108 SPECIAL SENSES. 



see the meshes of the gauze distinctly, when the impression 

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



Our knowledge of the action of the ciliary muscle is only 

 to be arrived at theoretically and by studying the effects pro- 

 duced upon the lens. This muscle, it will be remembered, 

 arises from the circular line of junction of the cornea and 

 sclerotic, which is undoubtedly its fixed point, passes back- 

 ward, and is lost in the tissue of the choroid, extending as 

 far back 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, probably, a slightly spiral motion of the lens, 

 the contents of the globe situated posterior to the lens are 

 compressed, and the suspensory ligament is relaxed. The 

 lens itself, the compressing and flattening action of the sus- 

 pensory ligament being diminished, becomes thicker and more 

 convex, by virtue of its own elasticity, in the same way that 

 it becomes thicker after death, when the tension of the liga- 

 ment is artificially diminished. 



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 adapted to the require- 

 ments of vision with exquisite nicety- In early life, the lens 

 is soft and elastic, and the accommodative power is at its 

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

 and less elastic, and the power of accommodation is neces- 

 sarily diminished. 



Changes in the Iris in Accommodation. The size of the 

 pupil is sensibly diminished in accommodation of the eye for 

 near objects. Although the movements of the iris under 

 these conditions are directly associated with the muscular ef- 

 fort by which the form of the lens is modified, the contrac- 

 tion of the pupil is not one of the essential conditions of ac- 

 commodation. Helmholtz cites a case in which the iris was 

 completely paralyzed, the power of accommodation remain- 



