or less tendinous or sinew-like in the middle. Being attached
to the spinal column behind and to the lower six or seven ribs, when
the muscle contracts it becomes less domed in shape--less convex
upward--and of course descends to a variable degree depending on the
extent of the muscular contraction. As to whether the ribs, and with
them the abdominal muscles, are drawn in or the reverse, is determined
wholly by the degree of force with which the contraction takes place
and the extent to which it is resisted. Throughout the body muscles
are arranged in sets which may either cooeperate with or antagonize
each other, as required. The forcible bending of one's arm by another
person may be resisted by one through the use of certain muscles. In
this the action of the muscles which bend the arm is imitated by the
agent seeking to perform this movement for us. The muscles acting in
opposition to certain others are said to be their _antagonists_.

Were the diaphragm to contract moderately the ribs would be but little
drawn in, even if no muscles acted as antagonists. But, as a matter of
fact, this domed muscle descends at the same time as the ribs ascend,
because of the action of the muscles attached to them. The diaphragm
being concave below toward the abdomen, the contents of this cavity
fit closely to its under surface. There are found the liver, stomach,
intestines, etc.--a part of great practical importance, as will be
shown presently.

Naturally, in breathing, the organs of the abdomen, especially those
above, are pressed down somewhat with the descent of the diaphragm in
inspiration, and, in turn, push out the abdominal walls. If, however,
the midriff contract so powerfully that the lower ribs are drawn
inward, the abdominal walls follow them. Although the actual extent of
the descent of the diaphragm is small in itself, since the total
surface is large it effects a very considerable enlargement of the
chest in the vertical diameter.

The capacity of the lungs for air is a very variable quantity:

1. The quantity of air taken in with a single inspiration in quiet
breathing (_tidal air_) is about 20-30 cubic inches.

2. The quantity taken in with the deepest possible inspiration
(_complemental air_) is about 100 cubic inches.

3. The quantity that may be expelled by the most forcible expiration
(_supplemental air_) is about 100 cubic inches.

4. The quantity that can under no circumstances be expelled (_residual
a