CH. LVIIL] 



KERVES OF THE IRIS 



833 



and reach the eyeball via the ophthalmic branch of the fifth, and 

 long ciliary nerves (fig. 530). 



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



I MID-BRAIN 



FIG. 530. Diagram of the motor nerves of the iris. Around the upper half 

 of the pupil the circular fibres (C) only are indicated. These are sup- 

 plied by the third nerve, one fibre of which (III.) is seen issuing from the 

 mid-brain ; the cell-station for these fibres is in the ciliary ganglion (C.G.). 

 Around the lower half of the pupil, the radiating fibres (R) are indicated ; 

 these are supplied by the cervical sympathetic (Sy), one fibre of which is 

 shown with its cell-station in the superior cervical ganglion (S.C.G.). 

 (After Dixon.) 



The experiments on the motor nerves are those of section and 

 stimulation of the peripheral ends ; the usual experiments by which 

 the functions of such nerves 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. 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 



3G 



