768 THE EYE AND VISTON. [CH. LV. 



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 dilatator 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. I. 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 4. When the accommodation is relaxed. 



physostigmine. 5. Under the local influence of atro- 



6. Under the influence of opium. pine. This drug also paralyses 



7. During sleep. 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 RETINA. 



The Retina 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 pene- 

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

 that it is the layer of the retina which is capable of stimulation 

 by light are the following : 



(i) The point of entrance of the optic nerve into the retina 

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

 called the blind spot. The phenomenon itself is very readily 

 demonstrated. If we direct one eye, the other being closed, 

 upon a point at such a distance to the side of any object, 



