Part 7
The great characteristic of the Hand, as distinguished from the Foot, is the mobility of the first digit, or thumb. Accordingly when this digit stands out apart from the others, and can be moved independently of them, so as to be more or less completely opposed to them, in the upper or Mammalian Class of animals, at least, we call the member a Hand. When this digit is absent, or is fixed in the same manner as the others, which is the case in each of the four limbs of Quadrupeds, we call the member a Foot. In Monkeys, or in most of them, the thumb is present and is separate and moveable in each of the four limbs; and these animals are, therefore, called “quadrumanous” or “four-handed.” Man, having the moveable thumb upon each of the two upper limbs only, is “bimanous” or “two-handed.” By this peculiarity, perhaps more definitely than by any other, he is distinguished in structure from all the rest of the animal series; and naturalists have, accordingly, given the epithet “Bimanous” to the class in which he is placed, and in which he stands alone.
The hand is the executive or essential part of the upper limb. Without it the limb would be almost useless. The whole limb is, therefore, so made as to give play and strength to the hand; and, in ever so brief a description of the hand, it is necessary, even more than in the case of the foot, to give some idea of the manner in which the other parts of the limb are constructed, and to dwell a little upon such points as have relation to its movements.
[Illustration: Fig. 53.]
The general plan of construction of the upper limb will readily be understood by means of the drawings (figs. 53 and 58, p. 122). It resembles very much that of the lower limb (see fig. 4, page 15). The one bone of the upper arm--the _humerus_--resembles the one bone of the thigh, and is jointed, above, with the shoulder-blade, which, with the collar-bone, corresponds with the pelvis. Below, it is connected with the two bones of the fore-arm--the _radius_ and _ulna_; and these correspond with the two bones of the leg. In the wrist there are eight bones, called _carpal_ bones, arranged in two rows. These are connected with five _metacarpal_ bones; and these, like the metatarsals of the foot, are jointed with the _phalanges_. Of the latter there are three in each finger; but in the thumb, as in the great toe (page 10), there are only two.
[Illustration: Fig. 54.]
The diagram shows how the bones of the hand are arranged in three divisions. Thus, the upper row of carpal bones (3, 4, 5) consists, practically, of three bones; the fourth (6), which is much smaller than the others, being rather an appendage to one of them than a distinct constituent of the wrist. (According to this view, the number of the wrist-bones corresponds exactly with that of the tarsal bones of the foot, viz. 7). The _outer_ of these three carpal bones (3) bears the thumb[7] and the fore-finger (I. and II.), and constitutes, with them, the outer division of the hand; the inner one (5) bears the ring-finger and the little finger (IV. and V.), and constitutes the _inner_ division of the hand; and the middle one (4) bears the middle finger (III.), and is the _middle_ division of the hand. The diagram shows, too, that the two outer bones (3 and 4), with the two outer divisions of the hand, are connected with the radius; whereas the inner bone (5) only, with the inner division of the hand, is connected with the ulna. Strictly speaking, even this bone is not directly connected with the ulna, but is separated from it, as will be shown presently, by a thick ligament.
[7] In deference to custom we call the palm the _front_ of the hand; and, therefore, we speak of the thumb as the _out_er and the little finger as the _in_ner digit: though it would better accord with the ordinary position of the part, with its correspondence with the foot and with comparative anatomy, to reverse these terms.
You frequently hear ignorant persons (and the greater number of persons are lamentably ignorant of the structure of their own body) speaking of the _small bones_ of the shoulder, or the _small bones_ of the elbow. You may think this a matter of no importance, and that it does not concern you and people generally to have any knowledge of human anatomy. But I will tell you what is very often happening, and will leave you to judge whether such complete ignorance on this subject is not attended with some practical disadvantage. A man meets with an injury, falls and hurts his shoulder. The immediate effects of the injury subside; but he does not quickly recover the use of the part; he still cannot raise his elbow, or put his hand upon his head, or put it behind him. Soon he begins to think that something more is wrong than has been suspected; and the notion creeps over his mind, and gradually takes possession of it, that some small bone is displaced. Not content with the assurances of his medical man, he resorts to a quack, called a “bone-setter.” The latter, taking advantage of the popular fallacy, gratifies the patient with the information that his fears are correct, affirms that “a small bone is out,” and proceeds forthwith to employ the requisite forcible measures for putting the said “small bone” in. I need not say with what result. Every year, in this civilized country, many persons are maimed for life by these attempts to put imaginary small bones in. I beg you, therefore, particularly to observe that _there is no small bone_ either at the shoulder or at the elbow. The only small bones are at the wrist; and these are so well fitted to one another, and so firmly bound together, that nothing short of a crushing force suffices to displace them. This remark respecting the small bones of the wrist is true of nearly all the small bones in other parts of the body. So that, in fact, small bones are very rarely dislocated; and when you hear it asserted that a small bone is out, you may pretty confidently conclude that the speaker does not know what he is talking about.
