The Mechanism of Respiration

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.

Pulmonated Vertebrata, 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 and Mammalia, 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, the 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, muscular 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. The diaphragm, 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.