564 STUDIES IN ADVANCED PHYSIOLOGY. 



coat underneath it with it, and there would be produced in 

 the coat a slack over the chest. The elastic foot-ball im- 

 bedded in the coat would at once take advantage of this 

 slack and try to return to its natural round position. As 

 soon as the forward pull on the sides of the overcoat 

 ceased, the overcoat and the coat underneath it would fall 

 back to their natural position under the regular pressure of 

 the enclosed body. Accommodation is thus the product of 

 two factors: first, the pulling forward of the choroid coat 

 and with it the hyaloid membrane next to it, and the conse- 

 quent production of a slack of the suspensory ligament and 

 capsule. Second, the elasticity of the lens which enables it 

 to take advantage of this slack to return, to some extent at 

 least, to its more rounded form. Hither the paralysis of the 

 muscles or the loss of elasticity of the lens would at once 

 make the accommodation of the eye impossible. 



This explanation is the one advanced by Helmholtz, and 

 is the one which is now almost universally accepted as cor- 

 rect. Recently there has been advanced, on not very good 

 grounds, however, a second explanation. According to 

 this second explanation the increase in the convexity of the 

 lens is brought about by the contraction of some circular 

 fibers which seem to lie among the radial fibers of the cili- 

 ary muscles, by the contraction of which circular fibers the 

 lens is rounded. As these circular fibers are said to sur- 

 round the lens like a rubber band, it is clear how a flat- 

 tened object would have its convexity increased by a 

 circular pressure exerted upon it in the manner indicated, 

 but as most of the fibers of the ciliary muscles are radial 

 fibers, and as even the few circular fibers do not lie suf- 

 ficiently close to the margin of the lens, this explanation 

 does not seem satisfactory, especially so in view of the 

 completeness of the other explanation. 



It will be remembered in the chapter on the distribu- 

 tion of nerves that the ciliary muscles are innervated with 

 branches from the oculi motores. 



With the process of accommodation are associated 

 changes in the size of the pupil. The enlarging of the 



