These observations of Weber have been deservedly celebrated by physiologists, as the foundation of an accurate mode of estimating the tactile sensibility of the skin. They have been extended by other observers, as may be seen in Dr. Carpenter's article on Touch in the Cyclopædia of Anatomy. It is necessary, however, for us to discuss more closely the points involved in them, and especially to discriminate the tactile, from the muscular element of the sensations.

Whenever two points produce a double sensation, it is to be presumed that one point lies on the area supplied by one distinct nerve, while the other point lies on the area of a second nerve. There is a certain stage of subdivision or branching of the nerves of touch, beyond which the impressions are fused into one on reaching the cerebrum. How many ultimate nerve fibres are contained in each unit nerve, we cannot pretend to guess; but on the skin of the back, the middle of the thigh, and the middle of the fore-arm, an area of three inches diameter, or between six and seven square inches, is supplied by the filaments of a single unit. On the point of the finger the units are so multiplied, that each supplies no more than a space whose diameter is the tenth of an inch. Such units correspond to the entire body of the olfactory or gustatory nerve, for these nerves give but one undivided impression for the whole area affected. If we had two different regions of smell, and two distinct olfactory nerves, we should then probably have a feeling of doubleness or repetition of smells, like the sense of two points on the skin.

The primitive or original impression of a plurality of points, sufficiently far asunder, can be nothing but a feeling of repetition, a sense that the same impression reaches us through different unknown channels. What the real distance of the points is, we have no means of judging, any more than we can tell previous to experience whereabouts on the body the impression is made. Hence in those of the experiments that relate to our sense of the relative interval of the points, as when they pass from a duller to a more sensitive region, there are involved perceptions that we have got at in some other

MUSCULAR IMPRESSIONS OF TOUCH.

185 way than through the sense of contact. This other means is the feeling of movement or the muscular sensibility, without which it is impossible to explain the vast majority of the sensations of Touch. We have already dwelt upon the perceptions growing out of the moving apparatus of the body, and we must here, and under the two following senses, Hearing and Sight, point out the combinations that are formed by sense and movement.

11. Sensations of Touch involving muscular perceptions. -In discussing these we shall begin with examples that are almost purely muscular, the tactile sensibility being a mere incident of the situation. The feeling of weight is of this description; depending on the sense of muscular exertion, aided perhaps in some cases by the feeling of compression of the skin. On this last point I quote from Todd and Bowman. 'Weber performed experiments to ascertain how far we are capable of judging of weight by the mere sense of contact [without muscularity.] He found that when two equal weights, every way similar, are placed on corresponding parts of the skin, we may add to or subtract from one of them a certain quantity without the person being able to appreciate the change; and that when the parts bearing the weights, as the hands, are inactively resting upon a table, a much greater alteration may be made in the relative amount of the weights without his perceiving it, than when the same parts are allowed free motion. For example, 32 ounces may thus be altered by from 8 to 12, when the hand is motionless and supported; but only by from 1 to 4, when the muscles are in action; and this difference is in spite of the greater surface affected (by the counter pressure against the support) in the Weber infers that the measure

former than in the latter case. of weight by the mere touch of the skin is more than doubled by the play of the muscles. We believe this estimate to be rather under than over the mark.'-p. 431.

That the discriminative sensibility of the skin to degrees of compression may operate in appreciating weight is further confirmed by the following statement. The relative power of different parts to estimate weight corresponds very nearly

with their relative capacities of touch. Weber discovered that the lips are better estimators of weight than any other part, as we might have anticipated by their delicate sense of touch and their extreme mobility. The fingers and toes are also very delicate instruments of this description. The palms and soles possess this power in a very remarkable degree, especially over the heads of the metacarpal and metatarsal bones; while the back, occiput, thorax, abdomen, shoulders, arms, and legs, have very little capacity of estimating weight.'—ib. p. 432.

What is said of weight applies to any other form of pressure, force, or resistance. The impetus of a push or a squeeze received on the hand is measured by the muscular exertion induced to meet it, and in some small degree, as above described, by the compression of the skin at the place of contact.

