=Kinds of Movable Joints.=--The movable joints are found chiefly in the limbs. When one end of the bone is rounded and fits into a cuplike hollow, the joint allows motion in all directions, and is known as a _ball-and-socket_ joint. The hip joints and shoulder joints are examples. A _hinge joint_ allows motion in only two (opposite) directions; for example, the to-and-fro motion of the elbow. A _pivot_ joint allows a rotary motion; examples, the first vertebra on the second, one bone of forearm upon the other. A gliding joint consists of several bones that slide upon one another, as at the wrists and ankles. =The Four Features presented by a Movable Joint= (Fig. 36).--If not held in place, the bones would slip out of their sockets, hence there are _ligaments_, or tough bands, to bind the bones together. Sudden jolts would jar the bones and injure them; shocks are prevented by a layer of elastic _cartilage_ over the end of each bone. The moving of one bone over another in bending a joint would wear the bone with friction unless the cartilages were very smooth and lubricated with a fluid called the _synovial fluid_. The synovial fluid would be constantly escaping into the surrounding tissues except for the collarlike ligament called the _capsule_, which surrounds the joint and is attached to each bone entirely around the joint (Fig. 36). [Illustration: FIG. 36.--DIAGRAM OF A JOINT.] THOUGHT QUESTIONS. =The Kinds of Joints.=--Write _B_, _H_, _G_, _E_, _P_, or _I_ after these names according to the kind of joint (ball-and-socket, hinge, gliding, elastic, pivot, immovable): between bones of skull, head nodding, head turning, vertebræ, lower jaw, ribs to breastbone (Fig. 75), shoulder, elbow, wrist, fingers, hip, knee, ankle, toes. =Growth of Bones.=--The blood vessels pass into the bones from the periosteum. _If the periosteum is removed, the larger blood vessels are taken away and the bone beneath it perishes._ If the underlying bone is removed and the periosteum left, the bone will be replaced. A curious proof of the active circulation in the bone is furnished when madder is mixed with the food of pigs. In a few hours the bones become a darker pink than usual; and if the madder is fed to the pigs for a few days, their bones become red. A child grows in height chiefly during three or four months in spring and summer; but its body broadens and becomes heavier during autumn. =Health of the Bones.=--It is plain that _a strong and free circulation of pure blood contributes to the health and strength of the bones_; good food and pure air make pure blood. Cases of “delayed union,” or slow mending of broken bones, occur more often with intemperate than with sober people. This is because the vitality of the bone cells has been weakened by the use of alcohol. Many surgeons dislike to operate on an old drunkard.