ORGANS OF THE BODY. 



637 



horse, and dog, the same variety in the distribution of both elements of 

 the retina may be observed. 



Strange to say, however, the retina of the bat is, according to Scliultze, 

 quite destitute of cones (although this is indeed 

 denied by Krause). The same is the case with 

 the hedgehog, the mouse, the Guinea-pig, and 

 mole, as well as many nocturnal and burrowing 

 animals. 



In the cat we find but ill-developed cones, while 

 in rabbit and rat traces merely of them exist 

 (Schultze). Whether, as appears probable, the 

 retina of the whale is quite without cones, is a 

 point which requires further investigation. 



The eyes of bony fishes approach those of man in 

 many respects ; their cones are of considerable size. 

 In rays and sharks rods alone are to be found. 



The case is quite different in birds and the naked 

 amphibia. In the first of these cones are present 

 in large numbers, reminding us of the yellow spot 

 in the human eye. In the lizard and chameleon, 

 on the other hand, rods are entirely absent ; perhaps 

 also in the snake. 



The cones of the bird's retina are remarkable for 

 peculiar spheroidal structures at the junction of style 

 and body, more or less imbedded in the latter. 

 They are glittering in appearance, and occupy the 

 whole breadth of the cone, so that no ray of light 

 can pass them by. They are usually of a yellow or red tint, but rarely 

 colourless. In owls (nocturnal fowl, as every one knows) the long rods 

 become again so prominent, and the cones so few in number, that the 

 ordinary structure of the bird's retina is reversed. Red spheroids are never 

 found here, and those yellow present become paler near the ora serrata. 



Globules similar to those just mentioned are to be met with also 

 among the scaly amphibia. In the cones of the 

 tortoise they are red, yellow, or colourless ; in 

 lizards yellow. 



Among the naked amphibia we find a few 

 very small cones standing amid a host of colossal 

 rods. 



The former present either colourless or pale 

 yellowish globules at the point of union of style 

 and body. 



The " twin cones," as they are called, discovered 

 some time ago by Hannover, present to us an- 

 other remarkable modification of these structures. 

 These are united to each other by their lateral 

 surfaces, remaining on the other hand quite dis- 

 tinct as regards their styles and bodies. They are frequently met with 

 in bony fishes; but even in birds and amphibia may be found inter- 

 spersed with the simple cones (Schultze). 



They, are probably the result of an uncompleted process of longitudinal 

 segmentation of the simple elements (Steinlin, Dobrowolsky). 



Those peculiar. lenticular bodies already mentioned as occurring within 



Fig. 592. Structure of the 

 rods of the retina. 1. 

 From the Guinea-pig ; a. 

 with internal and exter- 

 nal member; 6, still in 

 connection with a trans- 

 versely striped granule. 

 2. Macerated rods from 

 MacacwCynomolgus,v(\\\\- 

 altered external and in- 

 ternal members, and the 

 "fibres of Bitter'' in the 

 axes of the latter (after 

 Schultze). 



Fig. 593. The stratum bacilio- 

 sum viewed from without, a, 

 cones; &, cone-styles; c, ordi- 

 nary rods. 1. From the yellow 

 spot; 2. from the border of 

 the same; 3. from the middle 

 of the retina. 



