. Animal physiology. Physiology, Comparative; Physiology, Comparative. 74 GENERAL STRUCTURE OF BIRDS. opens into the lungs, e, which are themselves small, and are attached to the ribs, instead of lying freely in the cavity of the chest; but the space they would otherwise have occupied is filled up by the large air-cells,/,/, which communicate freely with the lungs and with each other, and which even occupy a large part of the cavity of the abdomen, as seen in the figure. The use of these air-cells in the respiration of Birds, will be explained hereafter (§. 356). 91. In the class of
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 74 GENERAL STRUCTURE OF BIRDS. opens into the lungs, e, which are themselves small, and are attached to the ribs, instead of lying freely in the cavity of the chest; but the space they would otherwise have occupied is filled up by the large air-cells,/,/, which communicate freely with the lungs and with each other, and which even occupy a large part of the cavity of the abdomen, as seen in the figure. The use of these air-cells in the respiration of Birds, will be explained hereafter (§. 356). 91. In the class of Reptiles we find a variety of form so re- markable, that, if we were influ- enced by this alone, we should ^ti\^z™::™^T-^™™y «*»* *> «**»]« it d the trachea, e the lungs, /// air-ceiis, contains as belonging to the same in which are also seen the tubes by which , ,, , , „ .. these air-ceiis communicate with the group; yet the structure of the lungs- internal organs, on which classifi- cation is founded, is essentially the same in all. Four obviously different tribes, turtles, lizards, serpents, and frogs, are brought together by the following characters. They are all oviparous (Chap, xv.), in this respect agreeing with Birds and Fishes; but they are cold-blooded, and have not a complete apparatus for the double circulation of the blood, in which respect they differ from Birds; and they breathe air by means of lungs, instead of breath- ing water by gills, in wThich respect they differ from Fishes. But by the lowest group, that of frogs and their allies, this class is united to that of fishes in a most remarkable manner; for these animals in their young state breathe by gills, and lead the life of a fish; and some of them retain their gills during the whole of life, even after the lungs are developed (§. 97). The first three of the tribes just mentioned, undergo no such change: and they further agree in this, that they breathe air during the whole of their li
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