

MECHANISM OF ACCOMMODATION. 571 



at near objects we must constantly accommodate our eyes afresh, 

 in accordance with the shallowness of our focal depth. 



The mechanisms by means of which the accommodation of 

 the eye is accomplished, differ from anything that can be 

 applied to an artificial optical instrument, and are much more 

 perfect. 



The changes which have been observed to take place are: (1) 

 The iris contracts so that the pupil becomes smaller ; (2) the cen- 

 tre of the anterior surface of the crystalline lens moves slightly 

 forward, pushing before it the pupillary margin of the iris, and 

 becomes more convex ; (3) the posterior surface of the lens also 

 becomes more convex, but without changing its position. 



These changes can be seen in the accompanying diagram, show- 

 ing a section of the lens, cornea and ciliary region (Fig. 225), 

 in the left-hand side of which the lens is drawn in the position 

 it assumes when accommodated for near objects. These move- 

 ments can be seen to take place in life by observing the changes 

 in relative positions, etc., of the reflections of a candle flame 

 thrown from the cornea and the two surfaces of the lens. On 

 the cornea is seen a bright upright flame ; next comes a large 

 diffused reflection from the anterior surface of the lens, and at the 

 other side of this a small inverted image of the flame reflected 

 from the posterior surface of the lens When the adjustment is 

 changed by looking from a far to a near object, the image on the 

 front of the lens becomes smaller and moves toward the centre 

 of the pupil. The image on the back of the lens also becomes 

 smaller, but does not change its position. The exact amount of 

 movement has been accurately measured by a special instrument 

 called an ophthalmometer, and the motions can be made more 

 obvious by means of the phakoscope, in which a dark box and 

 prisms are placed before the observed eye, and each image is 

 made double, so that the change in position of its two parts may 

 be more obvious than a mere change of size. 



The alteration in the shape of the lens is accomplished by the 

 action of the muscular ring already named, which radiates from 

 the edge of the cornea to the ciliary region of the choroid coat, 

 where it is attached. The junction of the cornea and sclerotic 



