The ball and socket joint is one that can move freely in all directions. We see it illustrated in the joints of the shoulder and hip. In these joints one of the bones has a deep depression in it, and this depression forms the _socket_. The other bone has a rounded head that fits into the depression. We call this rounded head the _ball_. [Illustration: FIG. 78. The structure of a bone.] [Sidenote: The hinge joint] The hinge joint is illustrated in the knee and elbow joints. These joints can move backward and forward in one plane like a hinge, but they cannot move in a circle like the ball and socket joints. You cannot swing your forearm about on a pivot at the elbow as you can your whole arm, nor will the knee joint bend in every direction as does the joint at the hip. [Sidenote: Serrated joints] Serrated joints do not move. The bones having serrated joints are fitted tightly together so that they form practically one bone. We find this kind of joint illustrated in the way the bones of the skull are put together. [Sidenote: Ligaments] The joints of the body are not held together by rivets, pins or bolts as are the joints of a machine, but by bands of very tough tissue placed about a joint in such a way as to allow it to move freely, although the bones are all the time held firmly together. These bands are called _ligaments_. Ligaments are much better than bolts or pegs would be, because they stretch a little, and thus prevent the breaking of the bones when the joint is put under a severe strain. [Illustration: FIG. 79. The muscles.] MUSCLES AND TENDONS [Sidenote: What muscle is] The lean meat of any animal is composed entirely of muscle tissue. It is the function of the muscles to move the body. [Illustration: FIG. 80. The biceps muscle contracted.] [Sidenote: How the muscles work] The muscles are nearly all attached to the bones. They are just long enough to let the joint straighten out when the muscles are at rest, but when the joint bends the muscle contracts. When a muscle contracts it becomes shorter and thicker. Sometimes it becomes very much thicker in one place. Every boy knows how much thicker the arm muscle (_biceps_) becomes when he bends his elbow hard. He calls this "showing his muscle." [Sidenote: Tendons] There is not room enough on most of the bones for all the muscles to be attached directly to them. Instead of being thus attached directly to the bone, they end in what we call _tendons_. These tendons are hard and strong, and a very small tendon will lift as much without breaking as quite a large muscle. The muscles are soft and would have to be attached over a large area in order to secure the required strength. The tendons, being so much stronger than the muscles, can be attached to a very small area and yet secure the same amount of power as would result from attaching the muscle itself.