598 HANDBOOK OF PHYSIOLOGY. 



mirrors, gives a reversed image. If now the eye under observation be made 

 to look at a near object, the second image becomes smaller, clearer, and 

 approaches the first. If the eye be now adjusted for a far point, the 

 second image enlarges again, becomes less distinct, and recedes from the 

 first. In both cases alike the first and third images remain unaltered in 

 size and relative position. This proves that during accommodation for 

 near objects the curvature of the cornea, and of the posterior of the lens, 

 remains unaltered, while the anterior surface of the lens becomes more 

 convex and approaches the cornea. 



Mechanism. Of course the lens has no inherent power of contrac- 

 tion, and therefore its changes of outlines must be produced by some 

 power from without; and there seems no reason to doubt that this power 

 is supplied by the ciliary muscle. It is sometimes termed the tensor 

 choroidece. As this name implies, from its attachment, it is able to 

 draw forwards the choroid and therefore slackens the tension of the sus- 



Fio. 406. Diagram representing by dotted lines the alteration in the shape of the lens on ac- 

 commodation for near objects. (E. Landolt.) 



pensory ligament of the lens which arises from it. The lens is usually 

 partly flattened by the action of the suspensory ligament; and the ciliary 

 muscle by diminishing the tension of this ligament diminishes, to a pro- 

 portional degree, the flattening of which it is the cause. On diminution 

 or cessation of the action of the ciliary muscle, the lens returns, in a 

 corresponding degree, to its former shape, by virtue of the elasticity of 

 its suspensory ligament (Fig. 406). From this it will appear that the 

 eye is usually focussed for distant objects. In viewing near objects the 

 pupil contracts, the opposite effect taking place on withdrawal of the 

 attention from near objects, and fixing it on those distant. 



Range of Distinct Vision. Near-point. In every eye there is a 

 limit to the power of accommodation. If a book be brought nearer and 

 nearer to the eye, the type at last becomes indistinct and cannot be 

 brought into focus by any effort of accommodation, however strong. 

 This, which is termed the near-point, can be determined by the follow- 



