442 SPECIAL PHYSIOLOGY. 



admitted to be the essential change in the accommodation of the eye 

 for near vision, is attributed by Helmholz, the originator of these re- 

 searches, to joint muscular and elastic action ; whilst the restoration 

 of the lens to its shape when at rest, is supposed to be due to elasticity 

 alone. According to him, the lens, when at rest within the eyeball, 

 is subjected to the tension of an elastic zone connected with its mar- 

 gin, which maintains its anterior surface somewhat more flattened 

 than it would be if not so acted upon ; the elastic zone here spoken 

 of, seems to coincide with the so-called highly elastic suspensory liga- 

 ment of the lens. When, however, the eye is turned to a near object, 

 the ciliary muscle is supposed to contract, to draw forward the choroid 

 coat, and, with it, the hinder margin of the elastic zone, which is thus 

 relaxed, and so its effect in flattening the lens is counteracted, and 

 the lens, by its own elasticity, becomes more convex in front. Ac- 

 cording to this view, therefore, the active agent in near accommoda- 

 tion is the ciliary muscle, which sets free the elasticity of the lens ; and 

 the feeling of eifort experienced in such adjustment must be chiefly 

 due to the action of that muscle; whilst distant accommodation in- 

 volves only the employment of the elastic force of the suspensory liga- 

 ment of the lens.v 



Though the explanations of Helmholz are generally accepted, it is 

 maintained by some, that, in near vision, the lens undergoes an altera- 

 tion both in shape and position, through the influence of the ciliary 

 muscle, or of some simultaneous pressure, exercised by the iris also, 

 upon the margin of the lens. By this pressure, the substance of the 

 lens is supposed to be subjected to the degree of tension necessary to 

 increase its convexity; whilst, in distant vision, the muscular parts 

 are supposed to relax, and the lens, by its own elasticity, to recover 

 its flatter form (H. Muller and Cramer). The swift forward move- 

 ment of the lens is supposed to be owing to the ciliary muscle drawing 

 forward the choroid coat, which acts on the vitreous humor, and this, 

 in its turn, on the lens. Lastly, an opinion recently entertained, is, 

 that the eye is at rest only when objects situated at medium distances 

 are looked at, and, that, in the production of the changes in the eye- 

 ball necessary for near vision, the circular fibres of the iris contract, 

 whilst the radial fibres contract when distant objects are viewed (Lang- 

 enbeck, Henke). If such be the case, it is difficult to account for the 

 fact, that the sense of fatigue, which, as already mentioned, always 

 attends near vision, immediately disappears on directing the eyes to 

 distant objects. Moreover, on opening the eyes after they have been 

 closed for any length of time, distant objects are those most clearly 

 perceived. 



There can be little doubt, however, that, whatever may be the na- 

 ture of the changes in the lens, by which the necessary adjustment is 

 effected, they are accomplished chiefly, as is now generally supposed, 

 by the agency of the fibres of the ciliary muscle, or so-called tensor of 

 the choroid, with which the iris may in some way co-operate. The 

 movements themselves, though instigated by the will, and assisted by 

 sensation, are automatic. As is well known, atropine, the active prin- 

 ciple of the atropa belladonna, or deadly nightshade, whether locally 



