Chap. XXXIX.] THE R-ETINA. 315 



above) the surrounding rods are wanting in the visual 

 purple. 



The visual purple stands in an intimate relation to 

 the pigmented epithelium of the retina, since a 

 retina regains its visual purple after bleaching, when 

 replaced on the pigmented epithelium (Kiihne). This 

 holds good, of course, only within certain limits. 



446. (10) The pigmented epithelium (Fig. 

 160), or tapetum nigrum, is composed of polygonal pro- 

 toplasmic cells, which, when viewed from the surface, 

 appear as a mosaic, in which they are separated from 

 one another by a thin layer of cement substance. Each 

 cell shows an outer non-pigmented part, containing the 

 slightly-flattened oval nucleus, and an inner part next to 

 the rods and cones, which is full of pigmented crystalline 

 rods (Frisch). This part is prolonged into numerous 

 fine fibrils, each containing a row of the pigmented 

 particles, and these fibrils pass between the outer 

 members of the rods, to which they closely adhere, and 

 which in reality become almost entirely ensheathed 

 in them (M. Schultze). Each cell supplies a number 

 of rods with these fibrils. Sunlight causes a protru- 

 sion of these fibrils from the cell body, whereas in 

 the dark they are retracted (Kiihne), in a manner 

 similar to what takes place in pigmented connective 

 tissue cells. (See par. 43.) The tint of this pigment 

 is darker in dark than in light eyes. It is bleached 

 by the light in the presence of oxygen (Kiihne), but 

 it persists in the absence of oxygen (Mays). 



447. The macula lutea (Fig. 161) of man and 

 ape contains a diffuse yellow pigment, between the ele- 

 ments of the retina (M. Schultze). In man and most 

 mammals, as mentioned above, there are hardly any 

 rods here, but cones only ; these are longer than in 

 other parts, and in the fovea centralis they are 

 longest, and, at the same time, very thin. Since 

 there are few rods here, the nuclei of the outer nuclear 



