In the pivot joint a kind of peg in one bone fits into a notch in another.  The best example of this is the joint between the first and second vertebrae (see sec. 38).  The radius moves around on the ulna by means of a pivot joint.  The radius, as well as the bones of the wrist and hand, turns around, thus enabling us to turn the palm of the hand upwards and downwards.  In many joints the extent of motion amounts to only a slight gliding between the ends of the bones.

55.  Uses of the Bones.  The bones serve many important and useful purposes.  The skeleton, a general framework, affords protection, support, and leverage to the bodily tissues.  Thus, the bones of the skull and of the chest protect the brain, the lungs, and the heart; the bones of the legs support the weight of the body; and the long bones of the limbs are levers to which muscles are attached.

Owing to the various duties they have to perform, the bones are constructed in many different shapes.  Some are broad and flat; others, long and cylindrical; and a large number very irregular in form.  Each bone is not only different from all the others, but is also curiously adapted to its particular place and use.

[Illustration:  Fig. 27.—­Showing how the Ends of the Bones are shaped to form the Elbow Joint. (The cut ends of a few ligaments are seen.)]

Nothing could be more admirable than the mechanism by which each one of the bones is enabled to fulfill the manifold purposes for which it was designed.  We have seen how the bones of the cranium are united by sutures in a manner the better to allow the delicate brain to grow, and to afford it protection from violence.  The arched arrangement of the bones of the foot has several mechanical advantages, the most important being that it gives firmness and elasticity to the foot, which thus serves as a support for the weight of the body, and as the chief instrument of locomotion.

The complicated organ of hearing is protected by a winding series of minute apartments, in the rock-like portion of the temporal bone.  The socket for the eye has a jutting ridge of bone all around it, to guard the organ of vision against injury.  Grooves and canals, formed in hard bone, lodge and protect minute nerves and tiny blood-vessels.  The surfaces of bones are often provided with grooves, sharp edges, and rough projections, for the origin and insertion of muscles.

[Illustration:  Fig. 28.—­External Ligaments of the Knee.]

56.  The Bones in Infancy and Childhood.  The bones of the infant, consisting almost wholly of cartilage, are not stiff and hard as in after life, but flexible and elastic.  As the child grows, the bones become more solid and firmer from a gradually increased deposit of lime salts.  In time they become capable of supporting the body and sustaining the action of the muscles.  The reason is that well-developed bones would be of no use to a child that had not muscular strength to support its body.  Again, the numerous falls and tumbles that the child sustains before it is able to walk, would result in broken bones almost every day of its life.  As it is, young children meet with a great variety of falls without serious injury.