THE hand, as we have said already, was originally used for walking upon when we were four-footed animals. In the course of evolution it started to be used for climbing trees. The great difference between the hand of man and the apes and that of the lower animals like the dog is that the thumb is ‘opposable ‘to the other fingers. This means that the front surface of the thumb can be brought to meet the front of any of the other fingers so that we can pick objects up and grasp them. This action was used first for grasping the branches of trees in climbing but it soon came to be used for picking things up.
In this connection the activities of a monkey are interesting, for this animal is notoriously one of the most curious of
beasts and will pick up and inspect anything it can lay its hands on. Climbing trees came first and then came curiosity and the ability to pick up and inspect. As movement depends on the action of the muscles, which are themselves told to move by the brain, all these new movements necessitated that the brain should become bigger and more efficient. The brain of the monkeys and apes is much bigger in proportion to their size than the brain of other animals, but the brain of man is much bigger still, and, of course, much more useful.
Only a few points in the anatomy of the hand can be given, for the subject is very long, although it is absorbingly interesting. If we look at the skeleton of a hand we shall see that beyond the two bones of the forearm are a collection—actually two rows—of small, irregular bones closely bound together by ligaments. These bones articulate at the wrist joint with the lower ends of the radius and ulna. Articulated with these bones are five small, long bones radiating like the spokes of a wheel and known as metacarpals. These together form the palm of the hand. With each metacarpal articulates a finger, each of which is composed of three bones, one beyond the other.
At the wrist joint the movements possible are those of bending the hand forwards and backwards, and to a slight degree from side to side. Rotation, or twisting of the hand, if incorporated in the same joint, would have made it weak, so it has been relegated, as we have seen, to the joints between the radius and the ulna.
The muscles which work the fingers are nearly all situated in the forearm where their fleshy bellies arise from the radius and ulna, both on the front and on the back of the two bones. As these muscles pass downwards to the hand, they become thinner and smaller to economise space and form long inelastic bands or tendons. They cross the wrist joint on its front and back and eventually are fixed to various points on the front and back of the fingers. When these muscles contract they have two actions, one on the fingers and one on the wrist. Those in front bend the wrist forwards and make the fingers curl up; those on the back bend the wrist backwards and straighten out the fingers.
THE MECHANISM THAT ENABLES US TO GRASP THINGS IF an object (such as a pencil) is grasped tightly with the fingers (not with the thumb), or if ‘a fist is made ‘as in boxing, it will be found that the wrist is bent backwards.
This is an essential of a good grasp and well illustrates the team work which the muscles must accomplish before they can act in a useful way. First the extensor muscles on the back of the forearm extend—that is, bend backwards the wrist so that the flexors on the front of the forearm can get a better pull on the fingers. This is the reason why the grasp is weakened when a broken wrist heals badly so that the extensors cannot do their work properly.
Flexion of the fingers and extension of the wrist are only one aspect of the mechanism of grasping and are done mainly by muscles lying in the forearm. The other activity which is so essential is opposition of the thumb, and this is accomplished by the actions of small muscles which lie in the palm of the hand. The bone which forms the end of the thumb nearest to the palm is so shaped that it can rotate or turn inwards and under the influence of the muscles the sensitive pads of each of the fingers can be met by the pad on the front of the thumb. This not only enables things to be picked up but as the sensitive pads are brought together we can tell by feeling alone the shape and texture of an article picked up.
An interesting point in the human hand is that whereas in the lower animals the flexors and extensors of all the fingers are combined into one single muscle so that all work together, in man and the apes the muscles of the thumb and forefingers are separate and so these fingers can move separately, with an obvious increase in skill. Such movements as tying a knot in a piece of string would be impossible if all our fingers moved together. That the other ringers move together can be seen at once by anyone who tries to make his fourth finger curl up alone. It is impossible, except for one who has long practised it, the middle and little fingers always move with it because the muscle controlling them is the same.
Much of our skill would be lost if we could only curl up our fingers under the influence of the flexor muscles. Writing, for example, would be impossible. Before this complicated and highly-skilled action can take place we must be able to keep our fingers nearly straight and yet have them bent at right angles to the palm at the knuckles. Special muscles lying in the palm have been evolved for this, their action being to bend the fingers at the knuckles and yet keep the fingers themselves straight. These are only a few of the interesting features of our hands, but sufficient has been said to illustrate their intricacies and to give an idea of how they work.