614 PHYSIOLOGY 



indispensable condition for the fineness of vision which we desire 

 when we examine any object closely. The fact that the amount 

 of light which will fall into the eye from any given object increases 

 inversely as the square of the distance of the object from the eye ensures 

 that sufficient light will pass through the constricted pupil for the 

 appreciation of the finer details of the object. 



(3) In sleep the pupils are always contracted. This fact seems 

 at first in opposition to the other conditions regulating the size of 

 the pupil, since during sleep no light is falling into the eye. If the 

 eyelid of a sleeping person be raised the pupil will be found to be 

 constricted ; as the person wakes up, in consequence of the inter- 

 ference, the pupil dilates and may then constrict again if the light 

 is held so as to fall into the eye. This behaviour of the pupil may 

 enable us to distinguish feigned from real sleep. The constriction of the 

 pupil is really, like that which accompanies accommodation, an 

 associated condition, and depends on the fact that during sleep the 

 axes of the eyeballs are directed upwards and inwards. 



(4) Contraction of the pupil is a marked effect of the action of 

 certain drugs, especially opium and its alkaloid, morphia, as well as 

 of the alkaloids eserine, or physostigmine, and pilocarpine. Contrac- 

 tion of the pupil also occurs in general excitatory conditions of the 

 central nervous system and is therefore found during the stage of 

 induction of chloroform and ether anaesthesia. 



Dilatation of the pupil occurs : 



(1) On the removal of light stimulus from the eye. If the 

 removal is complete the pupil remains dilated, but if there is any 

 light at all the pupil gradually constricts again as the eye becomes 

 dark- adapted. 



(2) Dilatation of the pupil can be reflexly excited by the stimula- 

 tion of many sensory nerves, and is constantly observed as a result 

 of severe pain. The presence or absence of dilated pupils may serve 

 therefore as a means of testing how far an emotional expression of 

 pain is to be credited to a physical cause. 



(3) The pupils are often dilated in emotional conditions such as 

 fear. 



(4) Dilatation of the pupils occurs in every condition of extreme 

 exhaustion, when the activities of the nervous centres are lowered. 

 It is therefore seen during the third stage of chloroform anaesthesia, 

 or in the comatose condition produced by excess of alcohol. Among 

 the drugs which cause dilatation of the pupil the belladonna alkaloids, 

 atropine and homatropine, are the best known. These alkaloids will 

 produce dilatation of the pupil when simply dropped into the con- 

 junctival sac, and are therefore largely used to dilate the pupil as a 

 preliminary to ophthalmoscopic investigation of the eye. 



