ACCOMMODATION 689 



Changes in the Iris in Accommodation. The size of the pupil is 

 sensibly diminished in accommodation of the eye for near objects. Al- 

 though the movements of the iris are directly associated with the 

 muscular effort by which the form of the lens is modified, the contrac- 

 tion of the pupil is not one of the essential conditions of accommodation. 

 Helmholtz reported a case in which the iris was completely paralyzed, 

 the power of accommodation remaining perfect ; and he described an- 

 other case, reported by Von Graefe, in which accommodation was not 

 disturbed after loss of the entire iris. 



It has already been noted that the pupil contracts when the eyes 

 are made to converge by the action of the muscles animated by the third 

 pair of nerves ; and it is evident that convergence of the eyes occurs in 

 looking at very near objects. Increased convergence of the visual lines 

 without change of accommodation makes the pupil contract, as is easily 

 proved by simple experiments with prismatic glasses. When accom- 

 modation is effected without converging the visual axes, " each stronger 

 tension is combined with contraction of .the pupil" (Bonders). Con- 

 traction of the pupil, therefore, occurs both in convergence of the visual 

 axes without accommodation and in accommodation for near objects 

 without convergence of the eyes. 



The action of the iris, as is evident from the facts just stated, is to 

 a certain extent under the control of the will ; but it can not be disas- 

 sociated, first, from the voluntary action of the muscles that converge 

 the visual axes, and second, from the action of the ciliary muscle. 

 Bonders, by alternating the accommodation for a remote and a near 

 object, was able to contract and dilate the pupil voluntarily more than 

 thirty times in a minute. Brown-Sequard, in discussing the voluntary 

 movements of the iris, has mentioned a case in which " the pupil could 

 be contracted or dilated without changing the position of the eye or 

 making an effort of adaption for a long or a short distance." As 

 further evidence of the connection of accommodation with muscular 

 action, cases are cited in works on ophthalmology in which there is 

 paralysis of the ciliary muscle, as well as cases in which the act of 

 accommodation is painful. 



A curious phenomenon connected with accommodation may be 

 observed in looking at a near object through a very small opening, like 

 a pinhole. The shortest distance at which one can see a small object 

 distinctly is about five inches (12.7 centimeters); but in looking at the 

 same object through a pinhole in a card, it can be seen distinctly at the 

 distance of about one inch (25.4 millimetres), and it then appears con- 

 siderably magnified. In this experiment, the card serves as a diaphragm 

 with a very small opening, so that only the centre of the lens is used ; 



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