THE ORGANS OF THE SPECIAL SENSES. 



261 



varies with the functional activities of the organ. When the eye 

 lias been exposed to light the pigment is found lying deeply between 

 the rods. When the eye has been at rest for some time the pigment 

 is retracted in greater or less degree within the body of the cell. 



2. The rods and cones are the terminal structures of cells which 

 extend from the fifth layer to the first. The nuclei of these cells 



FIG. 243. 



Diagram of the retina. (Kallius.) I., pigmented epithelial layer; II., layer of the rods and 

 cones ; III., external limiting membrane ; IV., outer granular layer ; V., outer molecular 

 layer; VI., inner granular layer; VII., inner molecular layer; VIII., ganglionic layer; 

 IX., layer of nerve-fibres, z, pigmented epithelial cells ; c, at the bottom of the external 

 limiting membrane, rods ; b, cone cells ; c-h, ganglion-cells of the sixth layer connecting 

 the fourth layer with the eighth ; i, horizontal cell sending a process into the seventh 

 layer; k-q, " spongioblasts," or neurons of the third type (Fig. 217); r-w, ganglion-cells 

 of the eighth layer; x, sustentacular cell of Miiller, with striated upper end forming 

 a part of the external limiting membrane ; y, y, neuroglia-cells. It should be borne in 

 mind that in sections of the retina numerous elements of the various sorts here rep- 

 resented are crowded together to form a compact tissue. The centrifugal fibres which 

 reach the retina from the cerebrum are omitted from this diagram. They are distributed 

 in the inner granular or sixth layer. The light entering the eye passes through the layers 

 represented in the lower part of this figure before it can affect the rods and cones. 



lie within the fourth layer, to which they give a granular appear- 

 ance (Fig. 243). 



3. The external limiting membrane is formed by the circularized 

 outer ends of certain sustentacular epithelial cells, the "cells of 



