118 



COMPARATIVE ZOOLOGY. 



exposed to the influence of the air, and, therefore, the 

 more active the respiration and the purer the blood. The 

 lungs are relatively largest in Reptiles, and smallest in 

 Mammals. But in the cold-blooded Amphibians and Rep- 

 tiles, the air-cells are few and large ; in the warm-blooded 

 Birds and Mammals, they are exceedingly numerous and 

 minute. 66 In Birds and Mammals, the blood in the capil- 

 laries is exposed to the air on all sides ; in the Reptiles, 

 on one only. Respiration is most perfect in Birds ; they 

 require, relatively to their weights, more air than Mam- 

 mals or Reptiles, and most quickly die for lack of it. In 

 Birds, respiration is not confined to the lungs; but, as in 

 Insects, extends through a great part of the body. Air- 

 sacs connected with the lungs exist in the abdomen and 

 under the skin of the neck, wings, and legs. Even the 

 bones are hollow for this purpose ; so that if the wind- 



FIQ. 85. Lungs of a Frog: a, hyoid 

 apparatus ; b, cartilaginous ring at 

 root of the lungs; c, pulmonary 

 vessels ; d, pulmonary sacs, having 

 this peculiarity common to all cold- 

 blooded air-breathers, that the tra- 

 chea does not divide into bronchial 

 branches, but terminates abruptly 

 by orifices which open at once into 

 the general cavity. A cartilaginous 

 net-work divides the space into lit- 

 tle sacs, on the walls of which the 

 capillaries are spread. 



FIG. 86. Distribution of Air-tubes in Mam- 

 malian Lungs : a, larynx; b, trachea; c, d, 

 left and right bronchial tubes ; e, /, g, the 

 ramifications. In Man the subdivision con- 

 tinues until the ultimate tubes are one twen- 

 ty-fifth of an inch in diameter. Each lobule 

 represents in miniature the structure of the 

 entire lung of a Frog. 



