RETINAL PHOTOMECHANICAL CHANGES 147 



rounding the visual cells and blocking off from them any light rays 

 which approach them at angles to their axes. Where the myoids are very 

 slender (as in most fish rods) the expanded pigment may close in densely 

 enough between the rod ellipsoids and the limitans to shut off even the 

 axial rays from the percipient outer segments of the rods (Fig. 62) . 



Fig. 63 — Visual-cell migrations in a catfish, Ameiurus nebulosus. x 500. 

 After Welsh and Osborn. 



a, depigmented section of light-adapted retina, showing rods elongated toward pigment 

 epithelium and cones retrarted toward external limiting membrane. 



b, depigmented section of dark-adapted retina; cones elongated, rods retrarted. 



Visual-Cell Movements — Cones always escape being thus shielded to 

 any extent by the expanded, light-adapted pigment. They either sit, 

 permanently, directly upon the limitans or, if migratory, contract into 

 that position — away from the advancing pigment — in the light. Rods how- 

 ever, if they migrate at all in bright light, do so in the direction toward 

 the pigment (Figs. 62 and 63), The effective covering of the rods by 

 pigment is thus the sum of the pigment expansion and the elongation of 