I have said that the upper limbs resemble the lower in their general construction. There are, however, some important differences; and one of the chief of these is the greater variety and freedom of the movements in the upper limbs. _Strength_, for the purpose of carrying the body, is the object in the lower limbs. _Mobility_ is the requisite in the upper limbs. Of this one example has already been given in the instance of the thumb as compared with the great toe.
_Movements at the Shoulder._
An equally striking example is afforded by the shoulder. In the first place, the “Shoulder-blade” itself can be moved in several directions--upwards, downwards, backwards and forwards;--whereas the “Pelvis,” i. e. the part which bears to the lower limb the same relation that the shoulder-blade does to the upper-limb, is immoveably fixed.
Secondly, the “Shoulder-joint” is so made as to permit a great variety and extensive range of movements to take place. We can move the arm forwards or backwards, as in throwing a ball, or, in sword exercise; we can raise it so that the limb points straight upwards; and we can swing it round in any direction. It is owing to the free movement in this joint that we are able to apply the hand to every part of the body, so as to remove sources of irritation. It is interesting to observe how other animals get on without hands, though they are much exposed to what we should consider great annoyance, as from flies, &c. The Cow, for instance, lashes its hide with its tail. The Cat licks itself with its tongue. The Sparrow dusts itself by the road-side. The Pig and the Donkey roll in the mud. And many of them, as the Horse and the Ox, have a thin muscle, called “panniculus carnosus,” spread out under the skin, which effects those sudden twitchings of the skin whereby they are enabled to jerk off anything that troubles them. In Man the hand answers better than all these methods combined; and it is necessary that it should do so, because his skin is more sensitive and less protected by natural covering than that of any other animal.
[Illustration: Fig. 55.
Chest and shoulders of man.]
For this freedom of movement of the arms, so important to the usefulness of the hand, we are much indebted to the “Collar-bones.” These bones, so called because they are placed at the lower part of the _collum_ or neck, extend, horizontally, from the upper edge of the breast-bone, to the processes of the blade-bones which overhang the shoulder-joint. Thus they hold the shoulders apart, and give width to the upper part of the chest. They also steady the shoulder-blades, and afford a _point d’appui_ to the muscles which effect the lateral movements of the arms,--for instance, to the muscles which tend to draw the arms together, as when we hold anything, between the hands, in front of us; and to those which separate the arms from one another, as when we stretch them out at right angles with the body.
[Illustration: Fig. 56.
Chest and shoulders of bird.]
Many animals--the ELEPHANT, the RHINOCEROS, the HORSE and the OX--have no collar-bones; and they are only able to swing their fore limbs to and fro. They cannot execute any lateral movements. They cannot throw the limbs out sideways, nor press their fore feet together, so as to hold anything between them. If the horse wants to seize or hold any substance he must do it with his mouth. The Elephant has a special provision for the purpose of prehension in his trunk, which enables him to provide himself with food by pulling down the branches of trees. The LION and the TIGER can press their fore paws together sufficiently to enable them to hold their prey, and fix it upon the ground, while they put the head down to it and pull at it and tear it with their teeth; and they are furnished with rudimentary, or half, collar-bones suspended in the flesh of the upper part of the chest; while the little SQUIRREL, which sits upon its hind legs, and holds up the nuts between its fore paws to be nibbled, has complete collar-bones. So has the flying BAT, the climbing SLOTH and the digging MOLE. In BIRDS the collar-bones (fig. 56, AA) are very large; and, for the purpose of giving them greater strength, they are united together in the middle line just above the breast-bone, forming what is commonly called the “merry-thought;” and, as this is not sufficiently strong to resist the force of the powerful muscles which flap the wings and sustain the animal in the air, there are, in addition, stout “side-bones,” called by anatomists “coracoid bones.” These (B) run, from the breast-bone (D), in the same direction as the collar-bones, one, on either side, to the shoulder-blades (C); and they afford even more efficient support to the shoulders than do the collar-bones. The coracoid bones are peculiar to oviparous animals, or nearly so. In some reptiles, as the CROCODILE, they quite supersede the collar-bones.
