208 ELEMENTARY PHYSIOLOGY 



between these structures and the cornea in front. The crystalline 

 lens measures about ^rd of an inch in diameter, and consists of 

 layers, one within another, like the onion. If the lens be hardened 

 in alcohol, these layers may be easily separated from each other. 

 The crystalline lens is surrounded by a membrane called the 

 capsule, and is kept firmly in its place by a strong and elastic frame 

 called the suspensory ligament, which extends from the capsule 

 to the ciliary processes of the choroid. This ligament keeps the 

 crystalline lens in a state of tension. 



Muscular fibres radiate backwards from the junction of the 

 cornea and the sclerotic, and are attached to the outer surface of 

 the choroid coat. These fibres form the ciliary muscle. When 

 they contract they pull the choroid forward, causing the suspen- 

 sory ligament to relax. This in turn causes the crystalline lens, 

 which is also elastic, to shorten its diameter and become more 

 convex. 



The space between the cornea in front and the iris and lens 

 behind is filled with a watery fluid known as the aqueous humour, 

 and the whole of the globe of the eye behind the crystalline lens 

 is filled with a transparent semi-solid substance called the vitreous 

 humour (Lat. vitreus, glassy). 



We shall now perform a few experiments to illustrate the uses 

 of the various structures of the eye : 



Hold a convex lens so that it faces the sun, and place behind it a sheet ot 

 paper or a piece of ground glass to act as a screen. If we move the lens to 

 or from the screen, we shall notice that, when at a certain distance from the 



Fig. 193. 



latter, a very bright spot is produced by the meeting of the rays which pass 

 through it. Hence we learn that, when rays of light pass through a convex 

 lens, they converge to one point (\hefocus~) (fig. 193). 



Now take the lens into a room which is illuminated only by a single candle- 

 flame, and turn the lens to face this flame at the distance of a few feet, again 

 holding the screen behind. We now notice that, at a certain distance from 

 the lens, we get a sharp, distinct, and inverted image of the flame on the 

 screen. Since the image is inverted, we know that the rays from the candle 

 have crossed each other previous to the formation of the image (fig. 194). If 

 we bring the candle-flame nearer to the lens we shall observe that the distinct 

 image is formed further behind (fig. 195)* and vice versd. Thus, if we want a 

 permanent distinct image of a moving object, we must keep the screen in a 



