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.
