CHAP, in.] SIGHT. 29 



being so placed in the several lamellae as to give rise to a star- 

 shaped figure on the back of the lens. Most of the fibres still 

 retain an elongated nucleus indicative of their epithelial nature 

 but in many this is lost ; in the adult as in the embryonic lens the 

 transition from the characteristic fibres of the body of the lens 

 to the cubical cells of the anterior layer may be observed in 

 sections. 



The lens developes around itself a cuticular membrane, struc- 

 tureless and elastic, called the capsule of the lens. This is much 

 better developed in front than behind, where it is said to be 

 partly wanting. It furnishes a distinct envelope for the lens, and 

 when it is ruptured the lens may be turned out leaving the cavity 

 of the capsule vacant. 



The lens thus constituted is a transparent body, of a certain 

 refractive power ( 705), possessing considerable elasticity ; its shape 

 may be altered by pressure, but when the pressure is removed it 

 regains its natural form. 



The chemical nature of the lens is peculiar. The proteids 

 which seem to form about 30 p.c. of the dry solids are of a globulin 

 nature, being apparently nearly allied to the vitellin found in 

 yolk of eggs and elsewhere ; albumin seems to be absent. Even in 

 healthy lenses a certain variable amount of cholesterin is present, 

 and in lenses which have become opaque, forming cataracts, 

 especially in what are called soft cataracts, this substance occurs 

 in considerable quantity ; it may amount to 5 p.c. of the dry 

 solids. 



720. The Vitreous Humour and Suspensory Ligament. As we 

 have said ( 703), the mesoblast which is carried into the retinal cup 

 at the involution of the lens is at first developed into the vascular 

 pupillary and capsulo-pupillary membranes, which are supplied 

 with blood by a continuation forwards of the central artery of 

 the retina (Fig. 134). In the adult eye these membranes have 

 been wholly absorbed, and the continuation of the central artery 

 obliterated, so that all that remains of the mesoblast filling up the 

 retinal cup is the jelly-like material known as the vitreous humour. 

 This consists of little more than water, containing in solution, like 

 the aqueous humour, about -1 p.c. of proteids, namely, serum- 

 albumin and globulin, as well as organic and inorganic salts ; 

 it seems chemically to resemble aqueous humour in spite of its 

 different origin. A few scattered branched cells as well as wander- 

 ing leucocytes are found in it. 



Where it is in contact with the retina, the vitreous humour is 

 defined by a structureless membrane, the hyaloid membrane, which 

 is adherent normally to the overlying retina. This hyaloid mem- 

 brane is continued forward beyond the ora serrata, and for some 

 little distance is adherent to the pars ciliaris retinas. A little 

 farther forward, however, it leaves the ciliary processes and stretch- 

 ing inward as an independent, structureless, or faintly fibrillated, 



