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MANUAL OF PHYSIOLOGY. 



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 forward. 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 forward 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 backward, because there it is in con- 



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 there- 

 fore tight, and compresses the anterior surface of the lens (A) so as to flatten it. On the 

 left the ciliary muscle (M) is contracting so as to relax the ligament, which allows the 

 lens to become" more convex. This contraction occurs when looking at near objects. 



tact with the vitreous humor, which, if anything, is held more 

 firmly against it by the increased tension of the hyaloid mem- 

 brane during the contraction of the ciliary muscle. 



The act of accommodation is a voluntary one, the nerve bear- 

 ing the impulse to the ciliary and iris muscles coming from the 

 third nerve by the ciliary branches of the lenticular ganglion. 

 The local application of the alkaloid of the belladonna plant 

 (atropin) causes paralysis of the ciliary muscle and wide dilata- 

 tion of the pupil ; and the alkaloid of the calabar bean (physos- 

 tigmatin) produces contraction of the muscle of accommodation 

 and extreme contraction of the pupil. 



