IMAGES UPON THE RETINA 



515 



From the cells of the ganglion layer optic fibers are given off, forming the 

 innermost layer of the retina, the nerve-fiber layer (H). 



The lateral extent of the outer tuft of dendrites of the bipolar cells (E), 

 both of those which correspond to the rods and those which correspond to the 

 cones, varies greatly. In general several rods or cones are connected with each 

 of the bipolar cells. But each cone of the fovea centralis is in contact with 

 the dendrites of but one bipolar cell. 



Compared with the end arborizations of the ganglion cells those of the 

 bipolar cells are very small; consequently the smallest ganglion cells must be 

 in touch with a relatively large number of bipolar cells. 



In addition to these elements the retina contains still other cells of a nerv- 

 ous nature, lying either in the inner granular layer (outer and inner horizontal 



FIG. 211. A section through the retina of a full-grown dog, after Cajal. A, layer of rods and 

 cones; B, outer granular layer, containing the bodies of the visual ceils; C, outer plexiform 

 layer; E, inner granular layer, containing the bipolar cells; F, inner plexiform layer; G, gan- 

 glion cell layer; H, layer of the optic fibers. a, cone fiber; b, body and fiber of a rod cell; 

 c, bipolar cell with " brush " of fibrils belonging to the cones ; /, giant bipolar cell with wide 

 spreading brush of fibrils; h, diffuse amacrine cell, the varicose processes of which lie for the 

 most part directly on the ganglion cells; i, ascending nerve fibers; /, centrifugal fibers; </ and 

 g f , specialized cells which are seldom impregnated; n, ganglion cell receiving within it the ter- 

 minal brush of a bipolar cell from the rods; m, nerve fiber which is lost in the inner plexiform 

 layer. 



cells) or in the inner plexiform layer (amacrine cells, h}. The former, accord- 

 ing to Cajal, are for the purpose of bringing definite groups of rods into 

 relation with other definite groups more or less remote from them. Nothing 

 positive can be said as to the significance of the amacrine cells. 

 Finally, the retina contains also centrifugal nerve fibers (;). 



Which of these layers of the retina is the one primarily acted upon by 

 the light? 



Certainly not the nerve-fiber layer, for the optic nerve is just as insensitive 

 to light as other nerve trunks. This is shown especially by the following 

 experiment first performed by Mariotte (about 1665). 



