CH. LVI.] USES OF THE IRIS 789 



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 v 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 

 often remains in cases of locomotor ataxy, after there is an entire 

 loss of the reflex to light (Argyll-Eobertson pupil). 



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

 dilatation of the pupil. This is accompanied by starting of the 

 eyeballs, due to contraction of the plain muscle in the capsule of 

 Tenon, which, like the dilator fibres of the iris, is supplied by the 

 cervical sympathetic nerve. 



We may sum up the principal conditions under which the pupil 

 contracts and dilates in the following table : 



Causes of 

 Contraction of the Pupil. Dilatation of the Pupil. 



1. Stimulation of third nerve. 1. Paralysis of the third nerve. 



2. Paralysis of cervical sympathetic. 2. Stimulation of the cervical sympa- 



3. When the eye is exposed to light. thetic. 



4. When accommodation occurs. ! 3. In the dark. 



5. Under the local influence of j 4. When the accommodation is 



physostigmine. relaxed. 



6. Under the influence of opium. 5. Under the local influence of atro- 



7. During sleep. pine. This drug also paralyses 



the ciliary muscle. 



6. In the last stage of asphyxia. 



7. In deep chloroform narcosis. 



8. Under the influence of certain 



emotions, such as fear. 

 , 9. During pain. 



There is a close connection of the centres that govern the activity 

 of the two irides. If one eye is shaded by the hand, its pupil will 

 of course dilate, but the pupil of the other eye will also dilate. 

 The two pupils always contract or dilate together unless the cause 

 is the local injury to the nerves of one side or the local action of 

 drugs. 



FUNCTIONS OF THE EETINA. 



The Ketina is the nervous coat of the eye ; it contains the layer 

 of nerve-epithelium (rods and cones) which is capable of receiving 



