joints are found at the elbows and knees and also
in the fingers. The hinge joint gives motion in but two directions--forward
and backward.

3. A _pivot_ joint is formed by the fitting of a pivot-like projection of
one bone into a ring-like receptacle of a second bone, so that one, or the
other, is free to turn. A good example of the pivot joint is found at the
elbow, where the radius turns upon the humerus. Another example is the
articulation of the atlas with the axis vertebra as already noted. The
pivot joint admits of motion around an axis.

4. The _condyloid_ joint is formed by the fitting of the ovoid
(egg-shaped) end of one bone into an elliptical cavity of a second bone.
Examples of condyloid joints are found at the knuckles and where the wrist
bones articulate with the radius and ulna. They move easily in two
directions, like hinge joints, and slightly in other directions.

5. _Gliding_ joints are formed by the articulation of plain (almost flat)
surfaces. Examples of gliding joints are found in the articulations
between the bones of the wrist and those of the ankle. They are the
simplest of the movable joints and are formed by one bone gliding, or
slipping, upon the surface of another.

*The Machinery of the Body.*--A machine is a contrivance for directing
energy in doing work. A sewing machine, for example, so directs the energy
of the foot that it is made to sew. Through its construction the machine
is able to produce just that form of motion needed for its work, and no
other forms, so that energy is not wasted in the production of useless
motion. The places in machines where parts rub or turn upon each other are
called _bearings_, and extra precautions are taken in the construction and
care of the bearings to prevent friction.

The body cannot properly be compared to any single machine, but must be
looked upon as a complex organization which employs a number of different
kinds of machines in carrying on its work. The majority of these machines
are found in the skeleton. The bones are the parts that are moved, and the
joints serve as bearings. Connected with the bones are the muscles that
supply energy, and attached to the muscles are the nerves that control the
motion. Other parts also are required for rendering the machines of the
body effective in doing work. These are supplied by the tissues connected
with the bones and the muscles.

HYGIENE OF THE SKELETON

Of chief concern in the hygiene