itutes _inspiration_.

481. After the expansion of the chest, the muscles that elevated the
ribs relax, together with the diaphragm. The elasticity of the
cartilages of the ribs depresses them, and the cavity of the chest is
diminished, attended by the expulsion of a portion of the air from the
lungs. At the same time, the muscles that form the front walls of the
abdominal cavity, contract, and press the alimentary canal, stomach,
and liver, upward against the diaphragm; this, being relaxed, yields
to the pressure, rises upward, and presses upon the lungs, which
retreat before it, and another portion of air is expelled from these
organs. This process is called _expiration_.

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478. Describe the action of the intercostal muscles upon the ribs.
479. How does the elevation of the tenth rib compare with the second?
What effect has this elevation upon the lateral diameter of the chest?
480. Describe the process of inspiration. 481. Describe the process by
which the air is forced out of the lungs.

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[Illustration: Fig. 96. A front view of the chest and abdomen in
respiration. 1, 1, The position of the walls of the chest in inspiration.
2, 2, 2, The position of the diaphragm in inspiration. 3, 3, The position
of the walls of the chest in expiration. 4, 4, 4, The position of the
diaphragm in expiration. 5, 5, The position of the walls of the abdomen
in inspiration. 6, 6, The position of the abdominal walls in expiration.]

482. Thus it is obvious that the enlargement of the chest, or
inspiration, is produced in two ways: 1st. By the depression of the
convex portion of the diaphragm; 2d. By the elevation of the ribs. On
the contrary, the contraction of the chest, or expiration, is
produced by the depression of the ribs, and elevation of the central
part of the diaphragm. These movements are successive during life, and
constitute _respiration_.

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Explain fig. 96. 482. In how many ways may the chest be enlarged, and
how is it accomplished? How is the contraction of the chest effected?

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[Illustration: Fig. 97. A side view of the chest and abdomen in
respiration. 1, The cavity of the chest. 2, The cavity of the abdomen. 3,
The line of direction for the diaphragm when relaxed in expiration. 4,
The line of direction for the diaphragm when contracted in inspiration.
5, 6, The position of the front walls of the chest a