CHAP, ii.] SIGHT. 499 



dation is lost. In the cases which have been recorded, where eyes 

 from which the lens had been removed seemed still to possess 

 some accommodation, we must suppose that no real accommodation 

 took place, but that the pupil contracted when a near object was 

 looked at, and so assisted in making vision more distinct. 



This increase of the convexity of the lens has been supposed 

 to be due to a compression of the circumference of the lens by a 

 contraction of the iris; but this is disproved by the fact that 

 accommodation may take place in eyes from which the iris is con- 

 genially absent. It has also been attributed to vaso-motor 

 changes, to increased fulness of the vessels of the iris or ciliary 

 processes, surrounding the lens ; but this also is disproved by the 

 fact that accommodation may be effected, after death in an eye 

 which is practically bloodless, by stimulating the ciliary ganglion 

 or ciliary nerves with an interrupted current or by other means. 

 The real nature of the mechanism seems to be as follows. 



The lens when examined after removal from the eye is found 

 to be a body of considerable elasticity. When the curvature of 

 the anterior surface of the lens is determined, as may be done by 

 appropriate means, in its natural position in the eye at rest, and 

 then again determined, after the lens has been removed from the 

 eye, the anterior surface is found to be more convex in the latter 

 than in the former case. There seems to be, in the eye in its 

 natural condition, some agency at work, keeping the anterior 

 surface of the lens somewhat flattened. The suspensory liga- 

 ment, attached to the choroid and ciliary processes behind, and 

 passing over the front of the lens, is just such a structure as 

 would produce this effect. In the natural position of the choroid 

 this ligament is tense, and tends to flatten the front of the lens. 

 When the choroid is pulled forward, the ligament becomes slack 

 and the lens bulges out forward. Further the ciliary muscle 

 attached on the one hand to a fairly fixed region, the junction of 

 the sclerotic and cornea, and on the other to the looser and more 

 moveable choroid, would naturally, when thrown into contraction, 

 pull forward the choroid and so slacken the suspensory ligament, 

 and hence permit the elastic lens to bulge out forwards. And we 

 have experimental evidence, carried out on lower animals, that 

 stimulation of the ciliary ganglion or of its so-called radix brevis, 

 does lead on the one hand to a contraction of the ciliary muscle 

 and pulling forward of the choroid, and on the other hand to an 

 increased curvature of the anterior surface of the lens. Hence we 

 may conclude that accommodation for near objects consists essen- 

 tially in a contraction of the ciliary muscle, which, by pulling 

 forward the choroid coat and the ciliary process, slackens the sus- 

 pensory ligament, and allows the lens to bulge forward by virtue 

 of its elasticity, and so to increase the convexity of its anterior 

 surface. 



Accommodation is in most cases a voluntary act ; since, however, 



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