THE NERVOUS TUNIC. 



349 



FIG. 398. Pigmented cells 

 from outer layer of retina ; 

 surface view. X 350. 



and, therefore, constitutes the outer portion of the neuroepithelial division of 

 the retina. It is composed, as its name indicates, of two elements, the rods 

 and the cones, which are the outer ends of the rod- and cone-visual cells. 

 They are closely set, with their long axes perpen- 

 dicular to the surface of the retina. The rods far 

 outnumber the cones, except in the fovea centralis, 

 in which location cones alone are found. In the 

 macula each cone is surrounded by a layer of rods; 

 elsewhere the cones are separated by intervals occu- 

 pied by three or four cones. 



The rods of the human retina (Fig. 399, B) have 

 an elongated cylindrical form and measure approxi- 

 mately 60 n in length and 2 ft in diameter. Each rod 

 is composed of an outer and an inner segment, of 

 about equal length. The outer segment possesses 

 a uniform diameter, is doubly refracting, and readily 

 breaks up into minute disks. It is invested with a delicate covering of 

 neurokeratin, contains myeloid and is the situation of the visual purple 

 that tinges the living retina. The inner rod-segment is somewhat thicker 

 and has an ellipsoidal form. It is singly refracting, homogeneous in structure 

 and from its inner extremity sends the delicate rod-fibre through the external 

 limiting membrane into the outer nuclear layer, 

 where the nucleus of the rod- visual cell is found. 



The cone-visual cell is composed of the same 

 general divisions as the rod-cell, including the 

 specialized outer part, the cone, and the body within 

 the external nuclear layer. The cones are shorter 

 than the rods and have a length of 35 /*. Each one 

 (Fig. 399, A) is composed of an outer narrow cone- 

 shaped segment, and an inner broader one, which 

 is distinctly ellipsoidal, with a diameter of 7 //-. The 

 inner segment is double the length of the outer and 

 continuous, as the cone-fibre, with its nucleus in the 

 outer nuclear layer. 



The outer nuclear layer, the inner portion 

 of the neuroepithelial layer, is about 60 f* thick and 

 composed of the bodies of the rod- and cone-visual 

 cells, which show chiefly as the nuclei, the rod- and 

 cone-granules. The rod-granules occupy an ellip- 

 tical enlargement of the attenuated rod-fibres, 

 exhibit a transverse striation and are placed at 

 varying levels within the layer. The rod-fibres are 

 continued as a thin protoplasmic process into the 

 outer reticular layer, where they form small end- 

 knobs which are associated with the outer terminals 

 of the small nerve-cells, the rod-bipolars. The cone- 

 granules are less numerous than those of the rods, 

 display no transverse markings, and are found only in the outer portion of the 

 nuclear layer. The cone-fibres, the attenuated bodies of the cone-visual cells, 

 are broader than the corresponding parts of the rods and continued through 

 the outer nuclear layer as far as the outer portion of the external plexiform 

 layer. Here they end with broad bases, from which delicate processes extend 

 inwards to interlace with the terminal arborizations of the cone-bipolars. 



e I m 



FIG. 399. Visual cells from 

 human retina; A, cone-cell, B, 

 rod-cell ; a, b, outer and inner 

 segments ; c, attenuated bodies 

 (fibres), with nucleus (d) and 

 central ends (e); elm, external 

 limiting membrane. \Greeff.) 