These few examples are enough to show that freedom of movement of the arms, especially of lateral movement, is closely associated with, and, indeed, is dependent upon the shoulder-blades being supported and steadied by bones, which extend from the breast-bone to the shoulder-blades, and fasten the one to the other.
But, even the powers and advantages conferred by nature have often some drawbacks; and this free play of the arm at the shoulder in man, of which we are speaking, and the provision for it afforded by the collar-bone, are no exceptions to the remark. It is necessary for so great a range of movement that the socket in the shoulder-blade should be shallow, and that the ligaments which connect the arm-bone with the blade-bone should be loose. Hence the shoulder-joint is weak as regards its ability to resist injury. The collar-bone also causes the shoulder to project so much that it is greatly exposed to injury and often bears the brunt of a fall. A man is thrown from a horse or is knocked down upon the ground, and, if anything prevents the hand being stretched out, the chances are that he falls upon the shoulder. True, the head is saved thereby; but the shoulder suffers. Hence the shoulder-joint is more often dislocated than any other; and no bone is more frequently broken than the collar-bone. Even in little children, in whom, notwithstanding their many tumbles, the other bones usually contrive to escape, the collar-bones are often broken; and in grown-up persons the shoulder is sometimes dislocated by the mere action of the muscles, as in swimming, or throwing, or lifting a weight above the head.
That you may understand the movements of the shoulder a little more fully, I will ask you to contrast the drawing (fig. 58), which shows the position of the blade-bone upon the chest in Man, with the drawing (fig. 57) of the corresponding parts of the Rhinoceros; and you will at once recognise several important differences, besides the presence of the collar-bone in the one and its absence in the other.
[Illustration: Fig. 57. Rhinoceros.]
In the RHINOCEROS the chest is deep, from the back-bone to the breast-bone, and is flattened at the sides; and the depth of this part of the trunk is increased, slightly, by the breast-bone projecting, keel-like, underneath, and, much more, by the spines of the back-bone running up into a high ridge, above. The blade-bone and the arm-bone are applied against the flat side of the chest, and lie, lengthways, between the spine and the breast-bone, nearly parallel with the broad flat ribs. The blade-bone has no process overhanging the shoulder-joint, and, as before said, there is no collar-bone. The short thick arm-bone descends nearly in a line with the blade-bone, and has huge processes at its upper end for the attachment of muscles. The parts are designed to bear the great weight of the animal, and to carry its ponderous head and horn; but the only movement of which they admit is a sliding of the blade-bone and arm-bone, backwards and forwards, upon the side of the chest.
In animals of similar construction to the Rhinoceros, but of lighter frame, and of greater fleetness, the blade-bone is placed more obliquely, which gives freer and easier movement both to it and to the arm-bone. This, for instance, is the case with the well-bred horse, and if we want a quick-going horse, one that can lift his fore feet well, we should observe whether the shoulder-blade is oblique, and whether the spines of the back rise well above it. Such a horse is said to have “a good shoulder” and to be “well up.” He will carry a saddle well, and is not likely to trip.
[Illustration: Fig. 58.]
In MAN the chest has proportionately less depth and length, and greater breadth, than in any other animal; the breast-bone is quite flat; and the spines of the back are sloped downwards, so that they do not project beyond the level of the ribs and the blade-bones. Hence he can lie easily either upon the stomach or the back--a privilege which is shared with him by very few of the lower animals. Scarcely any of them can lie upon the back, or even upon the stomach without the help of the fore limbs. The donkey enjoys rolling over and over upon a dusty road, but he cannot poise himself for a minute upon his back.
The sides of Man’s chest, moreover, are not _flat_, as in the Rhinoceros and Horse, but _rounded_, so that the blade-bones can revolve upon them to and fro, as well as slide upwards and downwards; and the long arms--comparatively long, that is, from the shoulder to the elbow--hang quite free of the chest and form sharp angles with the blade-bones.
The blade-bones are accommodated to the shape of the chest; for, instead of being elongated in a direction parallel with the ribs, they are prolonged downwards, along the sides of the chest, at right angles with the ribs. This prolongation of the lower part of the blade-bone is very important, inasmuch as it enables the muscles to hold the bone steady upon the wall of the chest, and so gives greater power to those muscles which pass from the blade-bone to the arm and act upon the shoulder-joint. Were it not for this provision the contraction of the muscles intended to raise the arm would quite fail to produce the desired effect, and instead of it would simply cause the shoulder-blade to revolve upon a transverse axis. That is to say, when we endeavoured to raise the arm our effort would merely have the effect of raising the hinder part of the shoulder-blade.
