CH. LV.] 



THE IRIS. 



767 



dilatation of the pupil as due to inhibition of the circular fibres. 

 But if the iris is stimulated near its outer margin at three 

 different points simultaneously the pupil assumes a triangular 

 shape, the angles of the triangle corresponding to the points 

 stimulated ; this must be due to contraction of three strands of 

 the radiating muscle ; inhibition of the circular fibres would 

 occur equally all round. 



The iris is supplied by three sets of nerve-fibres contained in 

 the ciliary nerves. 



(a) The third nerve supplies the circular fibres. 



(b) The cervical sympathetic supplies the radiating fibres. 

 The cilio-spiual centre which governs them is in the cervical 

 region of the cord (see p. 666). 



(c) Fibres of the fifth nerve which are probably sensory. 



The experiments on these nerves are those of section and 

 stimulation of the peripheral ends ; the usual experiments by 

 which the functions of a motor-nerve are discovered. 



Certain drugs dilate the pupil. These are called mydriatics ; 

 atropine is a well-known example. Others cause the pupil to 

 contract. These are called myotics ; physostigmine and opium 

 (taken internally) are instances. Different myotics and mydriatics 

 act in different ways, some exerting their activity on the muscular, 

 and others on the nervous structures of the iris. 



Reflex actions of the iris. When the iris contracts under the 

 influence of light, the sensory nerve is the optic, and the motor 

 the third nerve. The central connection of the two nerves in 

 the region of the mid-brain we shall see later on (fig. 598, p. 786). 

 The iris also contracts on accommodation ; and the reflex path 

 concerned in this action is a different one from that concerned in 

 the light reflex, as this reflex often remains in cases of locomotor 

 ataxy, after there is an entire loss of the reflex to light (Argyll- 

 Robertson pupil). 



On painful stimulation of any part of the body, there is reflex 



