RETINA OF ALLIGATOR MISSISSIPPIENSIS 221 
traction of their myoids and are thereby made freel}^ accessible 
to the stronger Ught stimulus, thus presenting optimum con- 
ditions for their stimulation. 
The fact that these photomechanical changes have not been 
demonstrated in the eyes of man and mammals does not con- 
stitute a denial of their taking place in the eyes of other animals, 
and there is no ground against explaining as above the phenomena 
in such animals. Adaptation of the mammalian eye is brought 
about by different means, viz., the formation and bleaching of 
visual purple. 
The process of light and dark adaptation is not dependent upon 
the phototropic movements of the elements of the retina, but 
these movements may take part in the process of adaptation, 
that is in the formation and bleaching of visual purple. That 
the pigment, as such, has anything to do with the formation of 
visual purple is not probable, because visual purple occurs in 
the eyes of albinos and in the pigment-free portions of the retina 
of many animals, for example, the cat. The significance of the 
pigment is probably a purely optical one concerned with the 
absorption of scattered light. 
In connection with the function of the retinal epithelium in 
the formation of visual purple, the paper by Kolmer ('09) is to 
be noted. Kolmer finds numerous droplets and granules on and 
between the rods in the retina of various vertebrates. These he 
regards as secretion products of the pigment epithehum. In 
the retina of frogs kept in darkness the droplets and granules are 
larger and more numerous than in the illuminated retina, and 
after illumination of the eye with direct sunlight are not to be 
seen at all. Since they are lacking in the eye of hzards and 
snakes, Kolmer assumes that they have something to do with 
the visual functions of the rods, the organs of twihght vision, and 
perhaps with the appearance of visual purple. 
It is interesting to note that pigment migration is still assumed 
by some to take place in the human eye. Ramon y Cajal ('11, 
p. 363) believes that the function of the pigment is to prevent 
the impression of halo, and that the dazzling sensation which one 
experiences on going from a dimly hghted place into a bright one 
