SENSE OF SIGHT. 553 



has a purplish-red color, and an inner slightly bulged segment, 

 which in part of its length is longitudinally striated. The nucleus 

 of the rod-element often has, in the fresh condition, a transversely 

 shaded aspect. The cone-elements are formed of a conical tapering 

 external part, the cone proper, which is directly prolonged into a 

 nucleated enlargement, from the farther side of which the cone- 

 fiber, considerably thicker than the rod-fiber, passes inward, to 

 terminate by an expanded arborization in the outer molecular 

 layer ; here it comes into relation with a similar arborization of 

 dendrons of a cone-bipolar. The cone proper, like the rod, is 

 formed of two segments, the outer of which, much the smaller, is 

 transversely striated, the inner, bulged segment being longitudi- 

 nally striated. The inner ends of the rod- and cone-fibers come, 

 as already stated, in contact with the peripheral arborizations of 

 the inner granules, and through these elements and their arbori- 

 zations in the inner molecular layer a connection is brought about 

 with the ganglionic cells and nerve-fibers of the- innermost layers. 

 There appears, however, to be no anatomic continuity between the 

 several elements, but merely an interlacement of ramified fibrils. 



10. Pigmentary Layer. This layer is called also tapetum 

 nigrum, and consists of a single layer of hexagonal, pigmented cells 

 in contact with the choroid. These cells send offsets inward be- 

 tween the rods. The pigment is in the form of minute crystals, 

 and when the cells have been exposed to light for a considerable 

 time they are found in the filaments just described as extending 

 between the rods. It is supposed that the function of these pig- 

 ment-granules is to restore the purple coloring-matter which has 

 been bleached by prolonged exposure to light. In the horse and 

 many carnivora these cells contain no pigment. 



Fibers of Muller. The layers of the retina between the in- 

 ternal and external limiting membranes are traversed by long, 

 stiff cells, the fibers of Muller. At their bases they expand, and 

 the tissue joining these expanded bases forms the membrana 

 limitans interna. At the outer nuclear layer the fibers branch 

 and form a fenestrated tissue supporting the fibers and nuclei of the 

 rod- and cone-elements. At the junction of the outer nuclear and 

 bacillary layers the fibers form the membrana limitans externa, 

 from which sheaths pass around the bases of the rods and cones. 

 In that portion of the fiber which lies in the inner nuclear layer 

 is an oval nucleus. The arrangement of these fibers of Muller 

 and the limiting membranes has been well described " as columns 

 between a floor and a ceiling." 



Macula Lutea (Figs. 326, 327). The retina at the yellow spot, 

 except at the fovea centralis, a part of the macula lutea, is thicker 

 than elsewhere ; the middle of the fovea is the thinnest part of the 

 retina. The ganglion-cells are more numerous in the macula, as 

 are the cones when compared with the rods. In the fovea the rods 



