MACULA LUTEA AND FOVEA CENTRALIS. 287 



observations, the percipient elements in the two fossae centralis of the 

 Falcon consist of cones of smaller thickness than in the adjoining 

 parts, which possess only yellow, but no red pigment spheroids, 

 though both are present in the other portions of the retina. The 

 rods are entirely absent.* The retina of the Chamaeleon presents 

 a very well-marked fovea, the minute anatomy of which has been 

 given with great exactness by H. Miillerf and Hulke.J As appears to 

 be the general rule in Reptiles, cones alone are found in the percipent 

 layer of the entire retina of the Chamaeleon. These, however, in the 

 fovea centralis are only about one-fifth as thick as in the peripheric 

 regions, but at the same time are much longer, so that the line of the 

 liinitans externa is here more distant from the choroid, just as I have 

 depicted it in Man. To these cones, obliquely running cone fibres 

 are attached, also closely resembling those of Man. But, whilst in 

 Man the connective tissue of the external granule and cone fibre 

 layers follows these fibres, H. Miiller has demonstrated the presence 

 in the Chamaeleon of a peculiar kind of radial supporting fibres, which 

 decussate at an acute angle with those of the cones. The delicacy of 

 the individual cones of the Chamaeleon, as seen in preserved eyes of 

 this animal, surpasses that of every other animal. 



In other Reptiles, as in Ophidia and Chelonia, it appears from the 

 statements of Knox and Hulke, that a fovea is present, though not very 

 well marked. On the other hand, in Amphibia and Fishes no indica- 

 tions of either a macula lutea or of a fovea centralis have been 

 observed. 



I have directed attention to the circumstance that the yellow 

 screen, which in the macula lutea is situated in front of the per- 

 cipent elements, must exercise an important influence on the amount 

 of violet and blue that we see in the spectrum, || and it naturally sug- 

 gests itself, that an increase in the intensity of the yellow pigment of 

 the retina must produce yellow vision, or violet blindness. In consider- 

 ing the effects of santonin, I overlooked, as I here desire to be particu- 

 larly noted, the fact that objects appear yellow, not only by direct, but 



* Archivfur Mikroskop. Anatomie, Band ii., p. 206. 



t Wiirzburg naturwiss. Zeitschrift, Band iii., p. 10, 1862. 



J Journal of A natomy and Physiology, No. 1, p. 104, 1866. 



Loc. cit.j pp. 103 and 104. 



|| See my treatise above cited, Ueber den gelben Fleck der Retina; seinen 

 Einfluss auf normales Sehen und auf Farbenblindheit. (" On the yellow 

 spot of the Retina : its influence on normal vision, and upon colour- 

 blindness.") 



