752 



SIGHT. 



slumber. The pupil is dilated (1) when stimulation 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. Dilatation 

 also occurs when there is an excess of aqueous humor, 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 chloro- 

 form, etc., and in all stages of poisoning by atropine and some other drugs, 

 and in pathological conditions of the nervous system. 



N 



Diagram to illustrate Accommodation. C, cornea ; S, sclerotic ; F, C, A; vertical plane of the 

 cornea ; B, C, D, axis of the eye ; s, s, canal of Schlemm ; p, angle formed by the iris and cornea, 

 or margin of anterior chamber ; m, position of iris and curvature of lens in an eye converged for 

 parallel rays, distant vision, or negative accommodation ; n, position of iris and curvature of lens 

 required for near objects or for positive accommodation.] 



[Fio. 171. 



Emmetropic Eye. The dotted lines show how accommodation for the diverging rays of near 

 objects is effected by the bulging of the lens. The dotted lines indicate the change in the 

 lens and the effect on the light rays.] 



629. Contraction of the pupil is caused by contraction of the circular 

 fibres or sphincter of the iris. Dilatation 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 favor 

 of their being really present. 



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 of depletion brought about by vasomotor 

 action or otherwise, the small or contracted pupil corresponding to the di- 

 lated and filled and the large or dilated pupil to the constricted and emptied 

 condition of the bloodvessels. 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 dilatation wholly the 

 result of depletion of the vessels of the iris, is shown 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 



