This is a projected image (focus stacked) of the vascular branches in the retina at the back of the eye. While all mammalians have vasculature behind the retina (choroid) that supplies it with oxygen and nutrients and takes away waste products and CO2, not all mammalians have vasculature inside the retina, like humans and mice. Mammalians like rabbits and guinea pigs do not have vasculature inside the retina, and we do not, as a scientific community understand why this is, as the psychophysical performance of these retinas does not seem to be that different.