REFRACTING MEDIA OF THE EYEBALL. 695 
an outer, in the inner nuclear layer. The inner network arises directly from the arteries 
and sends numerous small branches to the outer network, from w hich the veins take 
origin. The vessels do not penetrate deeper than the inner granular layer, nor do the 
arteries anastomose, except through the capillary plexuses. The veins follow the course 
of the arteries ; they have no muscular coats, but consist merely of a layer of endothelial 
cells, outside which is a perivascular lymphatic sheath, surrounded by delicate retiform 
tissue, 
REFRACTING MEDIA OF THE EYEBALL. 
The vitreous body (corpus vitreum) is a transparent jelly-like substance situated 
between the crystalline lens and the retina, and occupying the posterior four-fifths of 
the globe (Fig. 493). In front it presents a deep concavity, the fossa patellaris, for the 
reception of the posterior convexity of the lens. It is enclosed within a thin trans- 
parent membrane, the membrana hyaloidea, which is in contact with the membrana 
limitans interna of the retina and adherent to it at the optic entrance. The portion 
of the membrana hyaloidea in front of the ora serrata is thickened and strengthened 
by radial fibres, and is termed the zonule of Zinn, or 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 (recessus 
camerze posterioris); 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 covers that 
portion of the membrana hyaloidea which lines the fossa patellaris ; and (6) a thicker 
anterior layer, termed the suspensory ligament of the lens (Fig. 495), which blends 
with the front of the lens capsule a short distance from its equator. Scattered 
fibres of this hgament are also attached to the equator itself and to the regions 
unmediately anterior and posterior to it. By this suspensory ligament the lens is 
retained in position and its convexity 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 zonule of Zinn, 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 canal of Petit, and 
may easily be inflated on introducing a fine blow pipe through the suspensory liga- 
ment (Fig. 503). The hyaloid membrane of the 
vitreous body is not only adherent to the ciliary pro- 
cesses, but also at the optic entrance. ‘This is due 
to the fact that in the foetus a blood-vessel, termed 
the arteria hyaloidea, is continued from the arteria 
centralis retinze forwards through the vitreous body 
for the supply of the capsule of the lens. Its 
position, in the adult, is represented by a lymph 
channel, termed the canalis hyaloideus of Stilling 
(Fig. 493), the presence of which may be demon- 
strated 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 strie, 
together with numerous radial strive 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 984 per cent. of water, having in solu- 
tion about 1:4 per cent. of sodium chloride and traces of extractives and albumen. 
The crystalline lens (lens crystallina) lies in front of the vitreous body and 
behind the iris, and is a biconvex, transparent body (Fig. 493). It is enclosed in 
a thin, transparent, homogeneous capsule, the capsule of the lens (capsula lentis). 
The central points of its anterior and posterior surfaces are termed respectively its 
44 a 
_ Zonue 
of Zinw 
Canal 
of Petit 
Lens 
Fic. 503.—CanabL OF Prvit DISTENDED 
AND VIEWED FROM TH! FRONT (enlarged). 
