66o TEXT-BOOK OF PHYSIOLOGY. 



focalized on the retinal surface. Changes in the convexity of the lens, 

 either of increase or decrease, are attended by changes in the distinctness of 

 images. Coincidently with the lens change, the pupillary margin advances 

 and the pupil itself becomes smaller. By this means an indistinctness of the 

 image is prevented by cutting off the rays which would give rise, owing to 

 the angle at which they fall on the surface, to diffusion-circles, from spheric 

 aberration. 



The Function of the Ciliary Muscle. Though it is generally admit- 

 ted that the increase in the convexity of the lens is caused by the contrac- 

 tion of the ciliary muscle, the exact manner in which this is accomplished 

 is not clearly understood. According to Helmholtz, when the eye is in 

 repose and directed to a distant object the lens is somewhat flattened from 

 a traction exerted by the suspensory ligament. When the eye is directed 

 to a near object, the ciliary muscle contracts, thereby relaxing the ligament, 

 as a result of which the lens, by virtue of an inherent elasticity, bulges for- 

 ward and becomes more convex. In consequence of this latter fact the 

 refracting power is proportionally increased. In extreme efforts of accom- 

 modation it is believed by some observers that the circularly arranged 

 fibers, the so-called annular muscle, contract and exert a pressure on the 

 periphery of the lens and thus aid other mechanisms in relaxing the liga- 

 ment and in increasing the convexity. This view appears to be supported 

 by the fact that in hypermetropia, where a constant effort is required to 

 obtain a distinct image of even distant objects, the annular muscle becomes 

 very much hyper trophied, thus reinforcing the meridional fibers. In 

 myopia, on the contrary, where the accommodative effort is at a minimum, 

 the entire muscles possesses less than its average size and development. 



According to Tscherning, a different explanation of the action of the 

 ciliary muscle must be given. Thus, when it contracts, the antero-internal 

 angle, that portion in close relation with the suspensory ligament, recedes 

 and exerts on the ligament a pressure which in turn exerts a traction on the 

 peripheral portions of the anterior surface of the lens, which produces the 

 deformation observed. At the same time the postero-external portion of 

 the muscle exerts traction on the chorioid, thus sustaining the vitreous 

 and indirectly the lens. 



The reason for the flattening of the periphery of the lens from zonular 

 compression and the sharpening of the central convexity is to be found in 

 the fact that the convexity of the more solid central portion, the nucleus, 

 is greater than that of the lens itself. Hence it is easily understood why a 

 zonular traction would give rise to peripheral flattening. 



There is, however, one point which seems difficult to harmonize with 

 Tscherning's view; that is, the fact that during accommodation the lens 

 appears to be slightly tremulous, thus showing relaxation, and not increased 

 tension, of the suspensory ligament. 



Range of Accommodation. It has been stated that rays of light 

 coming from a luminous point situated at any distance beyond 65 meters 

 are so nearly parallel that no accommodative effort is required for their 

 focalization. So long as the luminous point remains between infinity and 

 65 meters, the eye, directed toward it, remains completely relaxed. The 

 point at which the object can be distinctly seen without accommodation 



