The mammalian cornea exposes for examination an uncomplicated field of the ramifying type of afferent innervation which mediates pain. Thus it offers a favorable site to attempt to define the unit for sensory reception in this, the most widespread sensory modality. To this end arrangements were made to amplify and record oscillographically action potentials in the long ciliary nerves of cats excited by mechanical stimulation of the cornea. In some instances, the nerves were dissected until only a few fibers registered. By these procedures a neuro-sensory unit was isolated the terminal ramification of one nerve fiber. The neuron unit of afferent innervation was distributed over a quadrant or more of the cornea, spreading onto the adjacent sclera and conjunctiva. This large terminal was spatially differentiated such that lowest threshold and highest frequency of response were obtained in the central area, the one increasing and the other diminishing toward the periphery in all directions, while adaptation was most rapid both on the periphery of the terminal and superficially. The total corneal sensory mechanism is an aggregate of such neuron units and not a continuum. Viewing the nerve ending as all the terminal tissue of one nerve fiber, the properties of this receptor then appear as peripheral determinants of the central processes resulting in the sensation of pain. Some observations are included on afferent impulses excited by stimulating the sclera, iris, and lens, and in response to increased intraocular pressure.