Now, on account of the large number and flexibility of the joints, the body could not be kept in an upright position without the cooperation of certain groups of muscles.  The muscles of the calf of the leg, acting on the thigh bone, above the knee, keep the body from falling forward, 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 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.