The power of tactile discrimination may be conceived to depend in part upon the mode in which the ultimate nerve-fibres are dis tributed in the skin, being greater in propor tion as contiguous parts are supplied from distinct central sources, and less when the central terminations of their nerve-tubes are the same. Thus, if two impressions be made along the course of the same nerve-tube, they will not be felt as two, but as one ; and this probably holds good of the parts of the in tegument supplied with branches from the central axis of any one tube. On the other hand, whenever two impressions be made upon two distinct nerve-tubes, or on the branches proceeding from them, they will probably be felt to be double ; and the dis tance at which these impressions seem to the tactile sense to be, appears to have a relation to the distinctness of the central connections of these nerve-tubes, as appears from the fact that the " limit of confusion " is less across the median line than on either side of it ; that it is less between two parts (such as the lips and eyelids) whose nervous supply is known to be distinct, than on either part separately ; and that it is less between two parts whose nervous supply may be presumed, from their difference of function, to be dis tinct (as the inner and outer surfaces of the lips), than on either part separately. More ever, it would not seem to be improbable that one use of the plexuses from which the limbs are supplied, is to produce such an inter mingling of the fibres from different gan glionic centres, that contiguous portions of the integument shall be connected with cen tres very remote from each other, and their discriminating power thus augmented. No such intermingling takes place in the nerves which supply the trunk, and the tactile dis crimination of its integument is (as we have seen) vastly inferior to that of the extremities. Thus it may happen that the common sensi bility of two parts may be the same, whilst their power of tactile discrimination may differ considerably ; and we may even have the common sensibility greatest where the tactile discrimination is least,— as we experience, for example, on the integument of the face, which is far more sensitive to a blow, and especially to a "fillip," than is the integument of the palmar surface of the fingers, although greatly inferior to them in discriminating power. The actual nervous supply, and the conse quent sensibility, of a part, may be greater in such cases ; but the unity, or close approxi mation, of the source from which this pro ceeds, may prevent its discriminating power from augmenting in the same proportion.
In like manner, we find that the tactile dis crimination of different parts bears no relation whatever to that peculiar modification of common sensibility (which yet appears the exclusive attribute of the external integuments) through which the feeling of " tickling " is excited. For the parts which are most sus ceptible to this feeling, such as the axillm and the soles of the feet, are possessed of a very low degree of discriminating power, and those which possess this power in the highest degree (such as the tips of the fingers) are the least " ticklish." Further, it is worthy of notice that the parts through which that peculiar sensation, which we have termed the genital sense, is specially excited,—namely, the penis and the mammary areola,—are remark able rather for the obtuseness than for the acuteness of their power of tactile discrimina tion.
That it is only through the skin and those parts of its internal reflexions which are in closest proximity with it (especially the lining of the mouth and nostrils), that we can dis criminate tactile impressions, appears from this ; that although the internal mucous and serous surfaces, the fibrous membranes, and the parenchyma of many organs, &c. &c„ are all capable of becoming acutely sensible to pain when irritated or inflamed, yet no foreign substance is ever distinctly felt by the touch through these parts. Thus, although a sensation of a pleasing or a painful nature is excited by certain substances immediately upon being swallowed, all consciousness of their presence (so far as it is dependent upon the sense of touch) soon ceases, and cannot be again recalled by the utmost exertion of the will. Further, a foreign substance, lodged in the alimentary canal, or in the trachea, may give rise to the greatest possible distress, through the irritation it produces ; but though it thus acts upon the nerves of the parts im mediately in contact with it, these nerves convey no idea to the sufferer of the shape or size of the body, or of any other of its physical qualities, concerning which we ceive information through the sense of touch.
Sense of Temperature. — This sense is called into action when there is a difference between the temperature of the sensory organ and that of the surrounding medium, or of substances with which it is specially brought into contact. It is one of which the intensity
is determined, more perhaps than that of any other sensation, rather by the relative than by the absolute condition of the body which excites it. Thus, if one hand be immersed for a time in hot water, and the other in cold, and both then be plunged into tepid water, this will seem cool to the former ant' warm to the latter. So, again, a person coming out of cold air into an atmosphere of moderate tem perature, derives from it the feeling of genial warmth, whilst another, coming into the very same atmosphere from one much hotter, com plains of its chillness. Again, when the tem perature of different substances is compared by the hand, the sense is not so much influ enced by the absolute amount of caloric possessed by each, as by their power of imparting cold or heat to the sensory organ. Hence substances which are good conductors (such as metals or marble) are felt to be colder than those which conduct heat badly (such as wood), although really of the same temperature, because they draw off the heat of the sensory surface more rapidly ; whilst, on the other hand, if both be warmer than the sensory surface, the best conductors will seem to be the hottest, because their caloric is most readily imparted. Further, the sense of temperature is influenced in a remarkable degree by the extent of surface on which the impression is made. Every one is familiar with the fact that hot water in which a single finger may be held without inconvenience, will be felt intolerably scalding when the whole hand is immersed in it. And it has been shown by Professor Weber, that if one vessel of water be heated to 98° and another to 104°, and the whole of the hand be im mersed in the former, while the finger alone is immersed in the latter, a wrong judgment of their relative temperatures will be probably given, that which is really the cooler being pronounced the hotter, on account of the larger extent of surface on which it acts. This mistake was made in some of his experi ments, when the difference was as much as eight degrees ; the cooler water being at 98°, and the hotter at 106°, and yet the former being esteemed the hotter. So, again, the immersion of the entire hand enables minute differences of temperature to be detected, which could not be recognised by the immer sion of a single finger. By the former method, a difference of only one-third of a degree may be distinguished ; the entire hand being im mersed, repeatedly and successively, in two vessels of water, differing only that much in their relative warmth. But it is remarked by Professor Weber, that these minute differ ences are best detected when the medium examined does not fall short of, or exceed very considerably, the usual temperature of the body ; just as the ear can best perceive a difference of tone in sounds which are neither very acute or very grave.* It is a remarkable fact, discovered by Pro fessor Weber, that the left hand is in most persons more sensible to variations of tem perature than the right. Thus, when the hands of a person lying in bed, and having exactly the same temperature, are plunged each in a separate vessel of hot water, the left hand is believed to be in the hotter medium, although the water in which it is immersed is really one or two degrees colder than the other. This difference is the more remarkable, as the power of tactile discrimina tion is usually greater in the right hand ; and it is attributed by Professor Weber to a difference in the thickness of the epidermis, the left hand usually having a thinner epi dermis than the right, especially in the palm, because it is less used. But this will only apply to the hand ; and since (as will lie presently shown) we possess a greater power of discriminating pressures through the entire surface of the left side than through that of the right, it would seem much more pro bable that there is an original difference in the tactile endowments of the two sides respectively. There is certainly a strongly marked difference between different parts of the trunk in regard to their sensibility to tem perature, as is experienced by those who sponge themselves over with cold water im mediately on leaving their bed in the morning. In the writer's case, the parts most sensitive to the cold are in the centre of the dorsal region behind; in front, between the lower end of the sternum and the umbilicus ; and the corresponding portions of the flanks. These spots are among the parts of the in tegument least possessed of tactile discrimi nation; and yet the cold sponge passing over them seems to be much lower in temperature than when it is applied to other parts.