238 ANATOMY FOK NURSES. [CHAP. XIX. 



The "posterior chamber" is filled with a semi-fluid gelatinous 

 substance, the vitreous humour or body, so called from its glassy 

 and transparent appearance. Its refractive power, though 

 slightly greater than that of the aqueous humour, does not 

 differ much from that of water. It distends the greater part 

 of the sclerotic, supports the retina, which lies upon its surface, 

 and preserves the spheroidal shape of the eyeball. 



The crystalline lens is a transparent refractive body, with con- 

 vex anterior and posterior surfaces, placed directly behind the 

 pupil, where it is retained in position by the counterbalancing 

 pressure of the aqueous humour and vitreous body, and by its 

 own suspensory ligament. It is a fibrous body, composed of 

 long riband-shaped cells and enclosed in an elastic capsule. Its 

 refractive power is greater than that of the aqueous or vitreous 

 humour, and it acts by virtue of its double-convex form as a 

 converging lens, bringing parallel or diverging rays to a focus 

 on the posterior surface of the retina. The function of the 

 crystalline lens is to bring to a focus all the rays of light 

 emanating from each separate point in the object seen, so that 

 all the light from each point falls on and stimulates a corre- 

 sponding point on the retina. For if the eye consisted only of 

 a sensitive retina, impressions of light could be received, but the 

 form of objects would not be distinguished. 



The action of the lens in thus focussing the rays of light at 

 a particular point may be illustrated in the following manner : 

 If a sheet of white paper be held at a short distance from a 

 candle-flame, in a room with no other light, the whole of the 

 paper will be moderately and uniformly illuminated by the 

 diverging rays. But if a double-convex lens, with suitable cur- 

 vatures, be interposed between the paper and the light, the 

 outer portions of the paper will become darker, and its central 

 portion brighter, because a portion of the rays are diverted 

 from their original course and bent inward. By varying the 

 distance of the lens from the paper, a point will at last be found 

 where none of the light reaches the external parts of the sheet, 

 but all of it is concentrated upon a single spot ; and at this spot 

 will be seen a distinct image of the candle and its flame, i.e. 

 each point of the flame is now represented by a single point on 

 the paper ; and if for the paper we were to substitute the 

 retina, each point would stimulate one, and only one, small area 

 of the retina. 



