STRUCTURE AND FUNCTION OF THE RETINA. 579 



altered from the light with which physicists experiment, but at 

 the retina this physical motion is stopped. The optic nerves no 

 more convey the light waves from the eye to the brain than the 

 tactile nerves carry the objects that stimulate their endings, but 

 only send a nerve impulse which the retina, on its exposure to 

 the light, excites in the optic nerve. As already stated, any form 

 of stimulation will cause the same kind of impulse to pass to the 

 brain and there set up the same sensation of light. Thus we are 

 told by persons who have had their optic nerves cut that the sec- 

 tion was accompanied by the sensation of a flash of light. Any 

 violent injury of the eyeball causes a flash of light to be experi- 

 enced. This fact has long since been arrived at in a practical 

 manner, for a blow implicating the eyeball is vulgarly said to 

 " make one see stars." Also, without violent injury, if we close 

 the eyes and turn them to the left side and then press with the 

 point of a pencil on the right side of the eyeball through the lid, 

 we have a sensation of a point or ring of light on the opposite side 

 of the eyeball. Thus we say that the specific energy of the optic 

 nerves excites a sensation of light, and the adequate stimulus of 

 the organ of vision is light. The first question that arises is, what 

 part of the retina does this important work of stimulating the 

 optic nerve when light impinges on its terminals? 



THE FUNCTION OF THE KETINA. 



The retina is not a simple sheet of nerve fibrils or of nerve cells, 

 but a most complex peripheral apparatus, which to histologists 

 has offered an endless field of study. This complex body is spread 

 all over the fundus of the eye except at the optic disk, where the 

 nerve pierces all the coats of the eye ; here there is nothing else 

 but nerve fibres and hence no retina properly so-called. 



The structure of the retina varies in different parts, but it may 

 be said to be composed in the main of the following layers. The 

 exceptions will be mentioned afterwards, viz. : (compare Fig. 216) 



Lying next to the hyaloid membrane is the layer of nerve fibres 

 which radiate from the optic disk to the ora serrata near the cili- 

 ary region. The fibres spread evenly over the fuudus except at 



