In our lessons on the human skeleton we found that some of the bones, such as those of the skull and face, are not meant to move. These bones are united firmly together.
The bones of the limbs, on the contrary, provide for every variety of movement. This freedom of action is due, partly to the large number of individual bones in the limbs, and partly to the manner in which they are arranged and jointed one with another. Wherever two bones are connected in such a way that they can move one upon the other, they form a joint. The variety of movement is brought about by a corresponding variety of joints.
I open and shut my hand, I bend and straighten my arm or my leg, and it is easy to compare these movements with the backward and forward movements of a door on its hinge. Such joints are known as hinge-joints. They are the most numerous in the body. We find them atthe elbow, the knee, the wrist, and the ankle, and in the fingers and toes. We have already had occasion to notice the admirable arrangement of the thumb. It has not onlya backward and forward movement, such as the fingers have, it can move also from side to side in a direction at right angles to that. It should be carefully noted that the remarkable freedom of movement in the thumb is not brought about altogether by the structure of the thumb itself. The thumb works upon a long bone, which forms the link to connect it with the wrist. It is the joint between this bone and one of the wrist bones which provides for the double movement.
Our lessons on the snake and its structure have made us already familiar with the nature of a ball-and-socket joint. You know that in jointsof this kind the moving bone has a rounded knob-a sort of ball-at its extremity, and this ball fits into a cup- shaped hollow in the other bone. The ball of the one bone plays freely in the hollow cup of the other, and this allows of very extensive movement in any direction.
The bone of the upper arm and the bone of the thigh have each a round knob or ball, which fits and moves freely in a corresponding cup or socket in the shoulder- blade and hip-bone respectively. This kind of joint allows of the widest freedom-a fact which is well illustrated in our drill exercises. The leg provides for the same kind of movement as the arm, but they differ in extent. We cannot swing the legs as freely as we can the arms. Thereason is that the cup in the hip-joint is very deep, and the ball of the thigh-bone is deeply lodged in it. Consequently the movement there is not so extensive as at the shoulder joint, where the cup is shallow.
Now think of the head and its movements on the neck. We are able to move the head forward and backward, as in the act of nodding, and to depress it towards either shoulder. This, you will readily see, is the work of a double hinge-joint. The joint is between the base of the skull andthe top-most vertebra on which it rests.
But we have only to turn the head right and left, to be reminded that there is another movement provided for,besides that of the hinge. In the top-most vertebra which carries the skull there is a small hollow ring, into which fits an upright peg or pivot of the second vertebra. When the head turns right and left, it is this top-most vertebra which really moves round on the peg of the second, and in moving it carries the skull round with it.
Such a joint is known as a pivot-joint.
There is one very important arrangement that must not be lost sight of in connection with all these joints. The joints are designed for movement, and all movementsmust be smooth and easy. There must be no friction, no grating.
If we examine some piece of machiner y, we shall find all the working surfaces perfectly smooth, and we know that the machinist is careful to keep these smooth surfaces well oiled, so that they may glide easily over each other. The movable bones are constructed on much the same principle. Their working surfaces are covered with a smooth coating of gristle, and each joint is enclosed in a membrane, which has the power of preparing and pouring into it a peculiar oily fluid known as synovia, or joint-oil.
In this way nature provides not only the machinery, butthe joint-oil to lubricate it, so that it may work easily and smoothly.
Lesson 09