DEFECTS OF ACCOMMODATION. 571 



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

 firmly against it by the increased tension of the hyaloid membrane 

 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. 



DEFECTS OF ACCOMMODATION. 



Myopia. It has been said that the "near limit" of distinct 

 vision differs in many persons from the twelve centimetres of the 

 normal emmetropic eye, and it is further found that the power of 

 accommodation varies very much in different individuals. Thus 

 in " short-sighted '' people, who have myopic eyes, i.e., in which dis- 

 tant parallel rays fall short of the retina, the near limit may only 

 be half the normal, i.e., five centimetres, and the far limit, which 

 is normally indefinite, is found to be within a comparatively short 

 distance of the eye. They, therefore, cannot see distant objects 

 clearly, since the rays are focussed before the retina is reached, 

 and then diverging, cause diffusion circles and a blurred picture. 

 The work of their accommodation is also much more laborious, 

 since they can only see in that part of the range of accommoda- 

 tion where the adjustment has to be altered for slight variations 

 of distance. The defect can be made much less distressing by 

 the use of concave glasses, which make parallel rays strike the 

 cornea as divergent ones, and thus allow them to be focussed on 

 the retina. 



Hypermetropia. Another abnormality is " long sight." In the 

 hypermetropic eye, parallel rays of light are brought to a focus 

 at a point beyond the retina, so that divergent or parallel rays 

 cause diffusion circles and a blurred image. This may be cor- 

 rected by means of convex glasses which make the rays conver- 



