118 



COMPARATIVE ZOOLOGY. 



blood is exposed to tlie influence of the air, and, therefore, 

 tlie 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 Amjjhibians and Rep- 

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

 Birds and Mammals, they are exceedingly numerous and 

 minnte." 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- 



FiG. S5.— Luu^i? of II Frog: o, hyoid 

 uppariitus ; b, cartilagiuous dug at 

 root of the lungs; c, pulmonary 

 veP8el8 ; (?, pulmonary sacs, having 

 this peculiarity common to all cold- 

 blooded air-breathers, that the tra- 

 chea does not divide iuto bronchial 

 branches, but terminates abruptly 

 by orifices which open at once iuto 

 the general cavity. A ciirtilagiuous 

 net-work divides the space into lit- 

 tle sacs, on the walls of which the 

 capillaries are Si)read. 



Fig. 80. — Distribution of Air -tubes iu Mam- 

 malian Lungs : a, larynx ; 6, ftachea ; t, <l, 

 left and right bronchial tabes ; e, f, g, the 

 ramifications. In Man the subdivision con- 

 tinues until the ultimate tubes are one twen.. 

 ty-flfth of an inch in diameter. Each lobule 

 represents in miniature the structure of the 

 entire lung of a Frog. 



