624 ORGANS OF THE SENSES. 



consists 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 laminaa are best demonstrated by 



Fig. 308. The Crystalline Lens, boiling, or immersion in alcohol. The same reagents 

 " d x ivide(L demonstrate that the lens consists of three triangular 

 segments, the sharp edges of which are directed towards 

 the centre, the bases towards the circumference. The 

 laminae consist of minute parallel fibres, which are 

 united to each other by means of wavy margins, the 

 convexities upon one fibre fitting accurately into the 

 concavities of the adjoining fibre. 



The changes produced in the lens ly age are the fol- 

 lowing : 



In the foetus, its form is nearly spherical, its color of 

 a slightly reddish tint, not perfectly transparent, and so 

 soft as to readily break down on the slightest pressure. 

 In the adult, the posterior surface is more convex than the anterior, it is color- 

 less, transparent, and firm in texture. 



In old age, it becomes flattened on both surfaces, slightly opaque, of an amber 

 tint, and increases in density. 



The suspensory ligament of the lens is a thin, transparent, membranous struc- 

 ture, placed between the vitreous body and the ciliary processes of the choroid : 

 it connects the anterior margin of the retina with the anterior surface of the lens, 

 near its circumference. It assists in retaining the lens in its position. Its outer 

 surface presents a number of folds or plaitings, in which the corresponding folds 

 of the ciliary processes are received. These plaitings are arranged round the 

 lens in a radiating form, and are stained by the pigment of the ciliary processes. 

 The suspensory ligament consists of two layers, which commence behind, at the 

 ora serrata. The external, a tough, milky, granular membrane, covers the inner 

 surface of the ciliary processes, and extends as far forwards as their anterior 

 free extremities. The inner, layer, an elastic transparent, fibro-membranous 

 structure, extends as far forwards as the anterior surface of the capsule of the 

 lens, near its circumference. That portion of this membrane which intervenes 

 between the ciliary processes and the capsule of the lens, forms part of the boun- 

 dary of the posterior chamber of the eye. The posterior surface of this layer is 

 turned towards the hyaloid membrane, being separated from it at the circum- 

 ference of the lens by a space called the canal of Petit. 



The canal of Petit is about one-tenth of an inch wide. It is bounded in front 

 by the suspensory ligament ; behind, by the hyaloid membrane, its base being 

 formed by the capsule of the lens. When inflated with air, it is sacculated at 

 intervals, owing to the foldings on its anterior surface. 



BLOODVESSELS AND NERVES OF THE EYE. 



The Vessels of the globe of the eye are the short, long, and anterior ciliary 

 arteries, and the arteria centralis retinae. 



The short ciliary arteries pierce the back part of the sclerotic, round the entrance 

 of the optic nerve, and divide into branches which run parallel with the axis of 

 the eyeball : they are distributed to the middle layer of the choroid, and ciliary 

 processes. 



The long ciliary arteries, two in number, pierce the back part of the sclerotic, 

 and run forward, between this membrane and the choroid, to the Ciliary muscle, 

 where they each divide into an upper and lower branch; these anastomose, 

 and form a vascular circle round the outer circumference of the iris ; from this 

 circle branches are given off which unite, near the margin of the pupil, in a 

 smaller vascular circle. These branches, in their course, supply the muscular 

 structure. 



