeball 

 where the cornea joins the sclerotic is a little vein called 

 the canal of Schlemm; it is seen in section at 8 in Fig. 124. 

 Lying on the inner side of this canal, just where the iris 

 and the ciliary processes meet, there is some plain muscular 

 tissue, imbedded mainly in the middle coat of the eyeball 

 and forming the ciliary muscle, which consists of a radial 

 and a circular portion. The radial part is much the larger, 

 and arises in front from the inner surface of the sclerotic; the 

 fibres pass back, spreading out as they go, and are inserted 

 into the front of the choroid opposite the ciliary processes. 

 The circular part of the muscle lies around the outer rim of 

 the iris. The contraction of the ciliary muscle tends to pull 

 forward (radial fibres) and press inward (circular fibres) the 

 front part of the choroid, to which the back part of the sus- 

 pensory ligament of the lens is closely attached. In this way 

 the tension exerted on the lens by its ligament is diminished. 



