VISUAL ACCOMMODATION. 



709 



The ordinary range of accommodation varies between a distance of about 

 five inches (12'7 centimetres) and infinity. 



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

 determine the extent and nature of the changes of the lens in accommoda- 

 tion ; and these changes have been accurately measured in the living subject. 

 As the general results of these measurements (Helmholtz), 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 in- 

 creased, 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 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 two persons, are as follows : 



The mechanism of the changes in the thickness and ,in the curvatures "of 

 the lens in accommodation can be understood only by keeping clearly in 

 mind the physical properties of the lens itself and its anatomical relations. 

 In situ, in what has been called the indolent state of the eye, the lens is ad- 

 justed to vision at an infinite distance and is flattened by the tension of its 

 suspensory ligament. After death, indeed, it is easy to produce changes in 

 its form by applying traction to the zone of Zinn. Remembering the exact 

 relations of the suspensory ligament, the ciliary muscle and the lens, and 

 keeping in mind the tension within the globe, it is evident that when the 

 ciliary muscle is in repose, the capsule will compress the lens, increasing its 

 diameter and diminishing its convexity. It is in this condition that the eye 

 is adapted to vision at an infinite distance. It is evident, also, that very 

 slight changes in the convexity of the lens will be sufficient for the range of 

 accommodation required. If any near object be fixed with the eye there is 

 a conscious effort, and the prolonged vision of near objects produces a sense 

 of fatigue. This may be illustrated by the very familiar experiment of look- 

 ing at a distant object through a gauze. When the object is seen distinctly, 

 the gauze is scarcely perceived ; but by an effort the eye can be brought to 

 see the meshes of the gauze distinctly, when the impression of the distant 

 object is either lost or becomes very indistinct. 



