800 



SPECIAL SENSES. 



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

 keep 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 we fix with the eye any near object we are 

 conscious of an effort, and the prolonged vision 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 we can bring the eye to 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 produced upon the lens. This muscle, it will be remembered, 

 arises from the circular line of junction of the cornea and sclerotic, w r hich is undoubtedly 

 its fixed point, passes backward, 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 for- 

 ward, with, probably, a slightly spiral motion of the lens, the contents of the globe situ- 

 ated posterior to the lens are compressed, and the suspensory ligament is relaxed. The 

 lens itself, the compressing and flattening action of the suspensory ligament being dimin- 

 ished, 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 ligament is artificially dimin- 

 ished. 



FIG. 253. 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 contracted and the suspensory liga- 

 ment of the lens consequently relaxed. 



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 

 requirements of vision with exquisite nicety. In early life, the lens is soft and elastic, 

 and the accommodating power is at its maximum ; but in old age the lens becomes flat- 

 tened, harder, and less elastic, and the power of accommodation is necessarily 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 are 

 directly associated with the muscular effort by which the form of the lens is modified, 

 the contraction of the pupil is not one of the essential conditions of accommodation. 

 Helmholtz cites a case in which the iris was completely paralyzed, the power of accom- 

 modation remaining perfect ; and he mentions another case, reported by Yon Graefe, in 

 which accommodation was not disturbed after loss of the entire iris. 



We have already noted the fact that the pupil contracts when the eyes are made to 

 converge by the action of the muscles animated by the third pair of nerves ; and it is evi- 



