48 



THE EYE. 



end-piece with the basal part ; consequently, the outer and inner segments of the 

 rods are nearly similar in size and shape, the inner being, however, slightly bulged, 

 whereas the inner segment of each cone far exceeds the outer one in size, the latter 

 appearing merely as an appendage of the inner segment (fig. 54). The two 

 segments both of the rods and cones exhibit well-marked differences in their 

 chemical and optical characters, as well as in the structural appearances which may 

 be observed in them. Thus, while in both the outer segment is doubly refracting in 

 its action upon light, the inner is, on the contrary, singly refracting : the inner 

 becomes stained by carmine, iodine, and other colouring fluids, whilst the outer 

 remains uncoloured by these reagents, but is stained greenish-brown by osmic acid. 

 The outer segment in both shows a tendency to break up into a number of minute 

 superposed disks. The inner segment of each is distinguishable into two parts 





[ ffllTF 1 



Figs. 55 and 56. SECTIONS OF FROG'S RKTINA SHOWING THE ACTION OF LIGHT UPON THE PIGMEXT- 

 CKLLS, AND UPON THE RODS AND CONES, (v. Grenderen-Stort. ) 



Fig. 55, from a frog which had been kept in the dark for some hours before death. 



Fig. 56, from a frog which had been exposed to light just before being killed. 



Three pigment cells are shown in each section. In Fig. 55, the pigment is collected towards the 

 nucleated part of the cell, in Fig. 56 it extends nearly to the bases of the rods. In Fig. 55 the rods, 

 outer segments, were coloured red (the detached one green), in Fig. 56 they had become bleached. In 

 Fig. 55, the cones, which in the frog are much smaller than the rods, are mostly elongated, in Fig. 56 

 they are all contracted. 



an outer part, composed, according to Max Schultze, of fine fibrils, and an inner parr, 

 homogeneous, or finely granular, and, at the membrana limitans externa, directly con- 

 tinued into a rod or cone-fibre, the disposition of which in the outer nuclear layer 

 has been already described. 



In the outer segments of the rods there can be detected, by the aid of a 

 powerful microscope, besides a delicate transverse striation (fig. 52), corresponding 

 to the superposed disks of which, as above mentioned, they appear to be formed, 

 also fine longitudinal markings which are due to slight linear grooves by which 

 they are marked in their whole extent. The ends of the segments are rounded and 

 project into the pigmentary layer, The purplish-red colour of the retina before 

 mentioned (p. 35), resides entirely in the outer segments of the rods (Boll, Kiihne). 

 A few of the rods are, however, at least in some animals, of a green colour. The 

 outer segments of the cones taper gradually to a blunt point, and do not exhibit 



