516 THE HUMAN BODY. 



The aqueous humor fills the space between the front of 

 the lens, 28, and the back of the cornea. This space is in- 

 completely divided by the iris into an anterior chamber, 30, 

 and a posterior, 31 (Fig. 142). Chemically, the aqueous 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 the inner edge 

 of the iris lies in contact with it in front. In consistence 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 attachment 

 around that organ is not straight but sinuous as represented 

 by the curved line between 28 and 26 in Fig. 142. 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 lymph-vessel called the 

 canal of Schlemm; it is seen in section at 8 in Fig. 142. 

 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 arid form- 

 ing the ciliary muscle, which consists of a radial and a 

 circular portion (Fig. 149). The radial part is much the 

 larger, and arises in front from the inner surface of the scler- 

 otic; 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 (circu- 

 lar fibres) the front part of the choroid, to which the back 

 part of the suspensory ligament of the lens is closely at- 

 tached. When this occurs the tension exerted on the margin 

 of the lens by its ligament is diminished. 



The Properties of Light. Before proceeding to the 



