ACCOMMODATION 



687 



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

 action is accessory and is not essential. 



The ordinary range of accommodation varies between a distance of 

 about five inches (12.7 centimeters) 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 accom- 

 modation ; and these changes have been accurately measured in the 

 living subject. It has been 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 de- 

 scribe minutely the methods employed in making these calculations, and 

 it is sufficient to state that it is done by accurately measuring the com- 

 parative size of images formed by reflection from the anterior surface of 

 the lens. The results obtained by Helmholtz, in observations on two 

 persons, are the following : 



The mechanism of the changes in the thickness and in the curva- 

 tures of the lens in accommodation can be understood only by keeping 

 clearly in mind the physical properties of the lens itself and its anatomi- 

 cal relations. In situ, in what has been called the indolent state of the 

 eye, the lens is adjusted for 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 at rest, 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 



