contracted. 1, The diaphragm in common expiration. 2, 2, The muscular
walls of the abdomen. 3, The position of the diaphragm in forced
expiration.]

These engravings show the diaphragm to be more convex, and the walls
of the abdomen more flattened, when the lungs are collapsed, than when
they are inflated.

473. The DIAPHRAGM is a flexible circular partition, that separates
the respiratory from the digestive organs, and the chest from the
abdomen. Its margin is attached to the spinal column, the sternum, and
cartilages of the lower ribs. The lungs rest upon its upper surface,
while the liver and stomach are placed below it, (fig. 88.) In a
state of repose, its upper surface forms an arch, the convexity of
which is toward the chest. In forced expiration, its upper point
reaches as high as the fourth rib. In an ordinary inspiration, it is
depressed as low as the seventh rib, which increases the capacity of
the chest.

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472. Describe the ribs. Explain figs. 93 and 94. 473. Describe the
diaphragm.

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474. The RESPIRATORY muscles are, in general, attached at one
extremity to the parts about the shoulders, head, and upper portion
of the spinal column. From these, they run downward and forward, and
are attached, at the opposite extremity, to the sternum, clavicle,
and upper rib. Other muscles are attached at one extremity to a rib
above, and by the opposite extremity to a rib below. These fill the
spaces between the ribs, and, from their situation, are called
_in-ter-cost'al_ muscles.

_Observation._ 1st. There are several actions of common occurrence,
that are intimately connected with respiration; such as hiccough,
sneezing, &c. Hiccough is an involuntary contraction of the muscles of
respiration, particularly the diaphragm.

2d. Sneezing is a violent, involuntary contraction of the respiratory
muscles, as in hiccough. When an acrid stimulant, as snuff, is applied
to the mucous membrane of the nose, an irritation is produced which is
accompanied by a violent expulsion of air from the lungs. This is
owing to the connection between the nasal and respiratory nerves.

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What is its form when not in action? 474. Where do the respiratory
muscles make their attachment? What name is given to those muscles
that fill the places between the ribs? What is hiccough? What is
sneezing?

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CHAPTER XXIV.

PHYSI