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_).