. The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners. Physiology, Comparative; Domestic animals. RESPIRATION. 567 (Fig. 244). The water is taken in by a process of swallowing, the mouth being first distended, and, as the muscles contract, the water is expelled through the aperture on either side of the pharynx into a cavity called the gill-cavity, and, as it passes over the gills, the oxygen of the atmos- phere held in solution is absorbed by the blood. Fish are thus admirably fitted for aquatic respiration, but die on removal from the wat
. The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners. Physiology, Comparative; Domestic animals. RESPIRATION. 567 (Fig. 244). The water is taken in by a process of swallowing, the mouth being first distended, and, as the muscles contract, the water is expelled through the aperture on either side of the pharynx into a cavity called the gill-cavity, and, as it passes over the gills, the oxygen of the atmos- phere held in solution is absorbed by the blood. Fish are thus admirably fitted for aquatic respiration, but die on removal from the water from the fact that, as the gills dry, absorption of oxygen is impaired, and the gills cling together, and so prevent ex- posure of their greater portion to the air. Under such circumstances fish then die from asphyxia. In some cases there is provided in addition an air-bladder or swimming-bladder, like a rudimentary sac of the air- breathing apparatus of the higher animals. In its simplest condition it is entirely closed, and can, therefore, serve no purpose except to C. Fig. 242.—Diagrammatic Section of a Lamellibranch (Fresh-water Mussel, Anodon) Through the Heart. (Huxley.) F, ventricle; G, auricles; C, rectum; P, pericardium : H, inner gill; I, outer gill; O Q, organ of Bojanus; 11, foot; A A, mantle-lobes. regulate the specific gravity during swimming. In others this bladder is connected with the alimentary canal by a short, wide tube, called the ductus pneumaticus, and is filled by the process of swallowing. If we admit, as seems perfectly reasonable, that the air-bladder of the fish is a rudimentary lung, it ma}' be stated that all vertebrates in the course of their life have two different kinds of respiratory apparatus. Every form of vertebrate breathes through gills during embryonic life; in the fish and a few reptiles the gills are permanent, but in others they disappear, and, while traces of a lung are seen in all vertebrates, they acquire full development only
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