Pyramidal Neurons of a Duck (1891)

Pyramidal Neurons of a Duck (1891), He arranged his descriptions of the primary visual cortex, the first processing center for visual signals within the brain, according to 9 layers clearly delineated by the types of cells present. A subset of these layers are shown above: medium pyramidal cells (B, D, F) in layer 6 give rise to recurrent axons (a) which reach back to the outermost layer of the cortex, while the giant pyramidal cells (A, E) in layer 7 form horizontal dendrite bundles as well as axons that descend into the lower layers of the cortex. Cajal was the first to realize how short the horizontal axonal/dendritic connections between the cells of the primary visual cortex were, and to hypothesize the significance of this fact: information flows vertically through the layers of the cortex with little lateral spread.

Below: Illustration of Cajal's scientific drawing of Pyramidal Neurons of a Duck by Dawn Hunter, 11" x 14," marker and pen on paper.