690 ORGANS OF SENSE. 



Fig. 373. The Crystal- inch in the transverse diameter, and about one-fourth in 

 line Lens hardened and the antero-posterior. It 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 laminee 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 towards the centre, the bases towards the cir- 

 cumference. The laminas consist of minute parallel 

 fibres, which are united to each other by means of wavy 

 margins, the convexities upon one fibre fitting accu- 

 rately into the concavities of the adjoining fibre. 

 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. 



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

 colorless, 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 correspond- 

 ing 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 

 whicn intervenes between the ciliary processes and the capsule of the lens, forms 

 part of the boundary of the posterior chamber of the eye. The posterior sur- 

 face of this layer is turned towards the hyaloid membrane, being separated from 

 it at the circumference 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. 



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

 arteries, and the arteria centralis retinas. 



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 to the ciliary processes. 



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

 and run forward, between that 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. 



The anterior ciliary arteries, five or six in number, are branches of the mus- 



