PHOTOMECHANICAL CHANGES IN THE RETINA 541 
Among mammals, the results of Stort (’87) stand alone in 
ascribing striking positional changes to the cones. This worker 
found the cone myoids of the pig to measure 5.7 » in darkness and 
2.3 w in light. Chiarini (’06) obtained negative results on the 
dog’s eye, although Garten (’07) working on apes, in a series of 
careful experiments, has been able to demonstrate slight changes 
in the region of the fovea. Garten suggests the possibility of the 
changes in all the retinal elements of mammals occurring so 
quickly that the slowness of penetration of the fixing fluid fails 
to preserve them in an extended condition. The rapidity of 
movement is perhaps correlated with the rich blood supply 
which these retinas receive in contrast with the condition in the 
lower vertebrates. It is significant to note in this connection 
that the eel’s retina, in which movements of the cones were not 
demonstrable, is the only one among the lower vertebrates which 
is supplied with blood vessels. 
The more distal portion of the inner member of the cone cell, 
the ellipsoid (ef. figs.3 and 4, ell. con.), has been recorded as chang- 
ing its shape. Thus Stort (’87) observed that the ellipsoid of the 
frog was broader and shorter in the light, and conceived of the 
change as being of an entirely passive nature. Chiarini (04 a) 
found the same condition and assumed that the lateral pressure 
of the rods on extended cones caused the elongation of the ellip- 
soid in the dark. Both of these views were in opposition to that 
of Angelucei (90), according to whom the ellipsoid and outer 
member actively shared in causing the movements of the cones. 
Hence it is seen that the greatest contractility of the cone 
myoid, through the action of light, is found in fishes, although 
well defined responses are also shown in the frog, toad, and in 
some birds, including one which is nocturnal in its habits. Among 
reptiles and mammals, changes in the length of the cone myoid 
are very difficult to observe, yet in both groups, responses to 
light have apparently been detected in a few instances. Changes 
in the outer member and in the ellipsoid of the vertebrate cone 
are presumably of a passive nature. 
The visible response of the vertebrate rod to the action of 
light is not identical throughout the various classes. Among 
