220 HENRY LAURENS AND S. R. DETWILER 
the more peripheral region of the retina could not be demon- 
strated. Near the margin of the retina the visual cells are 
greatly shortened and the pigment is found to extend down 
almost to the external limiting membrane. 
DISCUSSION 
Photomechanical changes of the retina. The question of the 
functional significance of pigment migration and the changes in 
position of the visual rods and cones in light and dark adaptation 
is one about which much has been written (Garten, '07, and 
Hehnholtz, '11). It may therefore appear redundant to add 
anything in the way of a theoretical consideration of this function. 
But there are a few points which still lack clarity. 
In the eyes of those animals in which these changes take jolace 
they represent a mechanism for the adaptation of the eye to day 
and to twihght vision (Herzog, '05; Exner and Januschke, '06). 
In dim light (twilight vision), when the rods alone are capable 
of being stimulated to any degree, or in complete darkness, the 
pigment moves back and leaves free the spaces between the 
rods, resulting in a less complete insulation of these elements. 
Under these conditions, with the entrance of a small amount of 
light the part played by the individual rods in the reception of 
the light is greater, owing to refraction and diffusion, than if the 
rods were covered over by a thick mantle of pigment, in which 
case only the light which passes through the retina in the direction 
of the long axis of the rods could enter them. The presence of 
a reflecting tapetum further enhances the favorable conditions. 
The cones in twilight vision are not functional, on account of 
their high threshold, and they elongate and thus move out of the 
way. The rods contract and thereby optimum conditions are 
presented for their stimulation. 
In bright light (day vision) the pigment by migrating forward 
protects the rods, which have a low threshold and which have 
been made particularly sensitive by the accumulation of visual 
purple in the dark, from too strong stimulation by absorbing 
the direct and scattered light. The rods elongate, while the 
less sensitive cones are drawn out of the pigment by the con- 
