834 THE EYE AND VISION [CH. LVII1. 



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

 loss of the reflex to light (Argyll-Kobertson 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. 



5. Under the local influence of 



physostigmine. 



6. Under the influence of opium. 



7. During sleep. 



3. In the dark. 



4. When the accommodation is 



relaxed. 



5. Under the local influence of atro- 



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 KETINA 



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

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

 the stimulus of light, and transforming it into a nervous impulse 

 which passes to the brain by the optic nerve. 



The bacillary layer, or layer of rods and cones, is at the back 

 of all the other retinal layers, which the light has to penetrate 

 before it can affect this layer. The proofs of the statement that this 

 is the layer of the retina which is capable of stimulation by light are 

 the following : 



(1) The point of exit of the optic nerve from the retina, 

 where the rods and cones are absent, is insensitive to light, and is 

 called the Uind spot. This is readily demonstrated by what is known 

 as Mariotte's experiment. If we direct one eye, the other being 



