500 MOVEMENTS OF THE PUPIL. [BOOK in. 



the change in the lens is always accompanied by movements in 

 the iris, it will be convenient to consider the latter, before we 

 discuss the nervous mechanism of the whole act. 



Movements of the Pupil. Though by making the efforts 

 required for accommodation we can at pleasure contract or dilate 

 the pupil, it is not in our power to bring the will to act directly on 

 the iris by itself. This fact alone indicates that the nervous 

 mechanism of the pupil is of a peculiar character, and such indeed 

 we find it to be. The pupil is contracted (1) when the retina (or 

 optic nerve) is stimulated, as when light falls on the retina, the 

 brighter the light the greater being the contraction, (2) when we 

 accommodate for near objects. The pupil is also contracted when 

 the eyeball is turned inwards, when the aqueous humour is deficient, 

 in the early stages of poisoning by chloroform, alcohol, &c.; in nearly 

 all stages of poisoning by morphia, physostigmin, and some other 

 drugs, and in deep slumber. The pupil is dilated (1) when stimu- 

 lation of the retina (or optic nerve) is diminished or arrested as in 

 passing from a bright into a dim light or into darkness, (2) when 

 the eye is adjusted for far objects. Dilation also occurs when there 

 is an excess of aqueous humour, during dyspnoea, during violent 

 muscular efforts, as the result of a stimulation of sensory nerves, as 

 an effect of emotions, in the later stages of poisoning by chloroform, 

 &c. and in all stages of poisoning by atropin and some other drugs. 



Contraction of the pupil is caused by contraction of the circular 

 fibres or sphincter of the iris. Dilation is caused by contraction of 

 the radial fibres of the iris; for though the existence of radial fibres 

 has been denied by many observers, the preponderance of evidence 

 is clearly in favour of their being really present. 



Considering how vascular the iris is, it does not seem unreason- 

 able to interpret some of the variations in the condition of the pupil 

 as the results of simple vascular turgescence or of depletion brought 

 about by vaso-motor action or otherwise, the small or contracted 

 pupil corresponding to the dilated and filled, and the large or 

 dilated pupil to the constricted and emptied condition of the blood- 

 vessels. Thus slight oscillations of the pupil may be observed 

 synchronous with the heart-beat and others synchronous with the 

 respiratory movements. But the variations in the pupil seem too 

 marked to be merely the effects of vascular changes, and indeed that 

 constriction of the pupil cannot be wholly the result of turgescence, 

 nor dilation wholly the result of depletion of the vessels of the iris, 

 is shewn by the facts that both these events may be witnessed in a 

 perfectly bloodless eye, and that the movements 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, 

 and indeed may be over before these have arrived at their maxi- 

 mum. Moreover the fibres of the sympathetic, which as we shall 

 see are concerned in causing dilation of the pupil, run a somewhat 



