scent of
the left diaphragm, for the abdominal walls relax and allow of the
increasing volume of the stomach to accommodate itself. The spleen, R,
is occasionally subject to an extraordinary increase of bulk; and this
organ, like the enlarged liver and the distended stomach, will, to some
extent, obstruct the movements of the diaphragm in the act of
respiration, but owing to its free attachments it admits of a change of
place. The abdominal viscera, one and all, admit of a change of place;
the peculiar forms of those mesenteric bonds by which they are
suspended, allow them to glide freely over each other; and this
circumstance, together with the yielding nature of the abdominal
parietes, allows the thoracic organs to have full and easy play in the
respiratory movements performed by agency of the diaphragm.

The muscles of respiration perform with ease so long as the air has
access to the lungs through the normal passage, viz., the trachea. While
the principle of the thoracic pneumatic apparatus remains underanged,
the motor powers perform their functions capably. The physical or
pneumatic power acts in obedience to the vital or muscular power, while
both stand in equilibrium; but the ascendancy of the one over the other
deranges the whole thoracic machine. When the glottis closes by muscular
spasm and excludes the external air, the respiratory muscles cease to
exert a motor power upon the pulmonary cavity; their united efforts
cannot cause a vacuum in thoracic space in opposition to the pressure of
the external air. When, in addition to the natural opening of the
glottis, a false opening is made in the side at the point K, the air
within the lung at I, and external to it in the now open pleural cavity,
will stand in equilibrio; the lung will collapse as having no muscular
power by which to dilate itself, and the thoracic dilator muscles will
cease to affect the capacity of the lung, so long as by their action in
expanding the thoracic walls, the air gains access through the side to
the pleural sac external to the lung.

Whether the air be admitted into the pleural sac, by an opening made in
the side from without, or by an opening in the lung itself, the
mechanical principle of the respiratory apparatus will be equally
deranged. Pneumo-thorax will be the result of either lesion; and by the
accumulation of air in the pleura the lung will suffer pressure. This
pressure will be permanent so long as the air has no egress fr