NERVOUS CONSTITUENTS OF THE RETINA. 261 



is impossible for the light to reach the outer segment without tra- 

 versing the spheroid in question. (See fig. 358, .) 



There are some colourless spheroids of this nature, but the majority 

 are chrome, Naples yellow, gamboge, or greenish-yellow and orange, 

 between which others are scattered at regular distances, of a ruby tint. 

 Their spheroidal form must exert a refractile influence upon the 

 course of the light traversing them, whilst certain portions cf the 

 rays corresponding to their colour must be absorbed. Their presence, 

 as Hensen * first pointed out, renders it highly probable that it is by 

 the external segments that perception is accomplished, since only in 

 this case can the object of the selective absorption be understood. 

 The circumstance that they are only present in the cones, and not in 

 the rods, proves that the former have more to do with the perception 

 of colour than the rods, which is also probable on other grounds for 

 Mammals and Man.f That these spheroids occupy the whole thickness 

 of the internal segment, demonstrates further r as Krause j has stated, 

 that we have here a solution of continuity, and that the outer seg- 

 ments are not of a nervous nature, even if this be true in regard to 

 the inner segments. As a result of my discovery,- that there are fibres 

 running upon the surface of the internal segments, which are prolonged 

 upon the outer ones, and are not interrupted by the fat spheroids, I 

 believed that I might be able to point out the mode in which the 

 outer segments take part in the process of perception. The fresh 

 complications, however, resulting from the discovery of the internal 

 fibre system of the rods and cones, does not at present allow us to 

 draw any definite conclusions upon the subject. 



As in Birds, so also in Reptiles, oil globules are found in the cones ; 

 in the Chelonia, in addition to a few colourless globules, there are 

 others that are red, orange, and yellow. Lastly, the very small cones 

 of the Anourous Batrachians are each characterized by presenting a 

 strongly refractive spheroid which is either colourless or of a clear 

 yellow hue. They are not present in Fishes, unless we may consider 

 them to be indicated by certain bodies observed by Leydig in the 

 Sturgeon. || Many of the cones in Birds (Pigeon) and Lizards contain 

 besides the coloured spheroid some diffused red or yellow colouring 



* Yirchow's Archiv, Band xxxiv., p. 405. 



t Max Schultze, Archiv fur Mikroskop. Anatomic, Band ii., p. 253. 

 Membrana fenestrata, p. 48. 



Archiv fur Mikroskop. Anatomic, Bandv., p. 400. 

 || Anatom. Histoloyische Untersuchungen iiber Fische und Reptilicn, p. 0, 

 1853. 



