STRUCTURE OF THE RETIXA. 



613 



mediate neighbourhood of the latter, and c, from the peripheral part of 

 the retina. 



The rods and the cones, although differing thus in shape and size 



Fiff. 423. 



Fig. 424, 



Fig. 423. — Outer Surface of the Colum- 

 nar Layer op the Eetina (Kollilcer). 

 350 Diameters. 



a, part witliin tlie macula lutea, where 

 only cones are ijresent ; b, part near the 

 macula, where a single row of rods inter- 

 venes between the cones ; c, from a part 

 of the retina midway between the macula 

 and the ora sen-ata, showing the preponder- 

 ance of the rods. 



Fig. 424. — A Rod and a Cone from the Human Retina (Jlax 

 Schultze). Highly Magnified. 



In the red the longitudinal striation of both the outer and inner 

 segments is shown ; in the cone the transverse striation of the outer 

 segment and the longitudinal of the inner. In both the fibrillation of 

 the inner segment is much more extensive than usual ; /, limitans 

 externa. 



aGjfee in many points of structure. Thus, each consists 

 of two distinct segments — an inner and an outer ; the 

 division between the t\yo occurring, in the case of the 

 rods, about the middle of their length (in man); in the 

 cones at the junction of the finer tapering end-piece with 

 the basal part ; consequently, the outer and inner seg- 

 ments of the rods are nearly similar in size and shape, 

 the inner being, however, slightly bulged as a rule, 

 whereas the inner segme^it of each cone far exceeds the 

 outer one in size, the latter appearing merely as an 

 appendage of the inner segment (fig. 424). The two 

 segments both of the rods and cones exhibit well-marked 

 differences, both 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 colourless. The outer segment in both shows a tendency to 

 break up into a number of minute super-imposed disks, whereas the 

 inner segment is itself again distinguishable into two parts — an outer 

 part, apparently composed of fine fibrils, and an inner part, homoge- 

 neous, or finely granular, and at the membrana limitans externa, directly 

 continued into a rod or cone-fibre, the disposition of which in the 

 outer nuclear layer has been already described. 



In the Older segments of the rods there can be detected by the aid 

 of a powerful microscope, besides a delicate transverse striation (fig. 

 422), corresponding to the superposed disks of which, as above indi- 

 cated, they appear composed, also fine longitudinal markings wdiich 

 are due to slight linear grooves by which they are marked in their 

 Avhole extent. Externally the segment projects into the pigmen- 

 tary layer with a somewhat rounded-off extremity. Internally it 

 is overlapped for a short distance by a delicate fibrillated prolongation 



