688 



SPECIAL SENSES 



an infinite distance. It is evident, also, that slight changes in the con- 

 vexity of the lens are sufficient for the range of accommodation re- 

 quired. If any near object is fixed with the eye there is a conscious 

 effort, and prolonged examination of near objects produces a sense of 

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

 looking 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 impres- 

 sion of the distant object is either lost or becomes indistinct. 



The ciliary muscle arises from the circular line of junction of the 

 cornea and sclerotic, passes backward, and is lost in the tissue of the 

 choroid, extending as far as the anterior border of the retina. Most of 

 the fibres pass directly backward but some become circular or spiral. 

 When this muscle contracts, the choroid is drawn forward, with prob- 

 ably a slightly spiral motion, of the lens, the contents of the globe, situ- 



Fig. 176. Section of the lens, etc., showing the mechanism of accommodation (Fick). 



The left side of the figure {F} shows the lens adapted to vision at infinite distances. The right side 

 of the figure (N) shows the lens adapted to the vision of near objects, the ciliary muscle being con- 

 tracted and the suspensory ligament of the lens consequently relaxed. 



ated behind the lens, are compressed, and the suspensory ligament is 

 relaxed. The lens itself, the compressing and flattening action of the 

 suspensory ligament being diminished, becomes thicker and more con- 

 vex, by virtue of its own elasticity, in the same way that it becomes 

 thicker after death, when the tension of the ligament is artificially 

 diminished. The theory of Tscherning that the ciliary muscle tightens 

 the suspensory ligament, compresses the lens near its borders and pro- 

 duces bulging of its central portion is not tenable in view of demonstra- 

 tions that the ligament actually is relaxed during accommodation for 

 near objects (Hess). 



This is in brief the mechanism of accommodation. Near objects 

 are seen distinctly by a voluntary contraction of the ciliary muscle, the 

 action of which is perfectly adapted to the requirements of vision. In 

 early life the lens is soft and elastic, and the accommodating power is 

 at its maximum ; but in old age the lens becomes flattened, harder and 

 less elastic, and the power of accommodation necessarily is diminished. 



