614 



THE EYE. 



of the margin of the inner segment. The outer segments of the cones 

 taper gradually to a blunt point and do not appear to exhibit 

 the superficial groovings, but the transverse markings are somewhat 

 more evident than in the rods (fig. 424), altliough the}- do not so 

 readily break up into the separate disks as in the latter. This 

 has been accounted for by supposing the existence of an excessively 

 delicate membrane covering the whole of the outer segment of the 

 cones. 



In the inner segments, th.Q 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 seg- 

 ment (fig. 422), 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 appearing fibrillated, it appears 

 homogeneous, but is nevertheless well marked off from the inner part 

 by its strong refractivity. 



This condition of a pcart of tlie inner segment cf tlie cones is much better 

 marked in most of the lower vertebrata, where it is met with as a distinct strongly- 

 refracting body, situated in the middle or outer part of the segment, and is known 

 from its shape as the " ellipsoid." Moreover, in these animals, the fibrils are 

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

 " lenticular " body being met with at their outer jiart, corresponding to the 

 ellipsoid of the cones. Further, in birds, reptiles, and amphibia, 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 colomdess, 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 liecome changed to blue. 

 Sometimes the whole inner segment is found to be slightly tinted of the same 

 colour as the " oil-globule." In all vertebrates below mammals, double- or twin- 

 cones are here and there mot with, which resemble two cones joined near their 

 base, but separate and distinct towards their apex. Numerous other differences 

 and peculiarities are found in animals : thus in birds the cones are more nume- 

 rous than the rods ; in many reptiles 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 rudimentary. 



Fi-. 425. 



Fig. 425. — Pigmented Epithelium of the Hum.vn Retina (Max Schultze.) Higuly 



Magnified. 



«, cells seen from the outer surface with clear lines of intercellular substance between ; 

 h, two cells seen in profile with fine offsets extending inwards ; c, a cell still in connec* 

 into with the outer ends of the rods. 



