The retina was a favorite subject for Cajal due to its accessibility, its orderly organization in alternate layers of cell bodies and intercellular contacts, and the easy identification of the main direction of nervous signal flow. Above is his first published illustration of the mammalian retina. His faithful depiction here highlights several structures that are now known or hypothesized to contribute to the optical properties of the tissue, in addition to their roles in visual signaling. The Müller cell (right portion of diagram), with its vertically oriented structure spanning the full thickness of the retina, has been proposed to guide light towards cone photoreceptors, thereby ensuring efficient photon delivery. Cajal also noted that the nuclei of rod photoreceptors (f) differ from those of their cone counterparts (e): animals with good night vision, like the dog from which this image was drawn, have evolved distinct chromatin patterns in their rod nuclei to facilitate light passage. In addition, Cajal highlighted in these photoreceptors large mitochondrial aggregates (l and n), which have been recently shown to act as microlenses concentrating light towards the absorptive photopigment in the outer parts of the cell.

3D print by Jeremy Swan based on an original illustration by Santiago Ramon y Cajal. Courtesy of the Cajal Institute, Spanish National Research Council or CSIC©