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HANDBOOK OP PHYSIOLOGY. 



length, but it resembles this in structure. There is, however, no visua! 

 purple found in the cone. The inner limb of the cone is broader in the 

 centre; each cone is in connection by its internal end with a cone fibre, 

 which has much the same structure as the rod fibre, but is much stouter. 

 This connects with a nerve-cell of the layer below (fig. 41 7 A, 4). 



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



Iff 



It 



Fig. 417A. Schematic diagram of the elementary structure of the retina, sz, Rods and cones; 

 e, membrana limitans externa; gr, external granules; e, external molecular layer); 6z, internal 

 granular layer; i, internal molecular layer; mz, multipolar cell layer (ganglion optici); nf, nerve- 

 fibre layer; Zt, membrana limitans interna. 



1, Rod; 2, rod granule; 3, cone; 4, cone granule; 1-1', rod visual cell; 3-3', cone visual cell; 5, 

 central termination of the visual cells and peripheral terminal arborization of the bipolar cells; 

 6, 6, two bipolar cells for rods; 6, one bipolar cell for cone; 7, 7, 7, 7, 7, 7, the cent ral processes of 

 bipolar cells with the terminal arborizations situated in the various layers of the internal molecular 

 layer; 7', central process of a bipolar cell for cone; 8, multipolar cells with their peripheral den- 

 drites and central neuraxons; 9, 9, 9, nerve-fibres and terminal arborizations of remote cells. 



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

 ous, except in the fovea centralis, where cones only are present, as is the 

 case at the anterior part of the retina near the ora serrata. The num- 

 ber of cones has been estimated at 3,000,000. In nocturnal birds, how- 

 ever, 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. The rods are absent in reptiles. 



External Limiting Membrane. A delicate membrane lies beneath 



