Sensation may be defined as 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), 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 afferent impulses are constantly being carried 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, hearing, taste, touch, temperature (or heat and cold), pain or general sensibility, the muscular sense, and those of hunger and thirst. While we have obtained a certain amount of knowledge of the nature of a nervous impulse resulting in muscular movement, the same cannot be said of a sensory nervous impulse. We cannot measure the rate of its transmission along the nerve; we have absolutely no notion of the change, if it is legitimate to employ such a term, by which a sensory nervous impulse becomes transformed into a sensation; we as yet know imperfectly at what part of the brain this transformation takes place, or by what paths the sensory impulses travel towards the brain. It has long been known, however, that if the posterior division of the bundle of nerve fibres constituting that part of the cerebrum which is termed the internal capsule (see BRAIN) is destroyed a complete loss of sensation on the opposite side of the body results. This has led to the investigation of the posterior part of the cerebrum, to which these fibres more specially pass, for 'centres,' the destruction of which would involve loss of the various forms of sensibility. The results are as yet conflicting. It is generally agreed that the 'centre' for sight is in the inner side of the occipital lobe; but there is in the angular convolution another centre in which visual sensations probably undergo a further elaboration. Above the corpus callosum there lies a long convolution, the gyrus fornicatus, the removal of which produces complete loss of tactile sensibility of the opposite half of the body; but in the 'motor area' there are probably other centres, as this is rarely diseased without the production of peculiar modifications of sensation. The centre for hearing has been located in the upper convolution of the temporal lobe, that for smell and taste on the anterior end of the temporal lobe. An exhaustive consideration of the difficulties of the problem will be found in Foster's Text-book of Physiology.
Each of the special senses 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 retina are all essential 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 of stimuli, and the nerves in connection with them convey those impulses only which give rise to their own special varieties 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. This law holds good also for the cutaneous nerves, there being reason to believe that the end-organs for touch are not the same as those for temperature. It is probable even that there are separate nerves for the perception of heat and of cold. All parts of the end-organs are not equally sensitive or capable of being similarly affected by the same kind of stimulus. Thus, the same substance may appear sweet at the anterior part and bitter at the posterior part of the tongue. The retina is most sensitive at the yellow spot, while the various colours are perceived over areas which differ considerably. Tactile sensibility varies exceedingly. The points of a compass can be felt as double at a distance of about part of an inch at the tip of the tongue, of of an inch on the front of the tip of the forefinger, but only at a distance of more than inches on the back.
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. Many authorities deny the existence of such a special sense altogether, and believe that it is merely a form of ordinary tactile sense. But that this is not so is frequently illustrated in a disease of the spinal cord, locomotor ataxy, in which there is, when the eyes are shut, a more or less complete absence of knowledge of the position of the limbs and of the power of regulating their movements, although the cutaneous tactile sensibility may be quite normal. 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 pressure in 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. What was at first a sensation of touch, or of heat or cold, becomes replaced by a painful sensation. It is very difficult to be certain whether there are special end-organs for the reception of such impressions. It is quite beyond doubt that the olfactory, optic, auditory, and gustatory nerves do not transmit painful impressions, and it might be supposed that the analogy would hold in the case of the senses of touch and temperature. It is found in some cases of disease that the prick of a pin may be felt as a touch one or two seconds before it becomes painful. But this is open to the explanation that the delay in the latter case may take place in the spinal cord owing to a different path of conduction rather than to a difference in the time that it takes to stimulate the end-organ. Painful sensations may result from excessive stimulation of a sensory nerve at any part of its course, which would seem to point in favour 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 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 muscular sense-impulses is by many regarded as lying in the posterior columns of the same side (see SPINAL CORD).
Within the medulla oblongata the obscurity 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. It is probable that the paths are not continuous, but are frequently interrupted by nerve cells, through the agency of which the nervous impulses undergo successive elaborations before reaching the cerebral cortex. The evidence for the existence of special upward conducting strands in the spinal cord, and the connections of these, will be fully considered under the article SPINAL CORD.
See Landois and Stirling's Physiology (for details of investigations in various senses); Quain's Anatomy (for end-organs and nervous apparatus); Foster's Physiology (for general consideration of the problems involved). See also TASTE, NOSE, EYE, VISION, PERCEPTION, NERVOUS SYSTEM, PSYCHOLOGY, Vol. VIII. p. 474.