232 ANATOMY FOE NUESES. [CHAP. XIX. 



is divided into two chambers by the crystalline lens and iris. 

 The " anterior chamber," the portion in front of the iris, is rilled 

 with a colourless, transparent watery fluid, the aqueous humour. 

 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 fibres 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 give perception of form and outline. 

 If the eye consisted only of a sensitive retina, impressions of 

 light could be received, but the form of objects would not be 

 distinguished. By focussing the rays emanating from an ex- 

 ternal object, the crystalline lens produces a distinct inverted 

 image of the object on the retina. 



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. 



