By a comparison of the retina of a frog kept in darkness with one exposed to light, it has been found that light causes not only the internal segments of the conesl and rods2 but also the pigment-cells of the retina to contract, so that the external parts of the rods and cones as well as the pigment are drawn away from the external towards the internal limiting membrane of the retina (Fig. 76b). A similar effect is produced by heat.2 The retina of a frog which has been tetanised by strychnine in complete darkness has an appearance

1 Tweedy, loc. cit. 2 Jessop, Proc. Boy. Soc., 1885.

similar to that of a retina which has been exposed to full daylight, the strychnine having caused extreme contraction of the rods, cones, and pigment-cells (Fig. 76c). A similar effect is produced by tetanising the eye itself either by induced currents in the dark, or while it is still in the head or immediately after its excision. Curare neither hinders this action nor produces it.

a

a

b

b

c

c

Fig. 76. - Shows the position of the rods and pigment-cells in the retina of the frog : a, after the animal has been kept in complete darkness for one or two days; b, after it has been exposed to diffused daylight for five or ten minutes, after being kept in darkness for twenty-four hours; c, after exposure to light as in b, but for half an hour instead of a few minutes. This also represents the position of the rods and pigment-cells in strychnine tetanus.