Section 39. The blood coming from all parts of the body, partly
robbed of its oxygen and containing much carbon dioxide and other
katastases, enters the right auricle of the heart through three great
veins, the median vena cava inferior from the posterior parts of the
body, and the paired venae cavae superiores from the anterior. With
the beating of the heart, described below, it is forced into the right
ventricle and from there through the pulmonary artery (p.a.) seen in
the figure passing under the loop of the aorta (ao.) to the lungs.

Section 40. The lungs (lg. Figure 1, Sheet 1) are moulded to the
shape of the thoracic cavity and heart; they communicate with the
pharynx by the trachea (tr. in Figure 1, Sheet 1) or windpipe, and
are made up of a tissue of continually branching and diminishing
air-tubes, which end at last in small air-sacs, the alveoli. The
final branches of the pulmonary arteries, the lung capillaries, lie
in the walls of these air-sacs, and are separated from the air by an
extremely thin membrane through which the oxygen diffuses into,
and the carbon dioxide escapes from, the blood.

Section 41. The mechanism of respiration will be understood by
reference to Figure 3, Sheet 2. It will be noted, in dissecting that the
lungs have shrunk away from the walls of the thorax; this
collapse occurs directly an aperture is made in the thorax wall, and
is in part due to their extreme elasticity. In life the cavity of the
thorax forms an air-tight box, between which and the lungs is a
slight space, the pleural cavity (pl.c.) lined by a moist membrane,
which is also reflected, over the lungs. The thorax wall is muscular
and bony, and resists the atmospheric pressure on its outer side, so
that the lungs before this is cut through are kept distended to the
size of the thoracic cavity by the pressure of the air within them. In
inspiration (or breathing-in) the ribs are raised by the external
intercostal (Anglice, between-ribs, e.i.c.m.) and other allied
muscles, and the diaphragm (dia.) contracts and becomes flatter;
the air is consequently sucked, in as the lungs follow the movement
of the thorax wall. In expiration the intercostals and diaphragm relax
and allow the elastic recoil of the lungs to come into play. The
thoracic wall is simultaneously depressed by the muscles of the
abdominal area, the diaphragm thrust forwards, as the result of the
displacement and compression of the alimentary viscera thus