There are many sensations of impressions, besides those derived from the five senses, which have been either in correctly arranged with those of touch, or have been alto gether overlooked. Of these we may point out three ; the sensations which attend the motion of the joints, the sen sations of heat and cold, and that of hunger. The least re flection upon our own feelings will convince us, that these are quite different from the impressions of resistance pro duced upon the ends of the fingers by a hard body. Some metaphysical writers have supposed, that we derive our idea of what is termed the third dimension of bodies, not so much by the touch, as from the sensations which we ex perience by opening the joints of the hand, and that from the same kind of sensations in the joints of the extremities we derive our ideas of distance and extension. We are probably much assisted by the sight on these occasions ; but with respect to the blind, there is every reason to sup pose that their ideas of tangible distance are principally derived from the associations which they form with parti cular motions of the joints.
The sensations of heat and cold are essentially different from those of touch, as well as from every other class of impressions. It is remarked by Darwin, that the sensa tions of touch, and those of heat and cold, are possessed in different degrees of acuteness by the same part of the body, and that the parts which are the most sensible to each class of sensations are different. Ile also relates a case, and other cases of a similar nature are upon record, where the body became insensible to mechanical impulse, but retain ed its full sensibility to heat and cold.
The sensations of heat and cold are caused, ultimately, by the passage of caloric into or out of the body ; but the degree of effect does not depend upon the absolute quan tity of it, but upon the quantity gained or lost, compared to the previous temperature of the body. The general fact seems to be, that we feel the sensation of heat, when the body is receiving caloric, more rapidly than it did the mo ment before ; and the sensation of cold, on the contrary, when it is losing its heat more rapidly ; and that these sen sations may take place, even although the body may be, in both instances, absolutely gaining or losing caloric. There are, however, many individual cases in which this rule does not apply, where sensations of heat and cold are quite in dependent of the actual temperature of surrounding bodies, but depend upon particular conditions of the sanguiferous and nervous systems. Some of these, where they are con
nected with morbid states of the body, arc among the most curious and inexplicable parts of pathology.
The peculiar sensations connected with the stomach, pecially that of hunger, is essentially different from that of touch, although frequently confounded with it. Yet every one who reflects upon his own feelings, must be conscious that hunger no more resembles the resistance of a hard body pressing upon the skin, than sight resembles pres sure upon the eyeball. The immediate cause of the sen sation of hunger has been much discussed, and there are two opinions that have been very prevalent, one among the mechanical, the other among the chemical pathologists. The former of these sects ascribed it to the friction of the sides of the stomach against each other, an opinion which is disproved by the slightest acquaintance with the anatomy of the organ. The chemical sect accounted lor the sensa tion of hunger to the action of the gastric juice, as they conceived, tending to corrode the part by its powerful ac tion upon organized matter. But upon this hypothesis we may observe, that we have no reason to suppose that the erosion of acrid chemical agents is at all similar to that of the gastric juice, or that any degree of actual erosion takes place upon the stomach during the life of this part. the whole, we are to regard the sensation of hunger as a specific effect produced upon the nerves connected with the stomach, the final cause of which is sufficiently obvious, but with the efficient cause of which we are to tally unacquainted. We may remark with respect to the stomach, that there is no organ possessed of specific sen sations, the feelings of which are more under the influence of habit. This is apparent, both with respect to the time of taking food, and the quantity of it which is required to satisfy the calls of hunger. The accidental associations of the stomach are no less remarkable than those of the pa late, so that we often experience the sensation of nausea, there is nothing in the body in question to produce except its possessing a taste or odour, or some other external quality, which resembles a substance that has a directly emetic effect.