840 THE ORGANS OF SPECIAL SENSE. 



the zonule of Zinn or zonula ciliaris. It presents a series of radially arranged 

 furrows, in which the ciliary processes are accommodated and to which they are 

 adherent, as evidenced by the fact that when removed some of their pigment 

 remains attached to the zonule. The zonule of Zinn splits into two layers, one 

 of which is thin and lines the fossa patellaris ; the other is named the suspensory 

 ligament of the lens ; it is thicker, and passes over the ciliary body to be attached 

 to the capsule of the lens a short distance in front of its equator. Scattered and 

 delicate fibres are also attached to the region of the equator itself. This ligament 

 retains the lens in position, and is relaxed by the contraction of the radial fibres 

 of the Ciliary muscle, so that the lens is allowed to become more convex. Behind 

 the suspensory ligament there is a sacculated canal, the canal of Petit, which 

 encircles the equator of the lens and which can be easily inflated through a fine 

 blow-pipe inserted through the suspensory ligament. 



In the foetus, the centre of the vitreous humor presents the canal of Stilling, 

 already referred to, which transmits a minute artery to the capsule of the lens. 

 In the adult, no vessels penetrate its substance ; so that its nutrition must be 

 carried on by the vessels of the retina and ciliary processes, situated upon its 

 exterior. 



III. Crystalline Lens. 



The crystalline lens, enclosed in its capsule, is situated immediately behind 

 the pupil, in front of the vitreous body, and encircled by the ciliary processes, 

 which slightly overlap its margin. 



The capsule of the lens is a transparent, highly elastic, and brittle membrane, 

 which closely surrounds the lens. It rests, behind, in the fossa patellaris in the 

 fore part of the vitreous body ; in front, it is in contact 

 with the free border of the iris, this latter receding from 

 it at the circumference, thus forming the posterior cham- 

 ber of the eye ; and it is retained in its position chiefly 

 by the suspensory ligament of the lens, already described. 

 The capsule is much thicker in front than behind, and 

 structureless in texture ; when ruptured, the edges roll 

 up with the outer surface innermost, like the elastic 

 lamina of the cornea. 



The anterior surface of the lens is covered by a single 

 layer of transparent, polygonal, nucleated cells. At the 

 circumference of the lens, these cells undergo a change in 

 form : they become elongated, and Babucin states that he 



can trace the gradual transition of the cells into proper lens-fibres, with which 

 they are directly continuous. There is no epithelium on the posterior surface. 

 In the foetus, a small branch from the arteria centralis retinse runs forward, 

 as already mentioned, through the vitreous humor to the posterior part of the 

 capsule of the lens, where its branches radiate and form a plexiform network, 

 which covers its surface, and they are continuous round the margin of the capsule 

 with the vessels of the pupillary membrane and with those of the iris. In the 

 adult no vessels enter its substance. 



The lens is a transparent, biconvex body, the convexity being greater on the 

 posterior than on the anterior surface. The central points of its anterior and 

 posterior surfaces are known as its anterior and posterior poles. It measures 

 from 9 to 10 mm. in the transverse diameter, and about 4 mm. in the antero- 

 posterior. It consists of concentric layers, of which the external in the fresh state 

 are soft and easily detached (substantia corticalis) ; those beneath are firmer, the 

 central ones forming a hardened nucleus (nucleus lentis). These laminae are best 

 demonstrated by boiling or immersion in alcohol, and consist of minute parallel 

 fibres, which are hexagonal prisms, the edges being dentated, and the dentations 

 fitting accurately into each other; their breadth is about 3-gVo" ^ an ^ ncn - 

 Faint lines radiate from the anterior and posterior poles to the circumference of 



