After what has been said of the necessity of the supply of blood, for the active exercise of common or general sensibility, and of the vas cularity of the special tactile organs, it is not requisite to lay any further stress on this point, in relation to the sense of touch, strictly so called. Another important condition, which is probably common to the whole sensory apparatus of the warm-blooded animal, and which has been already noticed under the head of Taste, is a temperature not too far removed from that which is natural to the body. It has been shown by Professor E. H. Weber, that if the fingers or the lips be im mersed for half a minute or a minute in water heated to 125°, or cooled to the power of tactile discrimination is so much impaired, that the power of distinguishing between a hot or cold fluid or solid body is for the time completely lost, a feeling of pain alone being experienced, which is the same whether the body be hot or cold. This, too, he found to be the case, when, instead of applying the heat or cold to the peripheral extremities of the nerve, he acted on its trunk. For this ex periment the ulnar nerve was selected, as its trunk, at the elbow, lies immediately beneath the surface. After immersing the elbow in a mixture of ice and water for about six teen seconds, Professor Weber observed that a peculiar painful sensation was experienced along the under side of the fore-arm, the wrist, the little finger, and the inner side of the ring-finger. This pain had no resemblance to that of cold. On continuing the immersion, the pain increased considerably, and eventu ally became almost intolerable ; then it gra dually diminished, and the middle and ring fingers became numb, as if " asleep," had no longer the power of distinguishing between heat and cold, and could only imperfectly per ceive the contact and pressure of bodies.
The exercise of the sense of touch .may be first considered under its simplest mode, namely, that in which the object is simply ap plied to the tactile organ ; and in this we have specially to consider the power of Tactile _Discrimination, and the Sense of Temperature.
Tactile Discrimination. — A very ingenious method was devised by Professor Weber t, for determining the relative power of tactile dis crimination in different parts of the skin, which is by no means accordant with their general sensibility. His mode of ascertaining this, was to touch the surface with the points of a pair of compasses, guarded by bits of cork or seal ing-wax ; the eyes being closed at the same time, the legs of the compasses were approxi mated to each other; until they were brought so near that the points could be no longer felt to be distinct from each other. The smallest distance at which this can be perceived (en titled "the limit of confusion," by Dr. Graves), is found to differ remarkably on diffbrent parts of the cutaneous surface ; and the com parison of these diversities affords us the means of estimating, — not their relative tactile sensibility (for this it cannot measure), but their relative discriminating power. The figures in the first column of the following TABLE represent these distances, as determined by Professor Weber on his own person, stated in Paris lines. The inquiry has been more re cently pursued by Professor Valentin t, whose results on the whole correspond very closely with those of Weber. He found, however, a considerable extent of individual variation ; some persons being able to distinguish the points at half, or even one-third of the dis tances required by others. In the following table, the second column expresses the maxima of the "limit of confusion," the third column the minima, and the fourth column the mean of all the observations made by Professor Valen tin ; it will be observed that his maxima cor respond almost exactly with the measure ments of Professor Weber on his own person. In the fifth and sixth columns are shown the relative acuteness and relative obtuseness of the discriminating sense in different parts, calculated from the mean results ; " the limit of confusion" at the tip of the tongue being taken as 1.000. Thus the co-efficient
of the acuteness of the discriminating sense in the palm of the hand, calculated by this standard, is 0.126 ; that of its obtuseness is The co-efficient of acuteness for the crown of the head, is 0.050; that of obtuse ness, for the same part, is 19.827. The co efficient of acuteness for the middle of the dorsal spine is ; that of its obtuseness is 50.086. Or, in other words, its acuteness is only that of the tip of the tongue ; its obtuseness fifty times as great.
Similar experiments, with the like results, have been made by M. H. Belfield-Lefevre* ; and from all these, the following general pro positions may be laid down ; — 1. On almost any part of the integument, the interval between the two points is more clearly dis tinguished, when the line which joins them is transverse (i.e. perpendicular to the axis of the body or member), than when it is longitudinal, or parallel to that axis. According to Weber, however, the tips of the fingers and of the tongue constitute an exception to this rule; the discriminating power being greatest in them, when the line joining the points of the com passes is longitudinally directed. 2. When two points, applied simultaneously to any part of the integument, are clearly distin guished, the distance which separates them seems to be greater, in proportion to the acute ness of the discriminating sense in the part of the surface which is the subject of examina tion. Thus, as Weber remarks, if the points of the compasses, set at a distance of two or three lines, be applied to the cheek just before the ear, and be then moved gradually towards the angle of the mouth, the points will seem to recede from one another, in consequence of the increase of the discriminating sense in the parts to which they are applied. 3. When the two points are successively brought into contact with the skin, they seem to be at a greater distance from each other than if they are simultaneously applied ; and, in general, the distance will seem greater in proportion to the interval between their application. 4. Two points applied on different sides of the median line, seem more remote from one another than two points equally distant, but applied on one and the same side of the median line ; in other words, the power of discrimination is greater when the two points are applied on the two sides of the median line, than when they are both applied on the same side. 5. If two parts of the tegumentary surface be selected, whose relative position is subject to variation (such as the two eyelids, the two lips, &c.), and the two points of the pair of compasses be applied respectively to these two surfaces, the distance which separates them will seem to be much greater than if the two points rest at the same time on one or the other surface. 6. The same holds good, ac cording to Weber, when the two points are applied to parts of the surface, which, though in continuity with each other, differ remark ably, either in structure, in function, or in the use habitually made of them ; thus, the points will be more clearly distinguishable, and will therefore seem to be more distant from each other, when one is applied to the inner surface and the other to the red outer surface of the lips, than when they are both applied to the latter, although its discriminating power is much greater than that of the former ; and the same holds good of the margin and dorsum of the tongue, the palmar and dorsal surfaces of the last phalanges of the fingers, &c. 7. The discriminating sense is more acute in the in teguments of the head, than in those of the trunk ; and on the face, its acuteness di minishes as the distance from the mouth in creases. 8. The tactile discrimination of the integuments of the limbs augments with the distance of the part from the axis of the body; it is less in the integuments of the trunk, than in those of the members.