390 TACTILE SENSIBILITY OF THE SKIN. Sense of Touch. 490. By the sense of Touch is usually understood that modification of the common sensibility (§ 487) of the body, of which the surface of the skin is the especial seat. In some animals, as in Man, nearly the whole exterior of the body is endowed with it in no inconsiderable degree; but in others, as in the larger number of Mammals, most Birds and Reptiles, and many Fishes, the greater part of the body is so covered by hairs, scales, or bony plates, as to be nearly insensible; and the faculty is restricted to particular portions of the surface, which often possess it in a very high degree. The sensory impressions, by which we receive the sensation of Touch, are made by the objects themselves upon the nerves which are distributed to the Skin; the general structure of which has been already described (§§ 36-38). Of the papillæ which are thickly set upon many parts of its surface, some contain looped tufts of blood-vessels without nerves; and as these are found to be largest where the Epidermis is thickest (as, for example, in the pads on the under side of the Dog's foot), it seems obvious that they minister, not to sensation, but to the nutrition of that protective coating (§ 492). But in other papillæ the blood-vessels are comparatively scanty, their interior being chiefly occupied by little cushions of condensed areolar substance to which the sensory nerves proceed; and as their Epidermic coating is thin, and as the degree of sensibility of any part of the skin bears a close correspondence to the number of these papilla which are included within a given area of its surface, it can scarcely be doubted that they are the special instruments of the sense of Touch. 491. The true skin, or Cutis (§ 37), from which alone leather is prepared, is thicker in most Mammals than in Man; but the thickness of the skin does not by any means involve (as is commonly supposed) deficient sensibility. Thus, in the Spermaceti Whale it has been observed that the sensory nerves, which are destined to be distributed on the skin, pass through the blubber without giving off any considerable branches, but spread out into a network of extreme minuteness as soon as they arrive near the surface. It is a fact well known to Whale-fishers, especially to those who pursue this species, that these animals have the power of EPIDERMIC PROTECTION. OTHER ORGANS OF TOUCH. 391 communicating with each other at great distances. It has often been observed, for instance, that, when a straggler is attacked, at the distance of several miles from a "school," a number of its fellows bear down to its assistance in an almost incredibly short space of time. It can scarcely be doubted that the communication is made through the medium of the vibrations of water, excited by the struggles of the animal, or perhaps by some peculiar movements specially adapted for this purpose, and propagated through the liquid to the immense surface of the skin of the distant Whales. 492. The sensibility of the true skin would be too great, if it were not protected by the Epidermis (§ 38), the thickness of which varies considerably, according as the part is to be endowed with acute sensibility, or to be protected from impressions of too strong a nature. Thus it is particularly thin at the ends of the fingers, and on the surface of the lips, which are used for feeling; but is thick on the palm of the hand, which is used for firmly grasping, and which would be continually suffering pain if its sensibility were too acute; and it is still thicker on the sole of the foot, especially on the heel and the ball of the great toe, where pressure has to be sustained. 493. Although the fingers of Man and of the Quadrumana, being endowed with peculiar sensibility, are their special organs of touch, yet we find that they cease to be so in most of the other Mammalia, whose extremities are adapted only for support and locomotion, and are terminated by hard claws or hoofs that completely envelop them. In many of these, we find the lips and tongue employed as the chief organs of touch; in the Elephant, this sense is evidently possessed very acutely by the little finger-like projection at the end of its trunk; and in several other cases the vibrissa or whiskers are its special instruments, the bulbs of their long stiff hairs being copiously supplied with sensory nerves. 494. A curious modification of the sense of Touch appears to exist in Bats. It has been found that these animals, when deprived of sight and (as far as possible) of hearing and smelling also, still flew about with equal certainty and safety, avoiding every obstacle, passing through passages only just large enough to admit them, and flying through places with which they were previously unacquainted, without striking against the objects near which they passed. The same result happened 392 IMPROVEMENT OF TOUCH BY EXERCISE. when threads were stretched in various directions across the apartment. Hence some Naturalists were inclined to attribute to the Bat the possession of a sixth sense unknown to Man ; but Cuvier correctly pointed out that this idea becomes unnecessary, if we attribute to the delicate membrane of the wings (as we are justified in doing) a high degree of tactile sensibility, so as to receive impressions from the pulses of the air that are produced by the action of the wings and modified by the neighbourhood of solid bodies. 