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