668 DISSECTION OF THE EYE. 



and when these are washed away plaits (ciliary processes) come into view, 

 which are less prominent and longer than the processes of the choroid 

 coat, and do not quite reach the lens capsule internally. The two sets of 

 prominences are dovetailed together the projections of one being received 

 into hollows between the other. In this membrane arc; stiff longitudinal 

 and elastic fibres. The tenseness or laxness is influenced by the state of 

 the ciliary muscle ; for during its contraction the membrane is rendered 

 lax by the drawing forwards of the ciliary processes. 



Canal of Petit. Around the margin of the lens capsule is a small canal 

 (fig. 235, o) about one-tenth of an inch across, which is situate between 

 the suspensory ligament and the front of the hyaloid membrane. When 

 the canal has been opened, and filled with air by means of a blowpipe, it 

 is sacculated at regular intervals, like the large intestine, by the inflation 

 of the plaits of the anterior boundary. The margin of the capsule of the 

 lens projects into the space. 



LENS AND ITS CAPSULE. The lens is situate behind the pupil of the eye 

 (fig. 227, g), and brings to a focus on the retina the light passing through 

 that aperture. 



The capsitle is a firm and very elastic transparent case, which is per- 

 meable to fluid, and closely surrounds the lens : it is seated in a hollow on 

 the front of the vitreous body. The anterior part projects towards the 

 pupil ; whilst the posterior is received on the vitreous mass, to which it is 

 inseparably united. The circumference of the case corresponds with the 

 canal of Petit (o). 



Its anterior surface (fig. 227) is free, and touches the iris (/), but is 

 separated from it by a slight space at the outer part (posterior chamber, i) ; 

 it gives attachment towards the circumference to the suspensory liga- 

 ment (t). 



The fore part of the capsule is four or five times thicker than the poste- 

 rior, as far outwards as the attachment of the suspensory ligament, and 

 supports itself after the removal of the lens ; it is firm and quite trans- 

 parent, and remains clear for some time when immersed in spirit, acids, 

 and boiling water, like the elastic layers of the cornea. The posterior 

 part of the capsule is thin and membranous, and decreases in thickness 

 towards the centre : it is joined by the hyaloid membrane (n) of the vitre- 

 ous body. 



In the adult human eye the capsule of the lens is not provided with 

 bloodvessels ; but in the fuetus a branch of the central artery of the retina 

 supplies it (p. (MO). 



Dissection. The lens will be obtained by cutting across the thin mem- 

 branous capsule in which it is inclosed. 



The lens is a solid and transparent doubly convex body ; but the curves 

 are unequal on the two surfaces (fig. 227, #), the posterior being greater 

 than the anterior. Its margin is somewhat rounded ; and the measurement 

 from side to side is one-third of an inch, but from before back about one- 

 fourth of an inch. The density increases from the circumference to the 

 centre ; for whilst the superficial part may be rubbed off with the finger, 

 the deeper portion is firm, and is named the nucleus. 



On each surface an- three lines or stelhe diverging from the centre, and 

 reaching towards the margin ; they are the edges of planes or septa, and 

 are so situate that those on one side are intermediate; in position to those 

 on the other. In the human eye they are not distinctly seen, because they 

 bifurcate repeatedly as they extend outwards. 



