STUDIES IN THE RETINA. 69 



usual description they are oE the nature of cuticular forma- 

 tions. This is a very natural summing up of the facts — 

 (1) that they are almost certainly the end organs of tlie 

 nerveSj and (2) that their tips are filled with refractive 

 matter of the nature of keratin. But the parallel with 

 cuticular cells, although justifiable, is not very close. As 

 protoplasmic vesicles thrust out against the pigment cells 

 they absorb the pigment granules and (unless the quantity 

 absorbed be too great, and its colour too intense) clarify them 

 somewhat as the stratum lucidum of the epidermis receives 

 and clarifies the pigment brought to it through the skin. 

 Here, however, the parallel ceases, for while the cells of the 

 cuticle perish with the waste matter they receive, and ulti- 

 mately fall away as horn-cells, the rods get rid of their refrac- 

 tive contents, which stream away through the retina. 



The working out of the finer structural details of the rods, 

 taken up where the subject was left by Max Schultze thirty 

 years ago, need not be repeated here, but one or two of the 

 more important corrections of the current doctrine may be 

 mentioned. 



What are called the '' cones " of the vertebrate eye, to 

 which special functions distinct from those of the rods have 

 been assigned, are not always analogous structures. 



In the Amphibia they are the early stages in the forma- 

 tion of new rods, and their form-phases are due to the 

 squeezing of new vesicles between the already existing rods. 



In the fish analogous stages appear in very young eyes, 

 but in older eyes the inner limbs of the earlier formed rods 

 swell to such monstrous sizes that the conditions of the rod 

 layer are altered, and the protrusion of new vesicles can no 

 longer result in the formation of the same cone stages. 

 The rods with the swollen inner limbs have been regarded as 

 " giant cones,'^ although presenting no analogy whatever 

 with the cones in the frog. 



In the Primates, what are usually called the cones are, as 

 in the fish, merely rods with swollen inner limbs. In the 

 centre of clear vision, where the pigmentary matter is 



