ece, at a point on a level with the end of the balance hook.
(The nail or screw used for this purpose must pass loosely through the
lever, and serve as a pivot upon which it can turn.) The lever should
consist of a light piece of wood, and should have a length at least three
times as great as the distance from the hook to the turning point. Connect
the balance hook with the lever by a thread or string, and then hang upon
it a small body of known weight. Note the amount of force exerted at the
balance in order to support the weight at different places on the lever.
At what point is the force just equal to the weight? Where is it twice as
great? Where three times? Show that the force required to support the
weight increases proportionally as the weight-arm and as the distance
through which the weight may be moved by the lever. Apply to the action of
the biceps muscle in lifting weights on the forearm.

*A Study of the Action of the Biceps Muscle.*--Place the fingers upon the
tendon of the biceps where it connects with the radius of the forearm.
With the forearm resting upon the table, note that the tendon is somewhat
loose and flaccid, but that with the slightest effort to raise the forearm
it quickly tightens. Now transfer the fingers to the body of the muscle,
and sweep the forearm through two or three complete movements, noting the
changes in the length and thickness of the muscle. Lay the forearm again
on the table, back of hand down, and place a heavy weight (a flatiron or a
hammer) upon the hand. Note the effort required to raise the weight, and
then shift it along the arm. Observe that the nearer it approaches the
elbow the lighter it seems. Account for the difference in the effort
required to raise the weight at different places. Does the effort vary as
the distance from the tendon?

CHAPTER XVI - THE SKIN

Protective coverings are found at all the exposed surfaces of the body.
These vary considerably at different places, each being adapted to the
conditions under which it serves. The most important ones are the _skin_,
which covers the entire external surface of the body; the _mucous
membrane_, which lines all the cavities that communicate by openings with
the external surface; and the _serous membrane_, which, including the
synovial membranes, lines all the closed cavities of the body. In addition
to the protection which it affords, the skin is one of the means by which
the body is brought into proper r