achea. 5, The right
bronchial tube. 6, The left bronchial tube. 7, 7, 7, 8, 8, 8, The
subdivisions of the right and left bronchial tubes. 9, 9, 9, 9, 9, 9,
Air-cells.]

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What membrane lines the trachea and its branches? What is peculiar in
its structure? What does fig. 91 represent?

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_Observation._ The structure of the trachea and lungs may be
illustrated, by taking these parts of a calf or sheep and inflating
the air-vesicles by forcing air into the windpipe with a pipe or
quill. The internal structure may then be seen by opening the
different parts.

471. The lungs, like other portions of the system, are supplied
with nutrient arteries and nerves. The nervous filaments that
are distributed to these organs are in part from the tenth pair,
(par vagum,) that originates in the brain, and in part from the
sympathetic nerve. The muscles that elevate the ribs and the
diaphragm receive nervous fibres from a separate system, which is
called the respiratory.

[Illustration: Fig. 92. 1, A bronchial tube. 2, 2, 2, Air-vesicles. Both
the tube and vesicles are much magnified. 3, A bronchial tube and
vesicles laid open.]

_Observation._ When the mucous membrane of a few of the larger
branches of the windpipe is slightly inflamed, it is called a "cold;"
when the inflammation is greater, and extends to the lesser air-tubes,
it is called _bronch-i'tis_. When the air-cells and parenchyma become
inflamed, it is called inflammation of the lungs. Coughing is a
violent expulsory effort by which air is suddenly forced through the
bronchia and trachea to remove offending matter.

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How may the structure of the trachea and its branches be illustrated?
471. Are the lungs supplied with nutrient arteries? Where are the
respiratory nerves distributed? From what source do these organs
derive their nervous filaments?

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472. The RIBS are joined to the spinal column at their posterior
extremity; and in front, they terminate in cartilages, which unite
them to the sternum. They incline downward, from the spinal column to
the breast-bone, and form resisting walls that assist in producing the
partial vacuum necessary for inspiration.

[Illustration: Fig. 93. A section of the chest when the lungs are
inflated. 1, The diaphragm. 2, The muscular walls of the abdomen.]

[Illustration: Fig. 94. A section of the chest when the lungs are