42 MOVEMENTS OF THE PUPIL. [BOOK in. 



that the impulses along the long ciliary nerves are ordinary 

 motor impulses, leading to a contraction of this dilator muscle, 

 whereby the constricting action of the sphincter is overcome. 

 And it is further urged in support of this view that stimu- 

 lation of one or some only of the long ciliary nerves, in 

 contrast to an uniform stimulation of all of them together, such 

 as happens when the cervical sympathetic is stimulated in the 

 neck, leads to partial, uneven widening of the pupil ; the long 

 ciliary nerve stimulated acts only on a portion of the circle 

 of the pupil and draws this part outward, forming more or less 

 of a notch. Such a result it is argued could only be obtained 

 by the direct local pull of some radial dilator fibres. 



On the other hand, those who deny the existence of any such 

 radial dilator muscle are led to explain the pupil-dilating influence 

 of the sympathetic as due to the impulses along that nerve inhibit- 

 ing the previously existing contraction of the sphincter. These 

 argue that the sphincter may be compared to the heart, inas- 

 much as it possesses an automatic power of contraction, manifested 

 however not in a rhythmic but in a tonic manner, and that like the 

 heart its action may be either augmented or inhibited by nervous 

 impulses ; and we have seen ( 429) that a similar view may be 

 taken of the actions of the plain muscular fibres of the alimentary 

 canal and of the bladder. According to this view the sphincter of 

 the iris, when removed from all influences, is in a state of tonic 

 contraction, pulling against the radiate strain of the elastic tissue 

 of the iris and so maintaining a pupil of a certain size. Under the 

 influence of light falling on the retina, impulses reaching the 

 sphincter by the short ciliary nerves augment its contraction, 

 and narrow the pupil in proportion to their intensity. On the 

 other hand, impulses reaching the sphincter from the sympathetic 

 by the long ciliary nerves inhibit the activity of the sphincter, 

 diminish the force with which it is pulling against the elastic 

 tissue of the iris, and so lead to a widening of the pupil, thus 

 either diminishing the constriction which is being caused by 

 the action of light on the retina or otherwise, or, in the absence 

 of all external constricting influences, causing the pupil to become 

 wider than it naturally would when removed from all extrinsic 

 influences whatever. In support of such a view it is pointed 

 out that the muscular tissue forming the sphincter is peculiar, 

 since a slip of it when directly stimulated by a weak interrupted 

 current elongates, in this respect also shewing its analogy 

 with the heart whose activity may similarly be inhibited by 

 the interrupted current. Again, in the extirpated eye, or even 

 in the isolated iris, warmth dilates and cold constricts the pupil, 

 the one relaxing, and the other increasing the contraction of the 

 sphincter. Other arguments on the one side or on the other may 

 also be brought forward, and the conflict between the two views 

 cannot be regarded perhaps as definitely ended, though the 



