32 



ACCOMMODATION. 



[Boon in. 



The cavity of the eyeball behind the suspensory ligament is 

 filled with the vitreous humour. If this is sufficiently abundant 

 i^ will distend the cavity and render the suspensory ligament tense. 

 But since the suspensory ligament passes obliquely forwards, all 

 round, from the ciliary processes to the front of the lens, tension of 

 the ligament will tend to flatten the lens, altering its shape but 

 not its bulk. 



The choroid, of which, as we have seen, the ciliary processes 

 form the forward continuation, is loosely attached to the sclerotic 

 along the line of the lamina fusca and suprachoroidal membrane ; 

 the one can to a certain extent be slipped backwards and forwards 

 beneath the other. 



i.c.m. 



FAR NEAR 



FIG. 141. DIAGRAM TO ILLUSTRATE ACCOMMODATION. (After Helmholti.) 



C. P. Ciliary process. /. Iris. Sp. I. suspensory ligament. /. c. m. longitudinal 

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



The left half represents the arrangement for viewing far objects aud the right 

 half that of viewing near objects. 



The (longitudinal) ciliary muscle is, as we have seen, attached 

 on the one hand to the junction of the sclerotic and cornea, and on 

 the other hand to the front part of the choroid. If we suppose 

 the former to be a fixed point, the contraction of the muscle would 

 pull the moveable choroid and ciliary processes somewhat forward. 

 But the pulling forward of these structures would slacken the 

 suspensory ligament by bringing its ciliary attachment more 

 forward. And a slackening of the suspensory ligament by relieving 

 the pressure on the elastic lens would allow the front surface to 

 become more convex. This is shewn diagrammatically in Fig. 141, 

 one-half of which, the left half, is intended to represent the eye 

 directed towards distant objects, while the other half represents 

 the change taking place during accommodation for a nearer object. 



722. It seems possible then that the accommodation for near 

 objects may be brought about by a contraction of the (longitudinal) 

 ciliary muscle dragging forwards the choroid and ciliary processes, 

 thus slackening the suspensory ligament, and so permitting the 



