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HANDBOOK OF PHYSIOLOGY. 



is 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 number 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- 

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

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

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



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

 choroid. It consists of hexagonal and unbranched cells with a light 

 nucleus. 



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 



FIG. 397. FIG. 398. 



FIG. 397. Ciliary processes, as seen from behind. 8, posterior surface of the iris, with the 

 sphincter muscle of the pupil; 2, anterior part of the choroid coat; 3, one of the ciliary processes, of 

 which about seventy are represented. X. 



FIG. 398. The posterior half of the retina of the left eye. viewed from before; , the cut edge of 

 the sclerotic coat; ch, the choroid; r, the retina; in the interior at the middle, the macula lutea with 

 the depression of the fovea centralis is represented by a slight oval shade; towards the left side the 

 light spot indicates the colliculus or eminence at the entrance of the optic nerve, from the centre of 

 which the arteria centralis is seen spreading its branches into the retina, leaving the part occupied 

 by the macula comparatively free. (After Henle.) 



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

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

 number, or entirely absent. There 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. 396 represents the view of Max Schultze. Accord- 

 ing to this there are certain sustentacular fibres of connective tissue 

 (radiating fibres of Miiller) which spring from the mewbrana limitans 

 interna almost vertically, and traverse the retina to the limitans ex- 

 terna, whence very delicate connective-tissue processes pass up between 

 the rods and cones. The framework which they form is represented in 



