Sensory nerves are divided into two classes, those of general sensibility and those of special sense. The nerves of general sensibility are distributed to the skin, muscles, or viscera, and convey influences to the brain which give rise to sensations of touch, heat, &c., or to those vague sensations, not definitely localised, which we include under the name of the muscular sense. The nerves of special sense are endowed with special and individual physiological properties. When a nerve of this order is irritated in any way, either by mechanical, chemical, or electrical stimuli, an influence is conveyed to the brain which gives rise to the same kind of sensation as that produced by the normal stimulus on the terminal organ. For example, pressure on the eyeball, as shown by Newton and Young, electrical stimulation by a continuous current, as demonstrated by Pfaff, Helmholtz Ritter, Purkinje, Du Bois-Reymond, and Schelske, produce many of the phenomena of vision, including not only the perception of light, but the perception of various colours and tints. But while this is the case, it is equally certain that each terminal organ responds to its normal stimulus. Thus the retina, though capable of stimulation by pressure or electricity, is specially fitted by its histological structure for the reception of those minute vibrations of the ether which constitute light. But while the terminal organ is capable of receiving a most delicate action of the normal stimulus, the nerve in connection with it is not so affected. For example, although the retina is affected by light, the optic nerve is not so, as may be proved by Marriotte's well-known experiment, by which it may be demonstrated that when the image of an external object falls on the entrance of the optic nerve, there is no corresponding sensation. The nerve is thus insensible to the normal stimulus of the sense organ, the retina.