_deltoid_ at the upper part of the arm beyond the projection of
the shoulder.

[Fig. 119]

Fig. 119--Back and front views of important muscles.

_Of the forearm_: The _flexors_ of the fingers, on the front side, and the
_extensors_ of the fingers, on the back of the forearm (Fig. 119).

_Of the hand_: The _adductor pollicis_ between the thumb and the palm.

_Of the trunk_: The _pectoralis major_, between the upper front part of
the thorax and the shoulder; the _trapezius_, between the back of the
shoulders and the spine; the _rectus abdominis_, passing over the abdomen
from above downward; and the _erector spinae_, found in the small of the
back.

_Of the hips_: The _glutens maximus_, fastened between the lower back part
of the hips and the upper part of the femur.

_Of the upper part of the leg_: The _rectus femoris_, the large muscle on
the front of the leg which connects at the lower end with the kneepan.

_Of the lower leg_: The _tibialis anticus_ on the front side, exterior to
the tibia, and the _gastrocnemius_, the large muscle in the calf of the
leg. This is the largest muscle of the body, and is connected with the
heel bone by the _tendon of Achilles_ (Fig. 119).

The use of these muscles is, in most instances, easily determined by
observing the results of their contraction.

HYGIENE OF THE MUSCLES

The hygiene of the muscles is almost expressed by the one word _exercise_.
It is a matter of everyday knowledge that the muscles are developed and
strengthened by use, and that they become weak, soft, and flabby by
disuse. The effects of exercise are, however, not limited to the large
muscles attached to the skeleton, but are apparent also upon the
involuntary muscles, whose work is so closely related to the vital
processes. While it is true that exercise cannot be applied directly to
the involuntary muscles, it is also true that exercise of the voluntary
muscles causes a greater activity on the part of those that are
involuntary and is indirectly a means of exercising them.

*Exercise and Health.*--In addition to its effects upon the muscles
themselves, exercise is recognized as one of the most fundamental factors
in the preservation of the health. Practically every process of the body
is stimulated and the body as a whole invigorated by exercise properly
taken. On the other hand, a lack of exercise has an effect upon the entire
body somewhat similar