744 SIGHT. 



lens and its suspensory ligament. The anterior portion contains the aqueous 

 humor, the posterior contains the vitreous body. 



The crystalline lens measures about 7 mm. in transverse diameter, and 

 about 4 mm. antero-posterior diameter. It is a transparent biconvex body, 

 somewhat flattened anteriorly. It consists of a number of segments which 

 radiate from the centre, similar to the segments of an orange. These seg- 

 ments are composed of superimposed laminae of varying density. The most 

 superficial are soft and gelatinous ; the deeper are relatively hard, so that 

 they form a kernel or nucleus. The laminae are made up of parallel fibres, 

 with an undulating course, the convexities and concavities of the adjoining 

 fibres fitting accurately into each other. The lens is covered with a capsule 

 consisting of a transparent, elastic, fragile membrane, which has a tendency 

 to curl up, with its external surface innermost. 



The suspensory ligament of the lens is formed by a continuation of the 

 hyaloid membrane which lines the vitreous body. The hyaloid membrane is 

 a delicate transparent structure situated between the vitreous body and mem- 

 brana limitans interna of the retina. It is continued in front of the ora 

 serrata, where it divides into two layers. The posterior is attached to the 

 posterior portion of the capsule of the lens ; the anterior portion gradually 

 becomes thicker as it extends forward behind the ciliary processes, and is 

 attached to the anterior surface of the capsule. This thickened portion of 

 the membrane, which is corrugated where it has attached the ciliary pro- 

 cesses, is called the zone of Zinn. These two layers constitute the suspensory 

 ligament. Between them is a triangular canal, with its base corresponding 

 to the crystalline lens. This is called the canal of Petit. 



The vitreous body is contained within the cavity formed by the hyaloid 

 membrane and the posterior surface of the lens. It consists of a clear, color- 

 less, albuminous fluid, having an extremely delicate interlacement of fibres 

 extending in all directions through it. These fibres are not discernible in 

 the adult, but can readily be seen in the foetus. 



The aqueous humor is contained within the space formed by the posterior 

 surface of the cornea and the anterior surface of the lens. The space, which 

 is divided into two chambers by the iris, is filled with a clear, colorless, 

 limpid fluid containing saline and proteid substances in solution. This fluid 

 constitutes the aqueous humor. 



The anterior external portion of the eyeball, comprising the surface of the 

 cornea and about 6 or 8 mm. of the sclerotic coat, is covered by the conjunc- 

 tival mucous membrane.] 



DIOPTRIC MECHANISMS. 



The Formation of the Image. 



616. The eye is a camera, consisting of a series of surfaces and media 

 arranged in a dark chamber, the iris serving as a diaphragm ; and the object 

 of the apparatus is to form on the retina a distinct image of external objects. 

 That a distinct image is formed on the retina may be ascertained by removing 

 the sclerotic from the back of an eye, and looking at the hinder surface of 

 the transparent retina while rays of light proceeding from any external object 

 are allowed to fall on the cornea. 



617. A dioptric apparatus in its simplest form consists of two media 

 separated by a (spherical) surface ; and the optical properties of such an ap- 

 paratus depend upon (1) the curvature of the surface, (2) the relative refrac- 

 tive power of the media. The eye consists of several media, bounded by 

 surfaces which are approximately'spherical, but of different curvature. The 



