Sensation

Collier's New Encyclopedia (1921)

the change in consciousness which results from the transmission of nervous impulses to the brain. Such impulses may be generated within the nerves themselves (but only in diseased or may be produced by conditions), stimuli applied to such parts of the body as are provided with nerves. Such nerves are often styled sensory or afferent. It must be remembe remembered, however, that af- Ises are constantly being carferent impulses 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 2 the various senses of smell, sight, hearing, taste, touch, temperature (or (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 peculiar 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 papillæ in the tongue for taste; and the Pacinian corpuscles and the special ramifications of the cutaneous nerves in the epidermis for touch. The integrity of these and of the special non-nervous apparatus with which they are connected is necessary for the production of a sensation. Thus, the transparent media of the eyeball, and the rods and cones of the the eyebessential to the production of a visual sensation. In proportion as they are abnormal, the sensation is imperfect. Further, each end organ can be thrown into action only by certain kinds the connection with them convey those impulses only of stimuli, and nerves in which give rise to their own special varieties of sensation. The retina can only be stimulated by waves of light, never ig 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 affect any_object or to overcome any resistance. It would appear likely that the nerve endings connected with this sense are situated in the muscles and joints, and that these are stimulated by , tendons, changes in movement and mutual pres sure these structures. The sensation of pain (or general sensibility) 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 With regard to the special end organs. 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 tactile, 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 certainly determined. The path for the mus cular sense-impulses is by manu as lying in the posteni same side. scurity as to the upward sensory conducting 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.