Objects are most important to us when in direct contact. The chief function of our eyes and ears is to enable us to prepare ourselves for contact with approaching bodies, or to ward such contact off. They have accordingly been characterized as organs of anticipatory touch.
"The delicacy of the tactile sense varies on different parts of the skin; it is greatest on the forehead, temples, and back of the forearm, where a weight of 2 milligr. pressing on an area of 9 sq. millim. can be felt.
"In order that the sense of touch may be excited neighboring skin-areas must be differently pressed. When the hand is immersed in a liquid, as mercury, which fits into all its inequalities and presses with practically the same weight on all neighboring immersed areas, the sense of pressure is only felt at a line along the surface, where the immersed and non-immersed parts of the skin meet.
The Localizing Power of the Skin.—"When the eyes are closed and a point of the skin is touched we can with some accuracy indicate the region stimulated; although tactile feelings are in general characters alike, they differ in something besides intensity by which we can distinguish them; some sub-sensation quality not rising definitely into prominence in consciousness must be present, comparable to the upper partials determining the timbre of a tone. The accuracy of the localizing power varies widely in different skin regions and is measured by observing the least distance which must separate two objects (as the blunted points of a pair of compasses) in order that they may be felt as two. The following table illustrates some of the differences observed:
| Tongue-tip | 1.1 mm. | (.04 inch) |
| Palm side of last phalanx of finger | 2.2 mm. | (.08 inch) |
| Red part of lips | 4.4 mm. | (.16 inch) |
| Tip of nose | 6.6 mm. | (.24 inch) |
| Back of second phalanx of finger | 11.0 mm. | (.44 inch) |
| Heel | 22.0 mm. | (.88 inch) |
| Back of hand | 30.8 mm. | (1.23 inches) |
| Forearm | 39.6 mm. | (1.58 inches) |
| Sternum | 44.0 mm. | (1.76 inches) |
| Back of neck | 52.8 mm. | (2.11 inches) |
| Middle of back | 66.0 mm. | (2.64 inches) |
The localizing power is a little more acute across the long axis of a limb than in it; and is better when the pressure is only strong enough to just cause a distinct tactile sensation than when it is more powerful; it is also very readily and rapidly improvable by practice." It seems to be naturally delicate in proportion as the skin which possesses it covers a more movable part of the body.
"It might be thought that this localizing power depended directly on nerve-distribution; that each touch-nerve had connection with a special brain-centre at one end (the excitation of which caused a sensation with a characteristic local sign), and at the other end was distributed over a certain skin-area, and that the larger this area the farther apart might two points be and still give rise to only one sensation. If this were so, however, the peripheral tactile areas (each being determined by the anatomical distribution of a nerve-fibre) must have definite unchangeable limits, which experiment shows that they do not possess. Suppose the small areas in Fig. 25 to each represent a peripheral area of nerve-distribution. If any two points in c were touched we should according to the theory get but a single sensation; but if, while the compass-points remained the same distance apart, or were even approximated, one were placed in c and the other on a contiguous area, two fibres would be stimulated and we ought to get two sensations; but such is not the case; on the same skin-region the points must be always the same distance apart, no matter how they be shifted, in order to give rise to two just distinguishable sensations.
"It is probable that the nerve-areas are much smaller than the tactile; and that several unstimulated must intervene between the excited, in order to produce sensations which shall be distinct. If we suppose twelve unexcited nerve-areas must intervene, then, in [Fig. 25], a and b will be just on the limits of a single tactile area; and no matter how the points are moved, so long as eleven, or fewer, unexcited areas come between, we would get a single tactile sensation; in this way we can explain the fact that tactile areas have no fixed boundaries in the skin, although the nerve-distribution in any part must be constant. We also see why the back of a knife laid on the surface causes a continuous linear sensation, although it touches many distinct nerve-areas. If we could discriminate the excitations of each of these from that of its immediate neighbors we should get the sensation of a series of points touching us, one for each nerve-region excited; but in the absence of intervening unexcited nerve-areas the sensations are fused together.