696 TEXT-BOOK OF PHYSIOLOGY 



face of the lens becomes more convex, while at the same time it slightly ad- 

 vances, possibly as much as 0.4 mm. in extreme efforts. This change is 

 represented in Fig. 303. According to Tscherning, the increase in con- 

 vexity of the anterior surface is confined to the central portion, the re- 

 mainder of the surface becoming somewhat flattened. There is, moreover, j 

 no evidence that there is any advance of the surface or any increase in the! 

 thickness of the lens. A series of new and ingenious experiments lend! 

 support to Tscherning' s view, though of late Hess has brought forward defi- 

 nite experimental evidence in favor of the view of Helmholtz. The radius 

 of curvature in either case approximates 6 mm. in extreme efforts of ac- 

 commodation. The increase in convexity naturally increases the refracting 

 power. 



Whichever view is accepted, the nearer the object that is, the greater the 

 degree of divergence of the light rays the more pronounced must be the 

 increase in convexity in order that they may be sufficiently converged and 

 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 



FIG. 303 . THE LEFT HALF REPRESENTS THE EYE IN A STATE OF REST. THE RIGHT 

 HALF IN STATE OF ACCOMMODATION. 



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 anil 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 



