Page:Proceedings of the Royal Society of London Vol 69.djvu/139

each component of the visual field the intensity of the visual impulse transmitted to the central organ, much as the iris and the eyelids regulate it for the whole. It is acknowledged that we cannot as yet attribute definite functions to the several elements of the retina, and that we cannot demonstrate a structural continuity between the three layers of which it is composed, although we may conclude that changes set going in the rods and cones sweep through the inner nuclear layer and issue along the fibres of the optic nerve as nervous impulses.* There is, therefore, room between the extreme peripheral structures and the beginnings of the optic nerve, for a structure, which when stimulated by light falling on it may block wholly or in part the im- pulses coming to it from the periphery.

In fig. 4 three similar hypothetical elements of the retina are drawn in diagram side by side. In each A represents the sensitive peripheral structure giving rise, when stimulated by light, to a visual impulse, and

FIG. 4.

D represents the controlling structure, also sensitive to light, the func- tion of which is to block the visual impulse if necessary. D : is repre- sented as having cross-connections with D 2 , and vice versd. Suppose a flash of light, FI, coming in the direction of the arrow to fall sud- denly on all three retinal elements. It gives rise in AI, A2, and A 3 to a condition of excitation lasting longer than the flash itself, and dying gradually away, arousing in the central organ the sensation of light during its earlier and of the " positive after-effect " during its later stages. But at the same time D!, D 2 , and D 3 are excited, and

• Foster, ' Text-book of Physiology,' Part IV, p. 1203.