It is well known that anatomically the retina is not only a receptor organ but a "true nervous center," to use the words of Ramón y Cajal, who discovered and traced its synaptic connections. However, there has been little appreciation of the part this synaptic structure plays in the mechanism of vision. This is due in part to the fact that the method of approach to visual problems has been chiefly by the interpretation of sensory data, which used alone do not allow of any differentiation between retinal happenings and processes in the brain; and in part to the fact that the attractive simplicity of the theory of duplicity has focused attention on the mechanism of the rods and cones and has hidden from many workers the real complexity of the problem. Recent progress in two related fields of physiology, however, emphasizes the importance of the synaptic functions