2224. When the branchial apertures close, the vessels separate from the arches, and betake themselves to a glandular body lying in front of them. The thyroid gland is the remnant of the former branchial formation, and is therefore found only in Reptiles, Birds, and Mammalia.
Lungs.
2225. In Fishes the internal organ of respiration is already indicated by a cæcal eversion of the pharynx, which is surrounded by the branchial arches. This membranous cæcal eversion is called the swimming-bladder, which in the higher animals becomes double on account of their symmetry, and is then called lung.
2226. In the Fish the aquatic and aerial process of respiration are present together, the former being external, the latter internal.
2227. The branchial arches having coalesced, are converted in the higher animals into tracheal rings, into the larynx and the posterior cornua of the hyoid bone, if these are present, as in the Reptiles. The larynx is therefore no special organ, but only a remnant of the branchial respiratory apparatus.
2228. The laryngeal vessels are, like the thyroid glands, branchial vessels, and in Fishes, therefore, the branchial vessels do not correspond to the pulmonary vessels, but to those of the trachea. The pulmonic vessels of Fishes are the blood-vessels of the swimming-cyst, which convey blood directly into the heart, whereby this organ obtains the signification of the left or arterial heart.
2229. When the branchial apertures have coalesced, the nose then opens into the mouth or into the trachea, and in this way the nostrils assume the complete character of aerial foramina.
2230. The nose is therefore originally an organ of smell, and then a part of the respiratory system. It is the animal lung.
2231. As the secretion of bone is a product of the more powerful process of oxydation, so do the bony rings multiply beneath the branchial arches or the larynx, and are called tracheal rings. In the feebly respiring Reptiles the trachea is therefore still in a membranous condition, but in Birds and Mammalia, it is surrounded by many rings, or is a repetition of the larynx.
2232. In Birds even a kind of inferior larynx originates, which is provided with muscles and can produce tones.