The vertebrate retina is another good example of Cajal’s Law of Dynamic Polarization. Because he knew from which direction the stimulus to the photoreceptors (at the top of the diagram) originated, it was clear that information ought to be transmitted inwards, through axons of the photoreceptors to the dendrites of bipolar cells, then to ganglion cells, and onward into the optic nerve, as noted by the arrows on the diagram above.
In addition to the usual retinal information flow, Cajal illustrated some more unusual directions for information transmission.
He noted information flowing from c to I, along the long axon of a horizontal cell, although this subsequently has not been found to be a viable signal path. Likewise, visual signals in manyamacrine cells, shown to the right of D, need not pass through the cell body, contradicting the apparent intent of Cajal’s arrows.
While Cajal may have erred in these two instances, he correctly observed another unusual signaling pathway between G and H: these axons from the visual cortex are returning to the retina and transmitting information to the amacrine cells. This phenomenon has indeed been observed in present day, but not in all vertebrates; significant centrifugal input is a hallmark of avian eyes in particular.
