The present volume covers the physiology of the visual system beyond the
optic nerve. It is a continuation of the two preceding parts on the
photochemistry and the physiology of the eye, and forms a bridge from
them to the fourth part on visual psychophysics. These fields have all
developed as independent speciali- ties and need integrating with each
other. The processing of visual information in the brain cannot be
understood without some knowledge of the preceding mechanisms in the
photoreceptor organs. There are two fundamental reasons, ontogenetic and
functional, why this is so: 1) the retina of the vertebrate eye has
developed from a specialized part of the brain; 2) in processing their
data the eyes follow physiological principles similar to the visual
brain centres. Peripheral and central functions should also be discussed
in context with their final synthesis in subjective experience, i. e.
visual perception. Microphysiology and ultramicroscopy have brought new
insights into the neuronal basis of vision. These investigations began
in the periphery: HARTLINE'S pioneering experiments on single visual
elements of Limulus in 1932 started a successful period of neuronal
recordings which ascended from the retina to the highest centres in the
visual brain. In the last two decades modern electron- microscopic
techniques and photochemical investigations of single photoreceptors
further contributed to vision research.