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Show 36 ANIMALIA VERTEBRATA. Subdivision of the Vertebrata into Four Classes. We have just seen how far vertebrated animals. ~e~emble each other; they present, however, four great sub~IVISIO?s or cI asses, ch arac teri.sed by the kind or power of t. heir m.o tio.n s, which depend themselves on the quantity of their respiratiOn, inasmuch as it is from this respiration that the muscular fibres derive the strength of their irritability· The quantity of respiration depends .upo~ two age?ts: the first is the relative amount of blood which IS poured Int? the resp·u at ory organ in a ;ryri ven instant of time; the second IS• •th e relative amount of oxygen which enters into the composition of the surrounding fluid. The quantity of th~ for~er depends upon the disposition of the organs of Circulation and respiration. The organs of the circulation may be dou?le, so that all the blood which is brought back from the various parts of the body by the veins, is forced to circulate through the respi· ratory organ, previous to resuming its former course through the arteries· or they may be simple, so that a part only of the blood is 'o bliged to pass through that organ, the remam. de r returning directly to the body. . . The latter is the case with reptiles. The quantity of the1r respiration, and all their qualities which depend on it, vary with the amount of blood thrown into the lungs at each pulsation. Fishes have a double circulation, but their organ of respi· ration is formed to execute its function through the medium of water; and their blood is only acted on by the portion of oxygen it contains, so that the quantity of their respiration is perhaps less than that of reptiles. In the mammalia the circulation is double, and the aerial respiration simple, that is, it is performed in the lungs only; their quantity of respiration is, consequently, superior to that of reptiles, on account of the form of their respiratory organ, and to that of fishes from the nature of their surrounding ele· ment. ANIMALIA VERTEBRATA. 37 The quantity of respiration in birds is even superior to that of quadrupeds, not only because they have a double circulation and an aerial respiration, but also because they respire by many other cavities besides the lungs, the air penetrating throughout their bodies, and bathing the branches of the aorta, as well as those of the pulmonary artery. Hence result the four different kinds of motion for which the four classes of vertebrated animals are more particularly designed: quadrupeds, in which the quantity ofrespiration is moderate, are generally formed to walk and run, both motions being characterized by precision and vigour; birds, which have more of it, possess the muscular strength and lightness requisite for flight; reptiles, where it is diminished, are condemned to creep, and many of them pass a portion of their lives in a kind of torpor; fishes, in :fine, to execute their motions, require to be supported in a fluid whose specific gravity is nearly as great as their own. All the circumstances of organization peculiar to each of • these four classes, and those especially which regard motion and the external sensations, have a necessary relation with these essential characters. The mammalia, however, have particular characters in their viviparous mode of generation, in the manner by which the fretus is nourished in the uterus through the medium of the placenta, and in the mammm by which they suckle their young. The other classes, on the contrary, are oviparous, and if we compare them to the :first, we shall :find such numerous points ?f resemblance as announce a peculiar system of organization m the great general plan of the vertebrata. |