STRUCTURE AND FUNCTION OF THE RETINA. 581 



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 nerve 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 import- 

 ant work of stimulating the optic nerve when light impinges on 

 its terminals ? 



THE FUNCTION OF THE RETINA. 



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

 cells, but a most complex peripheral apparatus, which to histolo- 

 gists 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 afterward, 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 

 ciliary region. The fibres spread evenly over the fundus except 

 at the central point (fovea centralis), which they avoid by pass- 

 ing on each side of it. 



Next to the fibres comes a layer of nerve cells; these seem com- 

 monly to have one pole connected with a fibre from the optic 

 nerve, while from the other side two or three poles send processes 

 into the other layers of the retina. 



Then comes an indistinct layer made up of granular material 

 and two layers of peculiar nuclear bodies, with a layer of granu- 

 lar material between them. 



Outside of these, and separated by a fine limiting membrane, is 

 the most important layer of the retina. It consists of a layer of 

 rods and cones which are connected with those parts of the retina 

 already named, and seems to project into the protoplasm of pig- 



