orward, while
another set in front of the thigh helps hold the leg straight. These thigh
muscles also tend to pull the trunk forward, but in turn are balanced by
the powerful muscles of the lower back, which help keep the body straight
and braced.

The head is kept balanced on the neck partly by the central position of
the joint between the atlas and axis, and partly by means of strong
muscles. Thus, the combined action of these and other muscles serves to
balance the body and keep it erect. A blow on the head, or a sudden shock
to the nervous system, causes the body to fall in a heap, because the
brain has for the time lost its power over the muscles, and they cease to
contract.

[Illustration: Fig. 36.--Diagram showing the Action of the Chief Muscles
which keep the Body Erect. (The arrows indicate the direction in which
these muscles act, the feet serving as a fixed basis.) [After Huxley.]

_Muscles which tend to keep the body from falling forward._

A, muscles of the calf;
B, of the back of the thigh;
C, of the spinal column.

_Muscles which tend to keep the body from falling backward._

D, muscles of the front of the leg;
E, of the front of the thigh;
F, of the front of the abdomen;
G, of the front of the neck.
]

76. Important Muscles. There are scores of tiny muscles about the
head, face, and eyes, which, by their alternate contractions and
relaxations, impart to the countenance those expressions which reflect the
feelings and passions of the individual. Two important muscles, the
temporal, near the temples, and the masseter, or chewing muscle,
are the chief agents in moving the lower jaw. They are very large in the
lion, tiger, and other flesh-eating animals. On the inner side of each
cheek is the buccinator, or trumpeter's muscle, which is largely
developed in those who play on wind instruments. Easily seen and felt
under the skin in thin persons, on turning the head to one side, is the
sterno-cleido-mastoid muscle, which passes obliquely down on each
side of the neck to the collar bone--prominent in sculpture and painting.

The chest is supplied with numerous muscles which move the ribs up and
down in the act of breathing. A great, fan-shaped muscle, called the
pectoralis major, lies on the chest. It extends from the chest to the
arm and helps draw the arm inward and forward. The arm is raised from the
side by a large triangular muscle on the shoulder, the deltoid, so
called from its