The metacarpal bones and the phalanges require no special description. Like the great-toe, the thumb has only two phalanges, while each of the other digits has three.
We shall now notice the various movements of which the hand is capable. They may be divided into (1) the different directions in which the hand collectively can be moved; and (2) the movements of which the hand itself, without reference to the arm, is capable.
The scapula or shoulder-blade,with which the principal arm-bone articulates, is itself movable to a very considerable extent on the surface of the ribs on which it rests. Again, the socket in which the nearly spherical head of the humerus or arm-bone lies is very shallow—not unlike the cup in the well-known toy the arrange ments of the shoulder-joint generally are such as to permit so great a variety, and se extensive a range of movements, that we are able to apply the hand to every part of the body. This freedom of motion is due in a great degree to the clavicles or collar-bones, which, by steadying the shoulder-blades, and keeping the shoulders apart, afford a fixed point for the various muscles which we employ in raising the arms, in folding them over the chest, in the act of hugging, etc. The movement at the next junction of bones, the elbow-joint, is very different from that at the shoulder. The latter is termed, from its construction, a ball-and-socket joint, and admits of motion in all directions, within definite limits; while the elbow is a hinge-joint, and merely admits of bending and straightening, or, in other words, of motion in one plane. We have next to consider a class of movements of the forearm and hand, to which there is nothing analogous (at lea'st to any material extent) in the leg. The movements in question are called "prora tion and supination." In pronation (derived from pronus, with the face downwards),we turn the palm of the hand downwards, as in picking up any substance from the table; in supination (derived from supinus, with the face upwards), we turn the palm upwards, as for the purpose of receiving anything that may be placed in it.
These movements of pronation and supination are so important to the usefulness of the hand, that we must notice the three muscles by which they are chiefly effected. One of the three muscles passes from a projecting process on the inner side of the arm bone, at its lower end, to the outer edge of the middle of the radius. Its contraction causes the radius to roll over, or in front of, the ulna. It thus prorates the hand, and
is called a pro nator muscle. Another muscle passes from a projecting process on the other side of the arm-bone to the inner edge of the radius near its upper part. It rune therefore in an opposite direction to the former muscle, and produces an opposite effect, the radius and the hand back into the position of supination. Hence it is called a sum/a/tor muscle. The third is a very powerful muscle, termed the biceps (q.v.).which not only bends the elbow, hut from the mode in which its tendon is inserted into the inner side of the radius, "also rotates the radius so as to supinate the hand; and it gives great power to that movement. When we turn a screw, or drive a gimlet, or draw a cork, we always employ the supinating movement of the hand for the purpose; and all screws, gimlets, and implements of the like kind are made to turn in a manner suited to that movement of the right hand; because mechanicians have observed that we have more power to supinate the hand than to pronate it." Supination can only be performed to its full extent by man, and even in man it is not the natural or habitual position; monkeys can partially effect the movement, and in most of the lower animals the part corresponding anatomically to the hand is constantly in a state of pronation.
The movements of which the hand itself, without reference to the arm, are capable, are very numerous, and in this respect differ considerably from the corresponding move ments of the foot. Thus we can bend the fingers down upon the palm, or we can extend them beyond the straight line; we can separate them from one another to a con siderable extent, and we can close them with considerable force. The wrist and hand are bent forwards or flexed upon the forearm by three muscles which pass downwards from the inner condyle or expanded end of the humerus, and are termed the radial flexor, the ulnar flexor, and the long palmar muscles. The first two of these muscles are inserted into wrist-bones on the radial and ulnar sides respectively, while the third expands into a fan-like fascia or membrane in the palm of the hand, and thus serves both to support the skin of the palm and to protect the nerves and vessels which lie below it Beneath the palmar fascia lie two sets of flexor muscles of the fingers, and they present so beautiful a mechanical arrangement as to merit special notice.