292 THE NERVOUS SYSTEM 



jugate when the eyes are focussed on distant objects. 

 When near objects are looked at a certain amount of 

 convergence takes place. 



The muscles of the iris are controlled by two sets of 

 nerves, the ciliary branches of the third cranial nerve 

 which supply the sphincter pupillse, and the sympathetic 

 which supplies the dilator. These muscles are related to 

 one another reciprocally, contraction of one being accom- 

 panied by active relaxation of the other. 



Under normal conditions the pupil is contracted : — 



1. When the eye is exposed to light. This is the light 

 reflex, the afferent path being the optic nerve, the efferent 

 being the third nerve. When any part of this arc is 

 destroyed, e.g. by atrophy of the optic nerve, the light reflex 

 fails. The purpose of this reflex appears to be to protect 

 the retina from sudden changes in brightness. 



2. During accommodation for near objects. In this way 

 a sharper definition is obtained, owing to the cutting out 

 of the rays from the periphery of the lens {see later). 



3. During sleep. 



The pupil is dilated (1) in the dark; (2) on focussing 

 upon distant objects; (3) on sympathetic stimulation, 

 whether due to a sensory stimulus or to an emotional state. 



When the eye ceases to respond to hght but can stiU 

 accommodate the condition is known as the Argyll- 

 Robertson pupil. 



Action of Drugs 



The following drugs contract the pupil : — 



Opium and morphia, by stimulating the third nerve 



centrally ; 

 Pilocarpine and physostigmine, by stimulating the 



third nerve peripherally; 



while the following dilate it : — 



Atropine, by paralysing the third nerve peripherally ; 

 Adrenalin, by stimulating the sympathetic peripherally. 



