VISCACHA or BISCACHA, a large South American bur rowing rodent belonging to the family Chinchillidae. The viscacha (Viscaccia) is distinguished from the other members of that group by having only three hind toes; it is the heaviest-built and largest member with smaller ears than the rest. It has a long tail and shaggy fur; the general colour of the latter being dark grey, with black and white markings on the face. Viscachas inhabit the South American pampas between the Uruguay river and the Rio Negro in Patagonia, where they dwell in warrens covering from too to 200 sq.f t. and forming mounds penetrated by numerous burrows. The ground around the "viscachera" is cleared from vegetation, the refuse of which is heaped upon the mound. Any thing the rodents meet with on their journeys, such as thistle stalks or bones, are deposited on the viscachera. In frequented districts they seldom emerge till evening. Their chief food is grass and seeds, but they also consume roots. (See RODENTIA.) VISCERAL SENSATIONS, the sensations that arise from the viscera and other internal bodily organs and tissues. The visceral sensations belong to the general class of organic sensa tions (q.v.) and are distinguished among them only by the seat of their origin. They are characterized by their paucity of quali tative variety and by their functional importance in consciousness. On the side of quality they include only cold and warmth, and the dull pressures and the aches that come from muscle and other tissue beneath the skin. (Cf. SKIN, SENSORY FUNCTIONS OF.) On the side of function they form the sensory basis for many of the vague awarenesses or "conscious attitudes" that are ever present in the mental life. For instance, recognition is a perception coloured by a feeling of familiarity, and this feeling often has its seat in the viscera although it has no fixed sensory basis. Visceral sensations are also functionally important as carrying much of the conscious organic reverberation that is characteristic of strong emotion.
The oesophagus and stomach are sensitive to pressure on dis tension and to ache on extreme distension, as well as to warmth and to cold. This fact can be brought out only by research, because the stomach is ordinarily not often distended, without distension of the bodily walls and is altered appreciably in tem perature only by large quantities of hot or cold material The common belief in the insensitivity of the oesophagus comes about because its sensations are ordinarily localized as above or below the bony chest wall, i.e., either in the throat or in the stomach The intestines are less accessible to experimentation, but it is probable that they are sensitive to pressure and pain like the stomach, but that excitation of these sensations is rare.
It is well known that in surgical operations the viscera appear to be almost entirely insensitive. The meaning of this finding is
that cutting and ordinary pressure are not adequate stimuli to the sensations that they arouse on the skin. Apparently distension or muscular contraction of the viscera themselves are the proper stimuli. The peritoneum, its extension in the mesentery, and pos sibly the pleura are very sensitive to pain, even upon cutting.
The more usual attack upon the problem of visceral sensa tions has been by way of the perceptions which they mediate. These perceptions seem to involve no unique qualities, but never theless to carry very specific significance for the organism. Thirst arises from dryness of the membranes of the oral cavity, and is ordinarily caused by a lack of water in the system. It is no more a new quality than is the perception of dryness on the skin. Hunger is an ache of a peculiar temporal pattern, for it is caused by certain slow rhythmic contractions of the stomach that appear in the absence of stomachic contents or at regular intervals by habit. Any substance introduced into the stomach to inhibit these contractions abolishes hunger. The alimentary experiences of fullness, repletion and nausea, and the experiences of the excre tory processes, are simply internal perceptions, in terms of pres sure or pain, of the states or processes to which they correspond. Appetite is the desire for food in the absence of hunger, and is ordinarily associated with all normal food-taking, since the first food that enters the stomach inhibits the hunger contrac tions. Appetite, however, seems to have no peculiar sensory or perceptual basis at all. Its mechanism is purely unconscious as in the instincts and habits, although of course the eater perceives his desire for food by perceiving his behaviour toward it. Some psychologists have thought that there are sensations of oppression from the heart and of stuffiness from the lungs, but the matter remains undetermined. Sexual experience involves unique per ceptual patterns of pressure and pain. Like appetite, however, the psychology of sex is best understood functionally as an instinctive urge and a form of behaviour, although to the indi vidual the earlier perceptions of bodily state may seem to be the causes of subsequent behaviour. That emotion involves visceral sensations is well known, but we are still ignorant of their degree and nature, although a great deal is now known of the visceral state during emotion (q.v.).
See A. F. Hertz, Sensibility of the Alimentary Canal (i9i1) ; A. J. Carlson, Control of Hunger in Health and Disease (Chicago, 1916) ; W. B. Cannon, Bodily Changes in Pain, Hunger, Fear and Rage (London and New York, 1915) ; E. G. Boring, American Journal of Psychology, vol. xxvi., pp. (1915) ; vol. xxviii., pp. 453 (1917) ; Psychological Review, vol. xxii., pp. (1915).
(E. G. Box.)