resemblance to the Greek letter delta, [Greek: D]. The
biceps, or two-headed muscle, forms a large part of the fleshy mass
in front of the arm. Its use is to bend the forearm on the arm, an act
familiarly known as "trying your muscle." Its direct antagonist is the
three-headed muscle called the triceps. It forms the fleshy mass on
the back of the arm, its use being to draw the flexed forearm into a right
line.

On the back and outside of the forearm are the extensors, which
straighten the wrist, the hand, and the fingers. On the front and inside
of the forearm are the flexors, which bend the hand, the wrist, and
the fingers. If these muscles are worked vigorously, their tendons can be
readily seen and felt under the skin. At the back of the shoulder a large,
spread-out muscle passes upward from the back to the humerus. From its
wide expanse on the back it is known as the latissimus dorsi
(broadest of the back). When in action it draws the arm downward and
backward, or, if one hangs by the hands, it helps to raise the body. It is
familiarly known as the "climbing muscle."

[Illustration: Fig. 37.--A Few of the Important Muscles of the Back.]

Passing to the lower extremity, the thigh muscles are the largest and the
most powerful in the body. In front a great, four-headed muscle,
quadriceps extensor, unites into a single tendon in which the
knee-cap is set, and serves to straighten the knee, or when rising from a
sitting posture helps elevate the body. On the back of the thigh are
several large muscles which bend the knee, and whose tendons, known as the
"hamstrings," are readily felt just behind the knee. On the back of the
leg the most important muscles, forming what is known as the calf, are the
gastrocnemius and the soleus. The first forms the largest part
of the calf. The soleus, so named from resembling a sole-fish, is a muscle
of broad, flattened shape, lying beneath the gastrocnemius. The tendons of
these two muscles unite to form the tendon of Achilles, as that hero
is said to have been invulnerable except at this point. The muscles of the
calf have great power, and are constantly called into use in walking,
cycling, dancing, and leaping.

77. The Effect of Alcoholic Drinks upon the Muscles. It is found that
a man can do more work without alcohol than with it. After taking it there
may be a momentary increase of activity, but this lasts only ten or
fifteen minutes at the most. It is followed by a rapid reducti