ibs, (7, 7, 7,) with their intercostal muscles, (8, 8.) These
ribs, in their natural position, have their anterior cartilaginous
extremity at 4, while the posterior extremity is attached to the
vertebrae, (6,) which are neither elevated nor depressed in respiration.
1, 1, and 2, 2, parallel lines, within which the ribs lie in their
natural position. If the anterior extremity of the ribs is elevated from
4 to 5, they will not lie within the line 2, 2, but will reach the line
3, 3. If two hands extend from 1, 1, to 2, 2, they will effectually
prevent the elevation of the ribs from 4 to 5, as the line 2, 2, cannot
be moved to 3, 3.]

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What effect has a full inspiration on the ribs and diaphragm? How is
the chest enlarged between the spinal column and sternum? What is said
of the lateral diameter of the chest? Explain fig. 95.

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478. The central portion of the ribs is raised by the action of
intercostal muscles. The first, or upper rib, has but little movement;
the second has more motion than the first, while the third has still
more than the second. The second rib is elevated by the contraction of
the muscles between it and the first. The third rib is raised by the
action of two sets of muscles; one lies between the first and second
ribs, the other between the second and third. The motion of each
succeeding rib is increased, because it is not only acted upon by the
muscles that move the ribs above, but by an additional intercostal; so
that the movement of the twelfth rib is very free, as it is elevated
by the contraction of eleven muscles.

479. The tenth rib is raised eight times as much as the second rib,
and the lateral diameter of the lower portion of the chest is
increased in a corresponding degree. At the same time, the muscular
margin of the diaphragm contracts, which depresses its central
portion; and in this way, the chest is enlarged forward, laterally,
and downward, simultaneously with the relaxation of the walls of the
abdomen.

480. The lungs follow the variations of capacity in the chest,
expanding their air-cells when the latter is enlarged, and contracting
when the chest is diminished. Thus, when the chest is expanded, the
lungs follow, and consequently a vacuum is produced in their
air-cells. The air then rushes through the mouth and nose into the
trachea and its branches, and fills the vacuum as fast as it is made.
This mechanical process const