856 



ACCOMMODATION. 



[Book hi. 



half represents the change taking place during accommodation 

 for a nearer object. 



§ 535. It seems possible then that accommodation for near 

 objects may be brought about by a contraction of the (longi- 

 tudinal) ciliary muscle dragging forwards the choroid and ciliary 

 processes, thus slackening the suspensory ligament, and so per- 

 mitting the compressed elastic lens to bulge forward. And 

 experimental evidence shews that this is what does take place. 

 The ciliary muscle is governed, as we shall see presently, by the 

 ciliary nerves. If in a living animal (dog) or in an eye imme- 

 diately after removal from the body, an opening be made in the 

 sclerotic in order to watch the choroid, it may be seen that when 

 the ciliary nerves are stimulated the choroid does move forward 

 at the same time that the front surface of the lens becomes more 

 convex ; a needle, the point of which is carefully lodged in the 



FAR 



NEAR 



Fig. 146. Diagram to illustrate Accommodation. (After Helmholtz.) 



C.P. Ciliary process. I. Iris. Sp.l. suspensory ligament. I. cm. longitudi- 

 nal ciliary muscle, c.c.m. circular ciliary muscle. C.S. canal of Schlemm. 



The left half represents the arrangement for viewing far objects and the 

 right half that for viewing near objects. 



choroid, moves in such a way as to shew that the choroid moves 

 forward, though no appreciable movement can be seen in a needle 

 thrust into the front part of the ciliary muscle itself. If the cor- 

 nea be cut away so as to leave only at one place a small fragment 

 still connected to the junction of the sclerotic and cornea, this 

 piece moves backward when the ciliary nerves are stimulated, 

 shewing that the ciliary muscle does pull on the point of junc- 

 tion of the sclerotic with the cornea. When, however, the cor- 

 nea is intact, or even when a sufficiently large part of it is left, 

 the junction becomes a fixed point, at least relatively to the 

 moveable choroid. Moreover not only the contraction of the 

 ciliary muscle and movement of the choroid, but the actual 

 slackening of the suspensory ligament and change in the curva- 



