Each of the limbs has a single, long, rounded bone in the upper part, known in the arm as the _humerus_, and two bones in the lower part. These last are known as the _radius_ and _ulna_ (the "funny bone") in the forearm, and the _tibia_ and _fibula_ in the leg. The shoulder-joint is made by the rounded head of the humerus fitting into the shallow cup of the _scapula_, or shoulder-blade. It is shallower than the hip joint to allow it freer movement; but this makes it weaker and much more easily dislocated, or put out of joint,--the most so, in fact, of any joint in the body. [Illustration: A BALL-AND-SOCKET JOINT Hip joint.] [Illustration: A HINGE JOINT Knee joint, with the knee cap removed] The hip joints are deep, strong, cup-shaped sockets upon each side of the hip bones, or _pelvis_, into which fit the heads of the _femurs_ or thigh bones. When the hip joint does become dislocated, it is very hard to put back again, on account of its depth and the heavy muscles surrounding it. It is quite subject to the attack of tuberculosis, or "hip-joint disease." [Illustration: LENGTHWISE SECTION OF BONE] The _joints_, or points at which the bones join one another, look rather complicated, but they are really as simple as the bones themselves. Each joint has practically made itself by the two bones' rubbing against each other, until finally their ends became moulded to each other, and formed the ball-and-socket, or the hinge, according to whichever the movements of the "bend" required. The ends, or heads, of the bones which form a joint are covered with a smooth, shining coating of _cartilage_, or gristle, so that they glide easily over each other. [Illustration: CROSS SECTION OF BONE] Around each joint has grown up a strong sheath of tough, fibrous tissue to hold the bones together; and, inside this, between the heads of the bones, is a very delicate little bag, or pouch, containing a few drops of smooth, slippery fluid (_synovial fluid_) to lubricate the movements of the joint. This is sometimes called the "joint oil," though it is not really oil. Bones are covered with a tough skin, or membrane (_periosteum_). They are hardest and most solid on their surfaces, and hollow, or spongy, inside. The long bones of the limbs are hollow, and the cavity is filled with a delicate fat called _marrow_--just as an elderberry stem or willow-twig is filled with pith. This tubular shape makes them as strong as if they were solid, and much lighter.[25] The short, square, and flattened bones of the body, such as those of the wrist, the skull, and the hips, instead of being hollow inside are spongy; and the spaces in the bone-sponge are filled with a soft tissue called the _red marrow_ in which new red and white corpuscles for the blood are born, to take the place of those which die and go to pieces. FOOTNOTES: