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MANUAL OF PHYSIOLOGY. 



ment 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 ob- 

 vious 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, 



FIG. 225. 



Diagram showing the changes in the lens during accommodation. The 

 muscle on the right is supposed to be passive as in looking at distant objects, 

 the ligament (L) is therefore tight, and compresses the anterior surface of 

 the lens (A) so as to flatten it. On the left the ciliary muscle (M) is con- 

 tracting so as to relax the ligament, which allows the lens to become more 

 convex. This contraction occurs when looking at near objects. 



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

 being its fixed point, when the ciliary muscle contracts it draws 

 the choroid coat and the connections of the suspensory ligament 

 of the lens slightly forwards. Under ordinary circumstances, the 

 eye being at rest, the suspensory ligament is tense and exerts a 

 radial, traction on the anterior part of the capsule of the lens, 

 and thus tends to stretch it flat ; this affects the shape of the soft 

 lens and reduces its convexity. When the ciliary muscle shortens 

 it draws forwards the attachment of the suspensory ligament, 

 relaxes it, and removes the tension of the capsule, so that the 

 unconstrained elastic lens bulges into its natural form. The pos- 

 terior surface cannot extend backwards, because there it is in con- 



