SENSATION, the change in conscious ness which results from the transmission of nervous impulses to the brain. Such impulses may be generated within the nerves themselves (but only in diseased conditions), or may be produced by stimuli applied to such parts of the body as are provided with nerves. Such nerves are often styled sensory or afferent. It must be remembered, however, that af ferent impulses are constantly being car ried to the brain from all parts of the body, resulting in motor and other acts necessary to our life, without exciting any sensation at all. It is through our sensations that we gain our knowledge of the external world, and of the state of our body. The means by which these are produced are the elaborate nervous mechanisms developed in connection with the various senses of smell, sight, hear ing, taste, touch, temperature (or heat and cold), pain or general sensibility, the muscular sense, and those of hunger and thirst. For each special sense there is a particular nerve center (see BRAIN) ; and each special sense has its own pecu liar end organ; the special endings of the olfactory nerves in the upper part of the septum of the nose for that of smell; the retina in the eyeball for sight; the rods of Corti in the cochlea for hearing; the taste bulbs and the fibrils in the fungiform papillce in the tongue for taste; and the Pacinian corpuscles and the special rami fications of the cutaneous nerves in the epidermis for touch. The integrity of these and of the special non-nervous ap paratus with which they are connected is necessary for the production of a sen sation. Thus, the transparent media of the eyeball, and the rods and cones of the retina are all essential to the production of a visual sensation. In proportion as they are abnormal, the sensation is im perfect. Further, each end organ can be thrown into action only by certain kinds of stimuli, and the nerves in connection with them convey those impulses only which give rise to their own special va rieties of sensation. The retina can only
be stimulated by waves of light, never by those of sound, and the optic nerve if stimulated directly can give rise to visual sensations only.
The muscular sense is that by which we are made aware of the position of any part of the body, by which we gauge the amount of movement necessary to af fect any object or to overcome any resis tance. It would appear likely that the nerve endings connected with this sense are situated in the muscles, tendons, and joints, and that these are stimulated by changes in movement and mutual Ares. sure in these structures.
The sensation of pain (or general sen sibility) is produced when pressure on a part, or when the temperature of a body applied, exceeds certain limits. Painful sensations may result from excessive stimulation of a sensory nerve at any part of its course, which would seem to point in favor of the non-existence of special end organs. With regard to the paths by which these various impulses reach the brain, we know (if we except the fifth cranial and the vagus nerves) that they reach the spinal cord by the posterior roots of the spinal nerves, and that those impulses which produce tac tile, thermal, and painful sensations for the most part (though this has recently been questioned) travel up the side of the cord opposite to that at which they entered, but their exact course is not cer tainly determined. The path for the mus cular sense-impulses is by many regarded as lying in the posterior columns of the same side.
Within the medulla oblongata the ob scurity as to the upward sensory con ducting tracts is even greater than in the cord, not only in the case of the senses above mentioned, but also of the sense of hearing and taste.