The Sense of Feeling and of Touch in the Hand
I have mentioned three parts of the body as remarkable for the acuteness of the sense of touch, namely, the Tongue, the Lips, and the Hands. Now in each of these the skin is richly supplied with nerves and blood-vessels; and it is also thick and lies upon a soft cushiony substance, so as to be yielding and to admit of being applied accurately over any surface with which it is placed in contact, and of again resuming its shape when the pressure is removed. For instance, the tongue is so soft and yielding that, when it is applied to a tooth, it dips down between the inequalities and coves accurate information of the condition of the whole surface. The same is the case with the edges of the lips, though not in so marked a degree as in the tongue; and each of these parts is indebted for its great sensitiveness very much to the delicate soft supple nature of its structure. The palmar surface of the hand too, though, like the skin of the sole, it is strong and tough, so as to offer considerable resistance to injury and to prove no dainty morsel even to dogs, as we surmise from the narrative of the death of Jezebel, is yet very soft and yielding. It is also underlaid by a stratum of fat interwoven with strong fibres of tissue, just in the same manner as the skin of the sole of the foot (fig. 46, p. 99).
An accumulation of this fat and fibrous tissue under the skin forms the “Pulps” at the ends of the fingers. The slightly conical form and exquisite softness of the Pulps adapts them well for the examination of the surfaces of bodies; and the sense of touch is more acute in them than in other parts of the hand.
In connection with them it is interesting to observe that the last bone of each finger and of the thumb swells out, at the end, into a nodulated lump, which serves the purpose both of supporting the pulp and of giving breadth to the nail. It also, like the corresponding part of the toe (page 99), affords a basis of attachment for the fibres that run, from the bone, through the pad of fat, to the skin, and give firmness and consistence to the part. The bulbous enlargement at the ends of the phalanges of the fingers and toes is peculiar, or almost peculiar, to man. In most Animals these bones taper to a point; in many they are also curved. Hence the nails are, in them, comparatively unsupported, and they become bent in at the sides and curved in their length, that is to say, they are formed into claws. This is the case, to a considerable extent, in the Monkey. The terminal phalanges of the monkey’s digits are more tapering than in man; the nails are more claw-like; and the pulps are less well-formed. This constitutes a not unimportant feature of difference between the hand of that animal and the human hand, in addition to those I have already mentioned.
You have experienced the sensitiveness to cold of the pulps of the fingers and toes; and have, probably, remarked that it is more difficult to keep them warm than any part of the body. I may add that, notwithstanding the liberal allowance of the means of supporting life (that is, blood and nervous influence) which they enjoy, they are very liable to mortify from frost-bite and other causes. I have repeatedly known that to happen when all the rest of the hand has escaped. This must be attributed, perhaps entirely, to their exposed position as terminal parts; and they share their susceptibility to cold with other parts similarly circumstanced, such as the nose, the elbows, the knees and the buttocks.
It is necessary to make a distinction between the Sense of Touch and common Feeling or sensitiveness to pain; for they are not quite the same. They are, it is true, very nearly alike, so nearly that we may consider them to be modifications of one another; and it is probable that the same nerves minister to both. Still there is a difference. The sense of touch is the sense of contact with external bodies, and enables us to take cognisance of their presence and inform ourselves of their shape, consistence, smoothness or roughness, &c.; whereas common sensation, or the sense of feeling, has an internal relation. It imparts to us information respecting the condition of our own bodies or any part of them. By the sense of touch in the tongue, for instance, we judge of the size and hardness of the morsel in the mouth; and by common sensation we learn that the organ is being bruised or scratched by it. Sensation of pain commonly destroys the sense of touch. Put your finger into a vice, and you may feel both sides of it. Screw it up, and you have nothing but the sensation of pain. If you were to awake in this state you would not, from the mere sensation, know that you were touching anything.
As a general rule there is a relation between the degree in which sensation and the sense of touch are manifested in different parts of the body. For instance, I have just been remarking on the acuteness of the sense of touch in the Tongue; and we know that this part is very sensitive to pain. The pain caused by a bite of the tongue is horrible; and so effectually does it serve the good end of warning the tongue to keep within its proper bounds, that that organ very rarely suffers from the pressure of the teeth.
But, forasmuch as sensitiveness to pain serves a different purpose from the sense of touch, namely, as in the instance of the tongue just mentioned, it renders parts alive to injurious impressions, and gives them warning to escape or protect themselves; so it is, as we might expect, most manifested in those surfaces where a slight amount of injury would prove most detrimental.
Thus, the membrane (the conjunctiva) which lines the eyelids and covers the front of the eyeball is exquisitely sensitive to pain. We are reminded of this when anything touches the eye, or when a fly has lodged itself under the eyelid. And, when an operator wishes to ascertain whether his patient is sufficiently under the influence of chloroform he separates the eyelids and puts his finger gently upon the eye, knowing that if no indication be given, by flinching, that the impression made here is felt, it is probable that the patient will not be conscious of the more severe impression to be made by the knife elsewhere. Yet, this membrane is by no means pre-eminently endued with the sense of touch. Indeed, the very acuteness of its sensitiveness to pain quite unfits it for distinguishing the quality of the impressions made upon it. We know very quickly that something is in contact with the eye, but can form no idea what kind of substance it is, whether it be hard or soft, rough or smooth.