495. The only idea communicated to our minds by the sense of Touch, when this is exercised in its simplest form, is that of resistance; and we cannot form a notion either of the size or shape of an object, or of the nature of its surface, by feeling it, unless we move the object over our own sensory organ, or move the latter over the former. By the various degrees of resistance which we encounter, we estimate the hardness or softness of the body; and by the impressions made upon the papillæ, when they are moved over its surface, we form our idea of its smoothness or roughness. It is by attention to the muscular movements we execute, in passing our hands or fingers over its surface, that we acquire our ideas of its size and figure; and hence we perceive that the sense of touch, without the power of moving the tactile organ over the object, would have been of comparatively little use. 496. This sense is capable of improvement to a remarkable degree; as we see in persons who have become more dependent upon it in consequence of the loss of their sight. This doubtless results, in part, from the increased attention which is given to the sensations; and partly from the greater acuteness or impressibility of the organ itself, arising from the frequent use of it. Amongst other remarkable instances of this kind was that of Saunderson, who, though he lost his sight at two years old, acquired such a reputation as a mathematician, that he obtained a Professorship at Cambridge. He exhibited, in several ways, an extraordinary acuteness in his touch; but one of his most remarkable faculties was the power of distinguishing genuine medals from imitations, which he could do more accurately than many connoisseurs in full possession of their senses. 497. The sense of temperature is of a different character from common tactile sensibility; and either may be lost, SENSE OF TEMPERATURE.- -ANTENNE OF INSECTS. 393 without the other being affected. It is rather of a comparative than of a positive kind; that is, we form our estimate of temperature rather by comparing it with that to which our body (or the part of it employed to test the heat or cold) has been previously exposed, than by any knowledge which we derive through the sensation as to the actual degree of heat or cold to which the organ is exposed. Thus, if we plunge one hand into a basin of hot water, and the other into cold, and then transfer both of them to a basin of tepid water, this will feel cold to the hand which has been previously accustomed to the heat, and warm to the other. In the same manner, the temperature of Quito, which is situated half-way up a lofty mountain, is felt to be chilly by a person who has ascended from the burning plains at its base, whilst it seems intensely hot to another who has descended from its snow-capped summit; the residents in the town at the same time regarding it as moderate,—neither hot nor cold. It is a curious circumstance, that a weak impression made on a large surface seems more powerful than a stronger impression made on a small surface; thus, if the fore-finger of one hand be immersed in water at 104°, and the whole of the other hand be plunged in water at 102°, the cooler water will be thought the warmer; whence the well-known fact, that water in which a finger can be held will scald the whole hand. a a 498. Where any special organs of Touch exist in Invertebrated Animals, they are for the most part prolongations from the portion of the head near the mouth. This is the case with the arms of the Cuttle-fish, and with the tentacula of the lower Mollusca which are similar in position. Among Crustacea and Insects, the antennæ or feelers (fig. 198, a, a) appear to be the special organs of touch. These are frequently very long, and present an extraordinary variety in their forms, of which some are depicted in fig. 199. They contain, for the most part, a large number of joints (in the Mole Fig. 198.-CAPRICORN-BEETLE. 394 ANTENNE OF INSECTS. Cricket above 100), and are very flexible. This flexibility enables them to be turned towards any object under examination by the Insect; and when the animal is walking, we see them constantly being applied to the surfaces of the bodies which it approaches, in a manner which leaves little doubt that they are used as organs of touch. It is no objection to this view, to say that, as their surfaces are hard, no delicate sensations can be received through them; for the slightest Fig. 199.-VARIOUSLY-FORMED ANTENNE OF INSECTS. contact of their firmest points with a hard substance, may produce a sense of resistance which will afford to the animal the information which it requires. The stick used by the blind man in feeling his way, serves a very similar purpose. -It appears to be by sensations received through their antennæ, that Bees and other Insects which naturally work in the dark, are enabled to carry-on their labours without confusion or inaccuracy; and to be by the same means, that they communicate with each other. When the antennæ are cut off, the Bee at once ceases to work, and seems unable to direct its movements in any other way than towards the light. When any important event has happened in a community, such as the loss of the Queen, the spreading of the intelligence through the whole hive may be watched by a close observer. The working bees which were near her are observed to run about restlessly, applying their antennæ to those of the others they may meet, crossing them and striking lightly with them; these in their turn become agitated and do the same; and thus the intelligence is speedily propagated throughout the hive. In the same manner, when two bees meet each other out of their hives, they seem to reconnoitre one another for some time by the movements of their antennæ; and often |