DISSECTION OF THE EYEBALL. 273 



imbedded in its anterior part. By a combined examination of all the 

 preparations, the following points regarding the lens and vitreous body 

 may be made out. 



The Lens is situated behind the pupil, and is contained within a 

 capsule of its own. 



The Capsule is a close-fitting, firm, transparent membrane, which is 

 four or five times thicker on the front than on the back of the lens. 

 The anterior surface of the capsule forms the posterior boundary of the 

 cavity in which the aqueous humour is contained, and the iris in its 

 movements glides on it. At its periphery the suspensory ligament of 

 the lens blends with it. The posterior surface is in contact with the 

 vitreous humour. 



The lens is a transparent solid body of a biconvex shape, the con- 

 vexity of its posterior surface being considerably greater than that 

 of the anterior. It is maintained in a depression on the front of the 

 vitreous humour by a suspensory ligament. This ligament, which is also 

 known as the zonula of Zinn, arises behind and beneath the ciliary 

 processes, where it is connected with the hyaloid membrane of the 

 vitreous humour. It passes over the rim of the lens, and blends with 

 the anterior part of the lens-capsule. Behind the rim of the lens the 

 ciliary processes rest on the outer surface of the ligament ; and when 

 these are removed the ligament is there seen to have a fluted or 

 plaited appearance, each plait fitting into the depression between two 

 processes. At this same point the inner surface of the zonula forms 

 the outer boundary of a triangular chink which runs round the lens 

 behind its rim. This is the canal of Petit, which is bounded in front 

 by the lens-capsule, behind by the hyaloid membrane of the vitreous 

 humour, and outwardly by the zonula. 



Structure. — When removed from its capsule, the lens is found to be 

 soft and pulpy in its outer portion, but its density increases in passing 

 from the surface to the centre. Both its surfaces show some faint 

 white lines radiating from the central point of the surface. The 

 number of these lines varies in the adult, but in the foetus they 

 are three in number, and each line on the posterior surface is in 

 position midway between two of the anterior lines. 



A lens that has been hardened in spirit or by boiling may be broken 

 down into concentric laminae like the coats of an onion. Each of these 

 laminae, is composed of long riband-shaped fibres. These lens-fibres, 

 when examined microscopically, are seen to have finely serrated edges 

 by which adjacent fibres are interlocked. 



The foetal lens is nearly spherical, of a reddish colour, and not 

 quite transparent. In the young adult it is distinctly biconvex, firm, 

 colourless, and transparent. With advancing age it tends to become 

 flatter, denser, less transparent, and of a yellowish colour. 



