n
can the animal expel all the air completely from its lungs, since by no
effort of its own, can it contract thoracic space beyond the natural
limit. On the other hand, the utmost degree of expansion of which the
lungs are capable, exactly equals that degree in which the thoracic
walls are dilatable by the muscular effort; and, therefore, between the
extremes of inspiration and expiration, the lungs still hold closely
applied to the costal parietes. The air within the lungs is separated
from the air external to the thorax, by the thoracic parietes. The air
within and external to the lungs communicate at the open glottis. When
the glottis closes and cuts off the communication, the respiratory act
ceases--the lungs become immovable, and the thoracic walls are (so far
as the motions of respiration are concerned) rendered immovable also.
The muscles of respiration cannot, therefore, produce a vacuum between
the pulmonic and costal pleura, either while the external air has or has
not access to the lungs. Upon this fact the mechanism of respiration
mainly depends; and we may see a still further proof of this in the
circumstance that, when the thoracic parietes are pierced, so as to let
the external air into the cavity of the pleura, the lung collapses and
the thoracic side ceases to exert an expansile influence over the lung.
When in cases of fracture of the rib the lung is wounded, and the air of
the lung enters the pleura, the same effect is produced as when the
external air was admitted through an opening in the side.

When serous or purulent effusion takes place within the cavity of the
pleura, the capacity of the lung becomes lessened according to the
quantity of the effusion. It is more reasonable to expect that the soft
tissue of the lung should yield to the quantity of fluid within the
pleural cavity, than that the rigid costal walls should give way
outwardly; and, therefore, it seldom happens that the practitioner can
discover by the eye any strongly-marked difference between the thoracic
walls externally, even when a considerable quantity of either serum,
pus, or air, occupies the pleural sacs.

In the healthy state of the thoracic organs, a sound characteristic of
the presence of the lung adjacent to the walls of the thorax may be
elicited by percussion, or heard during the respiratory act through the
stethoscope, over all that costal space ranging anteriorly between B,
the first rib, and I K, the eight and ninth