The qualities of hardness and softness are appreciated by this combined sensibility; the one means a greater resistance to compression, and the other a less. From the unyielding stone or metal to the mobility of the liquid state, we have all degrees of this property; the entire class of soft, viscid, and fibrous substances lying between. It belongs to many of the manual arts to appreciate minute differences of consistence in the class of soft bodies; the pastrycook, the builder, the sculptor, &c. In this they are assisted by practice, which improves all sensibilities; but there are great varieties of natural endowment in the case, which varieties must have their seat principally in the muscular tissue, and secondarily, in the skin and nerves of the hand.

The feeling of elasticity is only a case of simple resistance to force, exerted in the particular circumstance of a rebound or reaction from pressure. The elasticity implies a perfect return to the original position; air is elastic, and so is steel and ivory, meaning that when in any way compressed or distorted, they recover themselves. The softness that is agreeable to rest upon must be an elastic softness; we can note the difference by comparing a hair cushion with a lump of clay.

We may next consider the sensations rising out of the qualities of roughness and smoothness. Simple contact, we

ROUGHNESS AND SMOOTHNESS.

187

have seen, gives the sense of a multiplicity of points. The finger resting on the end of a brush would make us aware of its character; that is we should have the feeling of a plurality of pricks. In this way we are sensitive to rough and pointed surfaces. We can distinguish between bluntly-pointed asperities, like a file, and sharp points like a horse-comb: the sensibility of a blunt point being distinct from a needleprick. We can also distinguish between thick-set points and such as are more scattered, provided they are not too close for the limits of sensibility of the part, that is one-tenth of an inch for the finger, and one-thirtieth for the tip of the tongue. On the back, the calf of the leg, and the middle of the forearm, where points are confounded up to the distance of three inches, roughness would be altogether imperceptible.

In these instances, the thing touched is supposed to lie at rest on the finger, or on the part touched. But this does not do full justice to the tactile sensibility; it is requisite that we should move the finger to and fro over the surface in order to give full range to the power of discrimination. By this means we may discriminate far nicer shades of roughness; we may in fact appreciate minuter intervals than in the resting position. Supposing the sensibility of the tip of the finger to be one line at rest, by motion we can extend this sensibility to an unknown limit. The case may be illustrated by the micrometer screw on an astronomical instrument. The divisions on the limb of the instrument extend we may suppose to one minute of a degree, and if the index lie between two divisions, its place can be measured by the number of turns of the screw required to bring it up to one of the divisions. So, if a point is undistinguished on the finger in consequence of not being a line removed from the neighbouring point, we may estimate its distance nevertheless by the amount of motion of the finger needed to bring it into the limit of sensibility. I will take as an example a row of points, one-fortieth of an inch apart, the extremes being one-tenth, which is the sensibility of the tip of the finger. This row would be felt as two points if the finger were stationary. But by the motion of the finger one point would pass away and

another would come up, and there would be a feeling of the interval moved over between the perception of the successive points, which would be a measure of the intervals. The sense of movement would thus be brought in to aid the tactile feeling, and to reveal a degree of closeness in asperities beyond the reach of touch unassisted by motion. It is agreeable to all experience that the roughness of a surface becomes far more apparent by drawing the hand over it; whether the sense of movement explains all that there is in this increased sensibility, I will not undertake to say. For we must consider that friction creates a new variety of pressure on the skin and nerves, and the kind of friction is so different for a smooth and for a rough body, that by it alone we might learn to distinguish between the rough and the smooth

contact.

If any one will make the experiment of drawing over the finger two points, so close that to the touch they seem one when at rest, it will be found that the motion gives the feeling of doubleness. What is the limit of this, for a limit there is, it would take a considerable amount of observation to decide. I venture to affirm that at least half the interval will become sensible by the motion of the points, the motion being by bringing them in train, and not abreast of one another.

Whatever may be the explanation of the increase of sensibility due to movement, the fact is an important one. A vast amount of discrimination turns upon it. From the variety of trace made by different kinds of surface, we can distinguish them or identify them at pleasure, up to a considerable limit of delicacy. Hence the power of telling substances by the feel, and of deciding on the qualities and merits of texture and workmanship. Degrees of polish in stone, metal, or wood, the fineness of cloths, wool, &c., the beat of a pulse, the quality of powdered substances, and many things besides, are matters of judgment and comparison to the touch, and put to the proof its natural or acquired delicacy.

The feeling of temperature is an element in many discriminations, as in the distinction between stone and wood. Clamminess is a distinct sensation arising from the adhesion