34 MOVEMENTS OF THE PUPIL. [BOOK in. 



assist in slackening the suspensory ligament. But no very decisive 

 explanation has been given why the circular fibres are often 

 largely developed in some eyes, it is said hypermetropic or long- 

 sighted eyes, and scantily present in others, myopic or short- 

 sighted eyes. And indeed there are several points in the whole 

 action of accommodation which still require to be cleared up. 



Accommodation is in a certain sense a voluntary act ; we can 

 by looking at near or far objects bring about the change whenever 

 we please. Since, however, the change in the lens is always 

 accompanied by movements in the iris, it will be convenient to 

 consider the latter before we speak of the nervous mechanism of 

 the former. 



The Movements of the Pupil. 



723. Although by looking at near or far objects, and so volun- 

 tarily bringing about changes in the accommodation mechanism, 

 we can call forth the accompanying changes in the iris, and can 

 thus at pleasure produce a constriction, narrowing, or a dilation, 

 widening, of 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 special character, and such 

 indeed we find it to be. 



The pupil is constricted, contracted, narrowed, (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 con- 

 stricted when the eyeball is turned inwards, when the aqueous 

 humour is deficient, in the early stages of poisoning by chloroform, 

 alcohol, and similar substances, in nearly all stages of poisoning by 

 morphia, physostigmin, and some other drugs, in the early part of 

 the day, in deep slumber, in the epileptic seizure, and in certain 

 nervous diseases. The pupil is dilated, widened, (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 distending the anterior 

 chamber, during dyspnoea, during violent muscular efforts, as the 

 result of stimulation of sensory nerves, as an effect of emotions, 

 as an effect of fatigue, in the later stages of poisoning by chloro- 

 form, alcohol and similar substances, in all stages of poisoning by 

 atropin and some other drugs, and in certain nervous diseases. 



724. Constriction of the pupil is caused by contraction of 

 the circularly disposed muscular fibres which form within the iris 

 the sphincter muscle ( 715). The more or less spongy body of the 

 iris being extensible, the shortening of the fibres and bundles of 

 fibres of the sphincter must necessarily narrow the ring of the pupil 

 of which the sphincter forms the almost immediate margin. Con- 



