562 



THE ORGANS 



lie in the outer part of the cell, while from the inner side thread-Uke 

 projections extend down between the rods and cones of the layer next 

 internal. The pigment has the form of rod-shaped granules. Its 

 distribution seems to depend upon the amount of hght being admitted 

 to the retina. When little or no light is being admitted, the pigment 

 is found in the body of the cell, the processes being wholly or almost 



A B 



1 ^^^.g!5.f ^|j^Sjf;?^r-v:^r?r 



r ,C^ 





''tM 



Fig. 373. — A, Scheme of retina as shown by the Golgi method. B, Vertical section 

 of retina to show layers as demonstrated by the hasmatoxylin-eosin stain. (Merkel- 

 Henle.) B. — i, Layer of pigmented epithelium; 2, layer of rods and cones; 3, outer 

 limiting layer; 4, outer nuclear layer; 5, outer molecular layer; 6, inner nuclear layer; 7, 

 inner molecular layer; 8, layer of nerve cells; 9, layer of nerve fibres; 10, inner limiting 

 layer. A. — i, Pigment layer; 2, processes of pigmented epithelial cells extending down 

 between rods and cones; 3, rods; 4, rod-cell nuclei and rod fibres; 5, cones; 6, cone fibres; 

 7, bipolar cells of inner nuclear layer; 8, ganglion cells of nerve-cell layer; 9, larger gang- 

 lion cells of nerve-cell layer; 10, fibres of optic nerve forming layer of nerve fibres; 11 

 and 12, t>-pes of horizontal cells; 13, 14, 15, and 16, types of cells the bodies of which He 

 in the inner nuclear layer; 17, efferent optic-nerve fibre ending around cell of inner 

 nuclear layer; 18, neuroglia cells; 19, Miiller's fibre; 20, rod-bipolar cell of inner nuclear 

 layer. 



m 



free from pigment ; when the retina is exposed to a bright light, some 

 of the pigment granules pass down into the processes so that the pig- 

 ment becomes more evenly distributed throughout the cell. 



The layer of rods and cones and the outer nuclear layer (Fig. 373, 

 S, 2, 4) are best considered as subdivisions of a single layer, the neuro- 

 epithelial layer. This consists essentially of two forms of neuro- 



