REFRACTING MEDIA OF THE EYE. 



819 



KEFRACTINO MEDIA. 



Corpus Vitreum. The vitreous body is a transparent, jelly-like substance 

 situated between the crystalline lens and the retina, and occupying the posterior 

 four-fifths of the bulb of the eye (Fig. 677). In front it presents a deep concavity, 

 the hyaloid fossa (O.T. fossa patellaris), for the reception of the posterior convexity 

 of the lens. It is enclosed within a thin transparent membrane, the membrana 

 hyaloidea, which is in contact with the membrana limitans interna of the retina, 

 and is adherent to it at the entrance of the optic nerve. The portion of the 

 membrana hyaloidea in front of the ora serrata is thickened and strengthened 

 by radial fibres, and is termed the zonula ciliaris. Situated behind the ciliary body, 

 the zonula is radially folded and presents a series of alternating furrows and 

 elevations. The ciliary processes are received into, and are firmly adherent to, the 

 furrows, with the result that, if removed, some of their pigment remains attached 

 to the zonula. The elevations of the zonula are not attached to the interciliary 

 depressions, but are separated by a series of lymph spaces named the recessus 

 cameras posteriores ; these may be regarded as diverticula of the posterior chamber 

 with which they communicate. As the zonula approaches the equator of the 

 lens it splits into two chief layers, viz. : (a) a thin posterior lamina, which lines 

 the hyaloid fossa ; and (&) a thicker anterior layer, termed the suspensory ligament 

 of the lens (Fig. 677), which blends with the front of the lens capsule a short 

 distance from the equator of the lens. Scattered fibres of this ligament are also 

 attached to the equator itself and to the regions immediately anterior and posterior 

 to it. By this suspensory ligament the lens is retained in position, and its con- 

 vexity varies inversely with the degree of tension of the ligament. The radial 

 fibres of the ciliary muscle, by pulling forward the 

 ciliary processes and the attached zonula ciliaris, 

 relax the ligament, and thus allow the lens to 

 become more convex. Behind the suspensory 

 ligament, a sacculated lymph space surrounds the 

 equator of the lens; it is named the spatia 

 zonularia (O.T. canal of Petit), and may be easily 

 inflated on introducing a fine blow-pipe through 

 the suspensory ligament (Fig. 677). In the foetus 

 a blood-vessel, termed the arteria hyaloidea, is con- 

 tinued from the arteria centralis retinae forwards 



through the vitreous body, for the supply of the 



T , J ' ... . r f, J , ,, FIG. 693. THE SPATIA ZONULARIA DIS- 



capsule of the lens. Its position, m the adult, TENDED AND VIEWED FROM THE 



is represented by a lymph channel, termed the FRONT (enlarged). 

 canalis hyaloideus of Stilling (Fig. 677), the 



presence of which may be demonstrated by shaking up the vitreous body in a 

 solution of picrocarmine, when some of the pigment may be seen to extend along 



the canal (Anderson Stuart). 



When the vitreous body is treated by a 

 weak solution of chromic acid it presents a 

 series of concentric, peripherally arranged 

 strise, together with numerous radial striae 

 converging towards its centre. Between 

 these the more fluid part lies, and it frequently 

 contains vacuolated amoeboid cells scattered 

 through it. The vitreous body consists of 

 98*4 per cent of water, having in solution 

 about 1-4 per cent of sodium chloride and 

 traces of extractives and albumen. 



Lens Crystallina. The crystalline lens 



SHOWITSCONCENTRIC LAM1 lies in front of the vitreous body and behind 

 the iris, and is a biconvex, transparent body 

 (Fig. 677). It is enclosed in a thin, transparent, homogeneous capsule, the capsule 

 of the lens. The central points of its anterior and posterior surfaces are termed 



KO ~ 



Substantia 

 corticalis 



Nucleus 

 lentis 



FIG. 694. LENS HARDENED IN FORMALIN AND 



