264 THE RETINA, BY MAX SCHULTZE. 



Birds is coloured yellow, but which even in them always presents an 

 essentially different form and refractive power from the coloured oil 

 globules of the other half of the cone.* Moreover there is often a 

 difference in the length of the two halves, so that the one containing 

 the oil globule extends farther back than the other, whilst the planes 

 of junction of the outer and inner segments of the two halves do not 

 coincide. If we regard the outer segment as the seat of distinct 

 vision, there would be a necessity for different accommodation for the 

 two halves of the twin-cones, supposing them to have the same 

 function, and to receive the rays of light under otherwise identical 

 conditions. This last, however, does not really occur, inasmuch as 

 there is an essential difference between the refractile lenticular body 

 of the internal segments of the two halves. 



From all this we may draw the conclusion that the lenticular bodies 

 are destined to give to the luminous rays a direction fitting them for 

 undergoing their final changes in the outer segment, which, it would 

 seem, cannot be given to them by the coarser refracting apparatus. 



The different distribution of the rods and cones met icitJi in the 

 animal kingdom is well worthy of note. Both kinds of percipient 

 elements can be replaced or represented by the other. Thus the 

 cones are entirely absent in the retina of Sharks and Rays, the 

 Lamprey (Flussneunaugen), and probably in the Sturgeon ;f amongst 



* Archiv fur Mikroskop. Anatomic, Band iii., Taf. xiii., fig. 6, c. 



t The Petromyzon demands closer investigation than it has as yet re- 

 ceived. In the river Lamprey (Flussneunauge) an opportunity presented 

 itself long ago, in which I found that in the fresh condition it possesses 

 only one kind of element in the bacillar layer, and these, on account of 

 the form of their outer segment, I termed rods. According to a provisional 

 communication by H. Mtiller (Auge des Chamaleon, p. 25), both rods and 

 cones occur mixed together in the Petromyzon. In the Sturgeon, ac- 

 cording to Bowman (on the Eye, p. 89) and Leydig (Fishes and Amphibia, 

 p. 9), only one kind of percipient element is present, and these in Leydig's 

 drawings resemble the rods in the form of their outer segments. In the 

 osseous Fishes, rods and cones as a rule both occur. Amongst a large 

 collection of Fishes of the Baltic which I examined in the fresh state in 

 reference to the distribution of the rods and cones, and which included 

 species of Pleuronectes, Gadus, Gasterosteus, Trachurus, Cottus, Crenila- 

 brus and Syngnathus, I found in the latter genus alone any remarkable 

 deviation from the ordinary type. The rods are here very thick and 

 short as in Amphibia, the cones are much less conspicuous, and the disks 

 into which the rods break up after short maceration in perosmic acid have 

 in some instances a very well-marked semihmar form resembling that 

 which I have described as occurring in the Triton. 



