CHAP. IIL] SIGHT. 1173 



versely, the body of the iris being elastic as well as extensible, a 

 relaxation of the muscular fibres of the sphincter will lead to a 

 widening of the pupil. We may therefore in the first instance at all 

 events consider the constricted pupil as the result of a contraction 

 of the sphincter muscle, and the dilated pupil as the result of a 

 diminution of that contraction ; whether the dilated pupil is merely 

 a negative result, whether it is simply due to a lessening of the 

 activity of the sphincter, or whether there is in addition an active 

 dilator muscle, we will discuss later on. 



We may here, however, remark that, considering how vascular 

 the iris is, it does not seem unreasonable to interpret some of the 

 variations in the condition of the pupil as the results of simple 

 vascular turgescence or depletion brought about by vaso-motor 

 action or otherwise. When the blood vessels are dilated and filled 

 they will cause the iris to encroach on the pupil, making the latter 

 small and narrow, and conversely a constricted and emptied 

 condition of the blood vessels would lead to the pupil being large 

 and wide. And indeed slight oscillations of the pupil, due to 

 greater or less fulness of the blood vessels, may be observed 

 synchronous with the heart-beat, and others synchronous with the 

 respiratory movements. But the variations in the pupil are, as a 

 rule, too marked to be merely the effects of vascular changes ; 

 and indeed that turgescence of the vessels of the iris is not the 

 only cause of constriction of the pupil, nor depletion the only 

 cause of dilation, is shewn by the facts that both these events may 

 be witnessed in a perfectly bloodless eye, and that the move- 

 ments of the pupil when brought about by agents which also 

 affect the blood vessels begin some time before the changes in the 

 calibre of the blood vessels begin, and indeed may be over and past 

 before these have arrived at their maximum. Moreover those 

 fibres of the sympathetic which are concerned in causing dilation 

 of the pupil, are said to run a somewhat different course from those 

 which govern the blood vessels ; it is stated that we can ex- 

 perimentally, by stimulating one or the other set, either dilate 

 the pupil without any marked change in the blood-supply, or 

 affect the blood-supply without materially changing the pupil. 

 We may therefore adhere to the view that the main changes 

 of the pupil in the direction of narrowing and widening are 

 brought about by means of plain muscular fibres in the iris 

 apart from those of the blood vessels. 



725. Of all conditions affecting the size of the pupil, the 

 one most important and most frequently at work is the falling of 

 light on the retina ; and to this we may now turn. But before 

 doing so it will be desirable to recall to mind the nervous supply 

 of the eyeball, omitting for the present the nerves governing the 

 six ocular muscles which move the eyeball as a whole. 



The eyeball is supplied, in the first place, by the short ciliary 

 nerves (Fig. 142 s.c.) coming from the ophthalmic or lenticular, or 



752 



