EODS AND CONES OF THE RETINA. 49 



superficial groovings, but the transverse markings are somewhat more evident than 

 in the rods (figs. 52, 54). There is a delicate covering of neurokeratin investing 

 the outer segments of both rods and cones, and this is somewhat more pronounced 

 on the cones, so that a post-mortem separation into disks does not take place so 

 readily as in the rods. From their behaviour to staining reagents and the readiness 

 with which they become altered after removal from the body, it has been conjectured 

 that the outer segment contains materials similar in chemical nature to those com- 

 posing the myelin of the medullary sheath of nerve-fibres. 



In the inner segments, the proportion which the fibrillated part bears to the 

 homogeneous basal part differs in the rods and cones. In the rods the fibrils 

 usually occupy only the outer third of the inner segment (fig. 52), ceasing 

 abruptly at its junction with the middle third ; in the cones, on the other hand, 

 they occupy about the outer two-thirds of the segment, only the part nearest the 

 membrana limitans remaining free from fibrils. The fibrils in question are for 

 the most part straight and parallel, and strongly refracting. Sometimes, in the 

 cones, instead of this outer part of the inner segment being fibrillated, it appears 



Fig. 57. PlGMENTED EPITHELIUM OF THE HUMAN RKTINA (Max Schultze). HIGHLY MAGNIFIED. 



(t, cells seen from the outer surface with clear lines of intercellular substance between ; b, two cells 

 seen in profile with fine offsets extending inwards ; c, a cell still in connection with the outer ends of the 

 rods. 



homogeneous, but is nevertheless well marked oft from the inner part by its strong 

 refractivity. 



This condition of a part of the inner segment of the cones is much better marked in other 

 mammals and in the lower vertebrata, where there occurs a distinct strongly refracting body, 

 situated in the middle or outer part of the segment, and known from its shape as the 

 ; ellipsoid," a name which is also extended to include the fibrillated part of the cone in the 

 human retina. In reptiles an oval body, coloured red by iodine, takes the place of the 

 ellipsoid (Merkel). In lower vertebrata, as well as in most other mammals, the fibrils are 

 absent from the inner segments of the rods also, a peculiar, strongly refracting, lenticular 

 body (" rod-ellipsoid ") being met with at their outer part, corresponding to the ellipsoid of 

 the cones. Further, in birds, reptiles, and amphibia, in ganoid fishes and in marsupials (but not 

 in other mammals), there is found in the extreme outer part of the inner segment of each 

 cone a minute globular body, apparently of a fatty nature, which in some is clear and colour- 

 less, but in many cones is brightly coloured of a tint varying in different cones from red to 

 green red and yellow being the most common. Blue and violet are not met with, but by 

 the action of iodine the colours of all become changed to blue. Sometimes the whole inner 

 segment is found to be slightly tinted of the same colour as the " oil-globule." In birds 

 there are two kinds of cone : in the one kind, the cone-fibre passes straight down to the outer 

 molecular layer ; in the second kind, obliquely. In all vertebrates below mammals, double- 

 or twin-cones are here and there met with ; these usually have, the one a straight the other 

 an oblique cone-fibre. Numerous other differences and peculiarities are found in animals : 

 thus in birds and reptiles the cones are more numerous than the rods ; in many reptiles (e.g., 

 lizard) only cones are met with ; while in some fishes (sharks and rays), in most nocturnal 

 mammals, and in the owl, the cones are either altogether absent or are but few and rudi- 

 mentary (M. Schultze). This statement has, however, been denied by W. Krause so far as 

 nocturnal mammals are concerned. In the size of the elements there is also much variation : 

 the rods being very large in amphibia, and especially long in fishes. In the frog there are three 



VOL. in., PT. 3. E 