In the hand, on the contrary, the sensitiveness to pain, though considerable, is not proportionate to the acuteness of the sense of touch. The sting of the rod on the palm, if my recollection serves me right, is not so sharply felt as it is upon that other region which shares with the hand the privilege of receiving the wrathful attentions of the master; and, yet, that other region is by no means distinguished for acuteness in the sense of touch.
The mode in which sensitiveness to touch and to pain are adjusted in the hand and in the eye in relation to the functions of these two organs is one of the admirable features of their construction. Suppose the disposition to have been reversed—suppose the hand to have been as tender as the eye—of what use would it have been? The contact of a particle of dust would have caused agony; or, had the eye been no more sentient than the hand, it would soon have been destroyed by the chafing of foreign bodies upon its delicate surface.
How important is the sense of Feeling! more important than any of the other senses; more so than all the others taken together. It is almost universal in the animal kingdom. Indeed, we can scarcely conceive animal existence without it, and are slow to admit that to be an animal which shows no sign of it. Several of the lower animals seem to be destitute of any of the other senses. The Polyps, for instance, have no sight, hearing, taste, or smell, and are dependent, therefore, entirely, upon feeling for their communication with the external world; and the range of this sense is extended in them by means of their “tentacles” or “feelers” which wave about in the water, and, when they come in contact with foreign bodies, close upon them and draw them towards the oral opening. Thus, the tentacle of the polyp is a sort of rudimentary hand, and, by the aid of feeling, fulfils one important function of the hand, viz. that of the supplying the mouth with food. The sprawling movements of an infant’s hands and the tendency which they have to close upon anything—dress, blanket, or whatever it be—and draw it to the mouth remind one forcibly of the feelers of a polyp.
In most of the lower animals, however, the sense of feeling, though present, serving for protection and giving notice of injury, is not very acute. It is not much employed by them for the purpose of obtaining information respecting external objects; and they can scarcely be said to enjoy that modification of it which we call the sense of touch in any high degree. Indeed, the skins of animals have, commonly, such a covering of thick, horny cuticle, scales, feathers, or hair, as is incompatible with a fine discriminating sense of touch.
In many of them, however, some other sense is highly developed. The Vulture is guided by the smell of carrion for miles and miles; and the dog will, by the same sense, track game where man cannot detect the trace of an odour. Some birds can distinguish objects which are quite out of the range of our sight. The Eagle, for instance, soars aloft, till it dwindles to a mere speck or is lost to our view, and, then, from that great height, will pounce, with unerring certainty, on an unhappy grouse upon the ground. The sense of hearing is a great means of protection to animals, and necessitates extreme stillness and caution on the part of their pursuers. The Deer, when feeding, directs his eyes upon the ground, and depends for safety, chiefly, upon his hearing, which is so acute that the huntsman is obliged to approach with all possible wariness.
In each of these instances, it may be observed, the acuteness of the particular sense is manifested chiefly in the power it gives to the animal of distinguishing objects at a distance. Whereas, in the ability to use the several senses for the nice discernment of the qualities of substances and to derive enjoyment from them, man stands quite unrivalled. He alone appreciates the perfume of a bouquet, or takes cognisance of the various shades of colour and of the notes of music; and the sense of touch, which is of especial service in aiding us to an accurate knowledge of bodies, is much more highly developed in man than in other animals.
Fine as the sense of touch usually is in the human hand, it becomes far more so when an unusual demand is made upon it in consequence of a deficiency, or absence, of other senses. The rapidity with which blind persons can read with their fingers is truly astonishing. Some are said to be able to distinguish colours by the feel. (It should rather be said that they are capable of recognising the nice differences in certain substances by which colours are caused; for one can scarcely conceive it possible to distinguish by feeling the colours in a ray of light separated by a prism.) I am acquainted with a lady who has been, not only blind, but deaf and dumb from infancy. The sense of touch is, therefore, almost her only avenue for impressions from without; and it is surprising how much information is conveyed through it, and how quickly. It enables her to hold converse with her relatives, by the language of the fingers, almost as freely and as briskly as others do with the tongue. A few touches are sufficient to transmit a series of thoughts. After one shake of the hand her friends told me that she would recognise me again; and, true enough, although several days elapsed before I again saw her, she made the sign for my name as soon as she touched my hand. At our next meeting I presented my left hand, but was, again, immediately recognised.
Persons who have had much experience in the instruction of the deaf and dumb find that the hand, by means of writing and “dactylogy”, or the language of finger-signs, is abundantly sufficient for all the intercourse to which a deaf-mute is equal; and they are, therefore, disposed to discourage the teaching of articulation. Dr Kitto, in his little book “On the Lost Senses,” which acquires so much interest from the fact of his being himself deaf and dumb in consequence of an accident, relates that, after he had, with great difficulty, reacquired considerable facility of speech, he found it stood him in little stead. So efficient a means of intercourse had the hand become that, he tells us, he had not occasion for the use of his tongue ten times in a year.
Not only may the hand thus serve, to some extent, as a substitute for some of the other senses; it is also a most important auxiliary to them. Particularly is it so to the sense of sight, by proving, or correcting, the impressions which we receive through the eye. Without its aid we should often fail to distinguish between a real object and a picture or a reflection in a mirror, and should have difficulty in judging of size, shape, distance, &c.