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  Section: Anatomy of Vertebrate Animals » The Muscles and the Viscera
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The Respiratory Organs


Vertebrated animals may pos sess either branchiae for breathing the air contained in water, or lungs for atmospheric respiration; or they may possess both kinds of respiratory organs in combination.

Except in Amphioxus, the branchiae are always lamellar, or filamentous, appendages of more or fewer of the visceral arches; being sometimes developed only on the proper branchial arches, sometimes extending to the hyoidean arch, or (as would appear to be the case with the spiracular branchiae of some fishes) even to the mandibular arch. The branchiae are always supplied with blood by the divisions of the cardiac aorta; and the different trunks which carry the aerated blood away, unite to form the subvertebral aorta, so that all vertebrated animals with exclusively branchial respiration have the heart filled with venous blood.

In the early life of many branchiated Vertebrata, the branchiae project freely from the visceral arches to which they are attached, on the exterior of the body; and in some Amphibia, such as the Axolotl (Siredon), they retain their form of external plume-like appendages of the neck throughout life. But in the adult life of most Fishes, and in the more advanced condition of the Tadpoles of the higher Amphibia, the branchiae are internal, being composed of shorter processes, or ridges, which do not project beyond the outer edges of the branchial clefts; and, generally, become covered by an operculum developed from the second visceral arch.

The lungs of vertebrated animals are sacs, capable of being filled with air, and developed from the ventral wall of the pharynx, with which they remain connected by a shorter or longer tube, the trachea, the division of this for each luug being a bronchus. Venous blood is conveyed to them directly from the heart by the pulmonary arteries, and some (Generally all. but in some Amphibia, such as Proteus, part of the blood supplied to the lungs enters the general circulation.) or all of the blood which they receive goes back, no less directly, to the same organ by the pulmonary veins.

The vascular distribution thus described constitutes an essential part of the definition of a lung, as many fishes possess hollow sacs filled with air; and these sacs are developed, occasionally, from the ventral, though more commonly from the dorsal, wall of the pharynx, oesophagus, or stomach. But such air-sacs - even when they remain permanently connected with the exterior by an open passage or pneumatic duct - are air-bladders, and not lungs, because they receive their blood from the adjacent arteries of the body, and not direct from the heart, while their efferent vessels are connected only with the veins of the general circulation.

The wall of each pulmonic air-sac is at first quite simple, but it soon becomes cellular by the sacculation of its parietes. In the lower pulmonated Vertebrata, the sacculation is more marked near the entrance of the bronchus; and when the lungsac is long, as in many Amphibia and in Snakes, the walls of the posterior end may retain the smooth condition of the embryonic lung.

In Chelonia and Crocodilia, the lung is completely cellular throughout, but the bronchi do not give off branches in the lungs. In Birds, branches are given off at right angles; and, from these, secondary branches, which lie parallel with one another, and eventually anastomose. In Mammalia, the bronchi divide dichotomously into finer and finer bronchial tubes, which end in sacculated air-cells.

Blind air-sacs are given off from the surfaces of the lungs on the Chamaeleonidae, and the principal bronchial tubes terminate in large air-sacs in Aves.

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