858 CHANGES IN THE PUPIL. [BOOK in. 



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 

 constricted when the eyeball is turned inwards, when the aqueous 

 humour is deficient, in the early stages of poisoning by chloro- 

 form, alcohol, and similar substances, in nearly all stages of 

 poisoning by morphia, physostigrnin, 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 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. Dilation also occurs when there is an excess of aqueous 

 humour distending the anterior chamber, during dyspnoea, dur- 

 ing violent muscular efforts, as the result of stimulation of sen- 

 sory nerves, as an effect of emotions, as an effect of fatigue, in 

 the later stages of poisoning by chloroform, alcohol and similar 

 substances, in all stages of poisoning by atropin and some other 

 drugs, and in certain nervous diseases. 



537. We may say at once that we are able to recognize 

 two separate nervous mechanisms regulating these changes in the 

 pupil. One of these regulates the size of the pupil according 

 to the amount of light falling upon the retina, and is by far 

 the more important of the two ; through the other, the size of 

 the pupil is modified by other influences. We will consider the 

 former mechanism first. During the action of this, constriction 

 of the pupil is undoubtedly caused by contraction of the circu- 

 larly disposed muscular fibres which form within the iris the 

 sphincter muscle. 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. 

 Conversely, the body of the iris being elastic as well as exten- 

 sible, a relaxation of the muscular fibres of the sphincter, 

 assisted by the return to their natural position of structures 

 displaced by the contraction, will lead to a widening of the pupil. 

 We may in respect to this mechanism 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 a dilated pupil is always a mere 

 negative result, due to a lessening of the activity of the sphinc- 

 ter, or whether in certain cases an active dilator muscle is 

 brought into play we will discuss later on in connection with 

 the other mechanism. 



