904 THE ORGANS OF SENSE. 



The Crystalline Lens and its Capsule. 



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

 the pupil, in front of the vitreous body, and surrounded 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 a depression 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 chamber 

 of the eye ; and it is retained in its position chiefly by the suspensory ligament of 

 the lens. The capsule is much thicker in front than behind, structureless in text- 

 ure, and 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 connected to 

 the inner surface of the capsule by a single layer of transparent, polygonal, nucle- 

 ated 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 

 retinae 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 

 FIG 536. The crystalline network which covers its surface, and they are continu- 

 &ar h g a ed d ) ned and *** ous 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, double-convex body, the convexity being greater on 

 the posterior than on the anterior surface. It measures about a third of an inch 

 in the transverse diameter, and about one-fourth in the antero-posterior. It con- 

 sists of concentric layers, of which the external in the fresh state are soft and 

 easily detached ; those beneath are firmer, the central ones forming a hardened 

 nucleus. These laminae are best demonstrated by boiling, or immersion in alcohol. 

 The same reagents demonstrate that the lens consists of three triangular segments, 

 the sharp edges of which are directed toward the centre, the bases toward the 

 circumference. The laminae 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 g-^oT^b- of an inch. They run from the sutures or 

 lines of junction of the triangular segments on the one surface to the periphery of 

 the lens, and, curving round its margin, they terminate at the line of junction of 

 the segments on the other. No fibres pass from pole to pole, but they are 

 arranged in such a way that fibres which commence near the pole on the one 

 aspect of the lens that is to say, near the apex of the triangular segment 

 terminate near the peripheral extremity of the plane on the other, or near the base 

 of the triangular segment, and vice versa. The fibres of the outer layers of the 

 lens each contain a nucleus, which together form a layer (nuclear layer) on the 

 surface of the lens, most distinct toward its circumference. The meridians, or 

 lines of junction of the three segments, are composed of an amorphous granular 

 substance which sometimes becomes opaque, when the lines are seen forming a 

 distinct star on the lens. The lines on one surface do not lie immediately opposite 

 those on the other, but are intermediate. 



The changes produced in the lens by age are the following : 

 In the foetus its form is nearly spherical, its color of a slightly reddish tint, it 

 is not perfectly transparent, and is so soft as to break down readily on the slightest 

 pressure. 



