. 



Accoi 



DIOPTKIC MECHANISM OF THE EYE 299 



Accommodation is always associated with constriction of the 

 pupils and convergence of the eyes. The first depends on the 

 contraction of the sphincter of the pupil, the second on the 

 contraction of the internal recti muscles, which are also inner- 

 vated by the oculo-motor nerve. This functional association 

 shows that there is a co-ordinating centre which simultaneously 

 innervates the ciliary muscle, the sphincter of the pupil, and the 

 internal recti muscles of both sides. 



The associated contraction of the ciliary muscles and the recti 

 interni is not inseparable. It is possible with a definite amount 

 of convergence to alter the degree of accommodation, and with a 

 constant degree of accommodation to vary the amount of con- 

 vergence of both eyes within certain limits. This dissociation is 

 easily effected by placing prisms or lenses in front of the eyes. If 

 an object 30 cm. distant is focussed with both eyes and a concave 

 lens of 4 D. is then brought in front of the eyes, a greater effort of 

 accommodation is naturally required in order to see this object at 

 the same distance ; this is easily effected without producing double 

 vision (diplopia), as would result if the increased contraction of 

 the ciliary muscle were associated with increased contraction of 

 the recti interni If, on the other hand, convex lenses of 2*5 D. 

 are brought in front of the eyes the contraction of the ciliary 

 muscle must be decreased in order to see the object clearly, 

 although the convergence of the two visual axes is maintained for 

 30 cm. distance, so that the contraction of the recti interni is not 

 altered. 



VI. Accommodation is confined between definite limits, within 

 which alone distinct vision of an object is possible : a near point 

 and a far point are to be distinguished. 



The near point is that distance at which, with maximal 

 curvature of the lens, the formation of a sharp image on the retina 

 is still possible. According to Hess, it is not necessary for the 

 production of the greatest lens curvature that the ciliary muscle 

 should be in maximal contraction, since a moderate degree of 

 contraction suffices. The correspondence between the near point 

 of distinct vision and the maximal contraction of the ciliary 

 muscle, as assumed by many, after Helmholtz, is therefore 

 erroneous. 



The far point of distinct vision is that point in space at which 

 the eye is accommodated with the least possible curvature of the 

 lens ; this probably does not correspond with complete inactivity 

 of the ciliary muscle (Hess). The distance between the near point 

 and the far point was termed by Donders range of accommodation. 

 Within this range the degree of accommodation, i.e. increase of 

 lens curvature, is greatest at the near point, least at the far point, 

 and increases gradually from the near to the far point. This is 

 the logical deduction from the preceding argument on the 



