450 



KESPIEATION. 



nated respiration. This function is variously effected in 

 the different grades of the zoological scale. In the lowest 

 orders of animals, as in zoophytes and infusoria, the 

 blood is sufficiently exposed to the air through the thin 

 and impermeable skin. In fishes and most aquatic animals, 

 aeration of the blood is effected by means of gills ; these are 

 thin, feathery-like tufts, of a highly vascular structure, at- 

 tached to the mucous membrane of the pharynx, and either 

 projecting externally to be waved in the surrounding liquid, 

 or remaining within the pharynx, and subjected to a constant 

 stream of water, which is taken into the mouth, and expelled 

 through two lateral pharyngeal orifices, in an act, some- 

 what analogous to deglutition. In terrestrial and air- 

 breathing animals, the apparatus of respiration is generally 

 situated internally, and is more or less complex, according to 

 the rapidity of the respiratory process. It may be composed 

 of two simple, undivided cavities or lungs, as in the sala- 

 mander ; each cavity may be incompletely divided into sepa- 

 rate cells as in frogs and the majority of reptiles; or the 

 single air-tube proceeding to each lung may repeatedly sub- 

 divide into numerous smaller tubuli, each of which ultimately 

 terminate in a variable number of cells (pulmonary cells 

 or vesicles). This last arrangement we find in warm-blooded 

 animals, the high temperature of which calls for great acti- 

 vity in the respiratory process. 



In our domestic quadrupeds, the respiratory apparatus is 

 formed, 1st, of the air passages with the lungs and pleura; 

 and, 2nd, of the thorax, including the respiratory muscles. 

 The air passages comprise the nasal chambers, pharynx, 

 larynx, trachea, bronchia, air sacs, and pulmonary cells. 



The nostrils are two cavities placed above the mouth, and 

 extending the whole length of the face. The anterior extre- 

 mities open externally ; the posterior conduct into the 



