. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. 568 PHYSIOLOGY OF THE DOMESTIC Fig. 243. — Transverse Sec- tion OF A BHANCHIOPOD, SHOWING THE LEAF-LIKE Gills, after Grube. (Jef- frey Bell.) C, heart; I, intestine; N, ventral nerve-cord; D, fold of the integument; BR BR', gills, which are appendages of the hody. Vertebrates are the only animals that breathe through the nostrils or mouth. Fishes inspire only, and all vertebrate animals in whom the ribs are absent or solidly fastened together swallow air. In reptiles we first meet with a complete
. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. 568 PHYSIOLOGY OF THE DOMESTIC Fig. 243. — Transverse Sec- tion OF A BHANCHIOPOD, SHOWING THE LEAF-LIKE Gills, after Grube. (Jef- frey Bell.) C, heart; I, intestine; N, ventral nerve-cord; D, fold of the integument; BR BR', gills, which are appendages of the hody. Vertebrates are the only animals that breathe through the nostrils or mouth. Fishes inspire only, and all vertebrate animals in whom the ribs are absent or solidly fastened together swallow air. In reptiles we first meet with a complete adaptation of a pulmonary structure for the direct aeration of the blood through the influence of the atmosphere. In them there is an internal prolongation of the external integument, constituting the lungs, though they exhibit great simplicity, being, for the most part, capacious sacs, occupying con- siderable bulk, but being but slightly sub- divided, so that the amount of surface exposed is really very small, the blood being exposed on one surface only to aeration. The greatest diversity is met with in these animals as regards the structure and mobility of the thorax. In the saurians, the thorax resembles that of mammals, with movable ribs; in the chelonia, the thoracic walls are rigid and immovable; in the ophidians, the ribs are very numerous and movable, the sternum being absent. A dia- phragm is met with only in the higher sau- rians. In reptiles inspiration is not accom- plished by inhalation, but by deglutition, air being drawn into the pharynx by depression of the hyoid apparatus, and the nares then being closed, the air is forced into the trachea. Expiration is accomplished mainly by the elasticity of the lungs, aided by the abdom- inal muscles, and in saurians and ophidians by the intercostal muscles and the elasticity of the chest-walls. In snakes, as a rule, there is a single, long, cylindrical lung, while the left lung is rudimentary. In birds, though the diap
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