ACCOMMODATION OF THE EYE. 



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the retina, and, at the same time, upon the illuminated plane of the image, the vessels and 

 the other anatomical details of the retina are seen with equal distinctness. But, when 

 the flame is brought considerably nearer, its image becomes confused, while the details 

 of the structure of the retina remain perfectly distinct." 



It is evident that there is a certain condition of the eyes adapted to vision at an infi- 

 nite distance, and that, for the distinct perception of near objects, the transparent media 

 must be so altered in their arrangement or in the curvatures of their surfaces, that the 

 refraction will be greater ; for, without this, the rays would be brought to a focus be- 

 yond the retina. 



The changes in the eye by which accommodation is effected are now known to con- 

 sist mainly in an increased convexity of the lens for near objects; and the only points in 

 dispute are a few unimportant details in the mechanism of this action. The simple facts 

 to be borne in mind in studying this question are the following : 



When the eye is accommodated to vision at an infinite distance, the parts are passive. 



In the adjustment of the eye for near objects, the convexities of the lens are increased 

 by muscular action. 



In accommodation for near objects, the pupil is contracted; but this action is merely 

 accessory and is not essential. 



The ordinary range of accommodation varies between a distance of about five inches 

 and infinity. 



Changes in the Crystalline Lens in Accommodation. It is important to determine 

 first the extent and nature of the changes of the lens in accommodation ; and, by the 

 ingenious experiments of the German physiologists, particularly those of Helmholtz, 

 these changes have been accurately measured in the living subject. As the general result 

 of these measurements, it was ascertained that the lens becomes increased in thickness 

 in accommodation for near objects, chiefly by an increase in its anterior curvature, by 

 which this surface of the lens is made to project toward the cornea. As the iris is in 

 contact with the anterior surface of the lens, this membrane is made to project in the act 

 of accommodation. The posterior curvature of the lens is also increased, but this is slight 

 as compared with the increase of the curvature of its anterior surface. The distance 

 between the posterior surface of the lens and the cornea is not sensibly altered. It is 

 unnecessary to describe minutely the methods employed in making these calculations, 

 and it is sufficient for our purposes to state that it is done by accurately measuring the 

 comparative size of images formed by reflection from the anterior surface of the lens. 

 The results obtained by Helmholtz in observations upon three different persons are as 

 follows : 



The mechanism of the changes in the thickness and in the curvatures of the lens in 

 accommodation can only be understood by keeping clearly in mind the physical proper- 

 ties of the lens itself and its anatomical relations. In situ, in what has been called the 

 indolent state of the eye, the lens is adjusted to vision at an infinite distance and is flat- 

 tened by the tension of its suspensory ligament. After death, indeed, it is is easy to pro- 

 duce changes in its form by applying traction to the zone of Zinn. If we remember, 



