PHOTOMECHANICAL CHANGES IN THE RETINA 547 
and the degree of migration is limited, and in the rather heavily 
pigmented retinas of birds. 
The movements of the cone cells are of doubtful significance 
with respect to light perception. Angelucci (90) naively sup- 
posed that the positional changes in the visual and pigment 
cells directly induced the sensation of light. This view has been 
sharply criticized since it fails signally to account for many 
commonplace facts of retinal physiology. The slowness of the 
reactions, the equal responses of the elements to colored lights, 
the absence of movements among mammals and the changes 
induced by stimulating agents other than light which do not 
produce the sensation of light, are the more important objections 
that have been advanced. 
Herzog (05) and Exner und Januschke (’06) have developed 
the theory that in dim light, when rod vision occurs in the sense 
which Max Schultze uses the term, the cones are elongated in 
order that the rods may fully utilize the ight rays which enter 
the eye.?. It must be assumed in such a theory that the cone 
cells remain elongated in very dim light. This, however, as 
Garten and Weiss showed on Abramis, is not true, and accord- 
ingly the whole theory is fundamentally undermined.* 
Granting that the movements of the retinal elements may 
have a significance in the physiology of light perception, the 
nature of which is yet quite obscure, I nevertheless believe that 
from the extent of our present knowledge the responses of these 
elements can only be interpreted in terms of the action of definite 
stimulating agents upon protoplasmic cells. The acceptance of 
such a conservative attitude does not entail the abandonment 
of theoretical suggestions provided they are based on. sound 
‘In the same way, the elongation of the numerous small rods of fishes and 
birds in the light, might be interpreted as a response which serves to remove the 
rod cells from the immediate vicinity of the retracted cones and thereby allows 
more direct illumination of the latter. However this may be, it is certain that 
in the highest vertebrates, where vision is best developed, the movements of the 
visual cells are very slight, if indeed they occur at all. 
5 If the rods of fishes and birds were also shown to move (elongate) in dim 
light, resulting in their removal far above the external limiting membrane, the 
last support of this theory would be destroyed. 