In each of these particulars--in the form of the chest, and in the shape and direction of the shoulder-blade--the Monkey is intermediate between Man and the inferior animals. The Monkey’s chest is broad and round, in proportion to its length, if we compare it with other animals; but this is less marked than in the human chest. And you perceive that the Monkey’s back-bones project, as they do in other animals, beyond the level of the ribs. The blade-bones are also like those of Man in being prolonged downwards, and in being carried, to a certain extent, across the ribs; but their lower angles do not run so far in this direction as they do in the human skeleton.
[Illustration: Fig. 59. Monkey.]
The movement of raising the arm, as in carrying the hand _outwards_, or pointing upwards, or putting the hand upon the head, is rather a difficult one, and requires the combined action of many muscles. It is, therefore, to be avoided by persons to whom muscular straining is likely to be injurious; and the power of effecting this movement is easily impaired by accident or disease. A long time often elapses even after a slight bruise of the shoulder, before the person recovers the power of putting the hand upon the head.
The exercise of raising the arms above the head is a good one for those in health, and is much, and wisely, recommended by the directors of gymnastics. It brings many muscles into play, not only those of the shoulder, but the muscles all round about the chest, viz. those which pass from the spine and ribs, as well as from the breast-bone, head, and pelvis, to the shoulder-blade and arm; and, thus, it tends to strengthen the spine and the chest, as well as the shoulders and arms. There is, perhaps, no exercise so good as this; and it is much to be regretted that the dress of young ladies, with its paraphernalia of stays and shoulder-straps, interferes so greatly with it. The frequency among them of “pigeon-breast” and “crooked spine” must, partly, be attributed to the confinement of the arms, caused by the mode of dress and the customs of life. One of the few opportunities afforded to the arms of availing themselves of this exercise is in the dressing-room during the process of brushing the hair. I would by all means, therefore, recommend young ladies to give sufficient time and attention to this part of the toilette, and not to delegate it to the lady’s maid. If, in addition, I suggest that it be commonly done with open window, I feel sure that I shall have a deservedly great authority among them--Miss Nightingale--on my side.
[Illustration: Fig. 60. Elbow-joint.]
The movement at the ELBOW is, merely, that of bending and straightening, in a hinge-like manner; yet there is a slight obliquity in the direction in which it takes place, an obliquity resembling that in the movement at the knee (page 39).
_Pronation and Supination of the Hand._
In the FOREARM and HAND there is a movement with which we have nothing exactly corresponding in the leg. It is called “Pronation and Supination.” In _pronation_ we turn the palm _down_wards, as in picking up any substance from a table; in _supination_ we turn the palm _up_wards, as a boy does when he holds out his hand for a caning, or for the more agreeable purpose of having a shilling put into it.
PRONATION and SUPINATION take place in the following manner. Each of the two bones of the forearm extends from the elbow to the wrist (fig. 53); but one of them--the “ulna”--is chiefly connected with the elbow; and the other--the “radius”--is chiefly connected with the wrist, and, by means of the wrist, with the hand. The two bones are separate from one another, except at their ends. There they touch, and are jointed together in such a manner that the large lower end of the radius can play round, or partly round, the small, button-like, lower end of the ulna; and, in so doing, it carries the hand with it. In this movement the upper end of the radius (A, fig. 60) does not leave its place, but simply revolves, upon its own axis, on the surface of the arm-bone; and its edge turns in a notch cut for it in the upper end of the ulna (B), which remains still.
[Illustration: Fig. 61.
Hand supine.]
[Illustration: Fig. 62.
Hand prone.]
In the drawings (figs. 61 and 62) the relation of the parts in the supine and in the prone state is shown by the aid of a plumb-line falling from the part of the arm-bone upon which the upper end of the radius revolves. The line traverses the upper end of the radius, then passes along the interval between the two bones, then traverses the lower end of the ulna, and, finally, takes the course of the ring finger. And, provided the limb be held vertically, the line traverses the same parts whatever be the position of the forearm and hand. It does so in complete supination, as shown in fig. 61; it does so in complete pronation, as shown in fig. 62; and it does so in every intermediate position. We may call it, therefore, the axis upon which the radius and the hand turn in pronation and supination; and, according to this representation, the ring finger remains stationary during the movement, while the other fingers and the thumb perform their partial revolutions around it.