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HAND-BOOK OF PHYSIOLOGY. 



distinctly conical, and surmounted externally by a thin rod-like body. 

 According to the researches of Max Schultze, the rods show traces of 

 longitudinal fibrillation, and, moreover, have a great tendency to break 

 up into a n timber of transverse discs like a pile of coins. 



In the rod and cone layer of birds, the cones usually predominate 

 largely in number, whereas in man the rods are by far the more numer- 

 In nocturnal birds, however, such as the owl, only rods are present, 



ous. 



and the same appears to be the case in many nocturnal and burrowing 

 mammalia, e.g., bat, hedge-hog, mouse, and mole. 



' FIG. 373. Section of the retina, choroid, and part of the sclerotic, moderately magnified, a, 

 membrana limitans interna; 6, nerve-fibre layer traversed by Mtiller's sustentacular fibres (of the 

 connective tissue system) ; c, ganglion-cell layer: d, molecular layer; e, internal granular layer;/, 

 intergranular layer ;</, external granular layer; 7i, membrana limitans externa, running along the 

 lower part of i, the layer of rods and cones ; 7c, pigment cell layer formerly described as part of the 

 choroid; I, m, internal and external vascular portions of the choroid, the first containing capillaries, 

 the second larger blood-vessels, cut in transverse section; n, sclerotic. (W. Pye.) 



10. Pigment cell layer, which was formerly considered part of the 

 choroid. 



In the centre of the yellow spot (macula lutea), all the layers of the 

 retina become greatly thinned out and almost disappear, except the rod 

 and cone layer, which considerably increases in thickness, and comes to 

 consists almost entirely of long slender cones, the rods being very few in 

 number, or entirely absent. Aere are capillaries here, but none of the 

 larger branches of the retinal arteries. 



With regard to the connection of the various layers there is still some 

 uncertainty. Fig. 370 represents the view of Max"Schultze. According 



