The mechanism by which
the aerating medium is renewed in these different respiratory
organs is very various. Among branchiated Vertebrata, Amphioxus
stands alone in having ciliated branchial organs, which
form a net-work very similar to the perforated pharyngeal wall
of the Ascidians. Most Fishes breathe by taking aerated water
in at the mouth, and then shutting the oral aperture, and
forcing the water through the branchial clefts, when it flows
over the branchial filaments.
, which have the thoracic skeleton
incomplete (as the Amphibia
), breathe by distending their
pharyngeal cavity with air; and then, the mouth and nostrils
being shut, pumping it, by the elevation of the hyoidean apparatus
and floor of the pharynx, into the lungs. A Frog, therefore,
cannot breathe properly if its mouth is kept wide open.
In most Reptilia
, and in all Aves
, the sternum
and ribs are capable of moving in such a way as alternately
to increase and diminish the capacity of the thoracico abdominal
cavity, and thereby to give rise to an inspiratory
and expiratory flow of air.
In the Reptilia
, the elastic lungs dilate with the inspiratory,
and contract with the expiratory, act; but, in Aves
air rushes through the principal bronchial passages of the fixed
and little distensible lungs, into the very dilatable and compressible
air-sacs. From these the act of expiration expels it
back through the principal bronchial passages to the trachea,
and so out of the body.
Both in Reptilia
(e. g., Chelonia
) and in Aves
fibres pass from the ribs to the surface of the lungs beneath
the pleuroperitoneal membrane, and this rudimentary diaphragm
acquires a very considerable development in the Ratitae
or struthious birds. So far as the contraction of these
fibres tends to remove the ventral from the dorsal walls of the
lungs, they must assist inspiration. But this diaphragmatic inspiration
remains far weaker than the sterno-costal inspiration.
Finally, in the Mammalia
, there are two equally-important
respiratory pumps, the one sterno - costal, the other diaphragmatic.
, though it makes its appearance in Sauropsida
, only becomes a complete partition between the
thorax and the abdomen in mammals; and, as its form is such
that, in a state of rest, it is concave toward the abdominal
cavity, and convex toward the thorax, the result of its contraction,
and consequent flattening, necessarily is to increase
the capacity of the thorax, and thus pump the air into the
elastic lungs, which occupy a large part of the thoracic cavity.
When the diaphragm ceases to contract, the elasticity of the
lungs is sufficient to expel the air taken in.
Thus, mammals have two kinds of respiratory mechanism,
either of which is efficient by itself, and may be carried on independently
of the other.