ORGANS OF RESPIRATION IN THE VERTEBRATA. 95 



AMPHIBIA. 



Progs and salamanders in their tadpole state, breathe by gills, 

 which during the earliest periods of their existence are placed ex- 

 ternally; they also possess rudimentary lungs, which become deve- 

 loped as the animals change from the aquatic to the aerial respira- 

 tion. The perennibranchiate amphibia, as the proteus, siren, and 

 axolotl also possess both sets of organs, and retain them through 

 life, but here the lungs always present the rudimentary type, and 

 it is even doubted whether they subserve at all to the function of 

 respiration. In these as well as in the larval condition of the 

 caducibranchiate genera, the pulmonary organ is in the form of a 

 mere sac, and it is only during the metamorphosis that it assumes 

 the cellular character. In the adult state of all the higher orders 

 of amphibia respiration is accomplished in a manner different from 

 all other air-breathing animals, viz., the mouth being fully distended 

 by the air which enters through the nostrils, these passages together 

 with the pharyngo-oesophageal are closed, the mouth then suddenly 

 contracts by the action of the surrounding muscles, and the air is 

 forced by an effort of deglutition through the glottis and trachea 

 into the lungs. Hence, one of the most effectual ways of killing a 

 frog is to keep its mouth open for a short period ; this mode of 

 inspiration is accounted for by recollecting that the ribs are 

 absent in these animals. 



REPTILIA. 



In most of the amphibia the trachea is very short and perfectly 

 membranous. In this class it is somewhat longer, and cartilaginous 

 plates begin to appear in it ; these plates which are first seen in the 

 dactylethra, do not form perfect rings, but present the appearance 

 of perforated lamellse irregularly disposed. In the bronchi of the 

 casciliae, however, the cartilaginous rings are much more complete. 

 In lizards and serpents the lung is a mere cavity with cellular 

 parietes, having perforations which communicate with the neigh- 

 bouring cells. Turtles have a more complicated structure, approach- 

 ing that of warm-blooded animals. 



AVES. 



The lungs, in this class, are confined to the back parts of the 

 cavities of the thorax and abdomen by the serous membrane com- 

 mon to these cavities ; they are of a flattened, elongated form, smooth 

 anteriorly, and grooved posteriorly by the ribs, between which they 

 are impacted ; they are of a bright red colour, and of a loose spongy 

 texture; on the surface of the lungs there are openings through 

 which air passes from the bronchial tubes into large neighbouring 

 cells. In birds not organised for flight these cells, are confined to 



