Sensation
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 conditions), stimuli applied to such parts of the body as are provided with nerves. Such nerves or may be produced by are often styled sensory or afferent. It must be remembered, however, that af- are often styled sensory or afferent. It as are provided with nerves. Such nerves stimuli applied to such parts of the body conditions), or may be produced by ried to the brain from all parts of the Ises are constantly being carbody, 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, hearing, taste, touch, temperature (or and cold), pain or general sensibility, the ing, taste, touch, temperature (or heat the various senses of smell, sight, hear- mechanisms developed in connection with are produced are the elaborate nervous the cord opposite to that at which they been questioned) travel up the side of (or heat SENSATION thirst. For each special sense there is 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 a particular nerve center (see BRAIN); fications of the cutaneous nerves in the retina in the eyeball for sight; the rods of Corti in the cochlea for hearing; the taste epidermis for touch. The integrity of bulbs and the fibrils in the fungiform papillæ in the tongue for taste; and the Pacinian corpuscles and the special ramiparatus with which they are connected these and of the special non-nervous ap- epidermis for touch. The integrity of sation. Thus, the transparent media of is necessary for the production of a senthe eyeball, and the rods and cones of the of a visual sensation. In proportion as is necessary for the production of a senthey are abnormal, the sensation is imperfect. Further, each end organ can be the the eyeball, and the rods and cones of the which give rise to their own special vathrown into action only by certain kinds with them convey those impulses only of stimuli, and the nerves in connection these and of the special non-nervous apig rieties of sensation. The retina can only which give rise to their own special va- with them convey those impulses only visual sensations only. rieties of sensation. The retina can only of stimuli, and the nerves in connection thrown into action only by certain kinds by those of sound, and the optic nerve part of the body, by which we gauge the The muscular sense is that by which we are made aware of the position of any joints, and that these are stimulated by amount of movement necessary to afchanges in movement and mutual pres fect any_object or to overcome any resistance. It would appear likely that the nerve endings connected with this sense are situated in the muscles , tendons, and sure these structures.
With regard to the joints, and that these are stimulated by are situated in the muscles, tendons, and nerve endings connected with this sense tance. It would appear likely that the applied, exceeds certain limits. Painful sensations may result from excessive stimulation of a sensory nerve at any The sensation of pain (or general senpoint in favor of the non-existence of a part, or when the temperature of a body special end organs. part of its course, which would seem to paths by which these various impulses reach the brain, we know (if we except that they reach the spinal cord by the the fifth cranial and the vagus nerves) special end organs. With regard to the point in favor of the non-existence of part of its course, which would seem to paths by which these various impulses 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 certainly determined. The path for the mus tile, thermal, and painful sensations for same side. the most part (though this has recently SENUSSI 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.