THE STRUCTURE AND FUNCTION OF THE ELEMENTARY LUNG 451 



intake through the skin is much less than the discharge of carbon 

 dioxid. 



In this connection mention should also be made of the fact that 

 the lungs of birds are beset with many appendages, or air-sacs, which 

 communicate with the bronchi by special tubules and frequently 

 extend into the bones, or for some distance between the muscles and 

 underneath "the skin. These air-sacs must be regarded as integral 

 parts of the respiratory apparatus, although they tend to render the 

 entire body more buoyant. The metabolism of birds is very intense 

 and subject to considerable fluctuations. Thus, this additional respira- 

 tory surface may be called upon at any time to 

 effect a more intense and rapid interchange of 

 the gases without necessitating an undue expan- 

 sion of the lung tissue itself. 



The interchange of the gases in the placenta 

 of the mammals is responsible for the difference 

 in the character of the blood of the umbilical 

 artery and vein. As the blood of the latter 

 vessel contains more oxygen and less carbon 

 dioxid than that of the former, it must be evi- 

 dent that this organ is the seat of diffusion pro- 

 cesses between the body fluids of the embyro 

 and mother. 



The Complex Lung. To begin with, the lung 

 consists of a single sac which possesses no divid- 

 ing septa and extends in many cases through 

 the entire length of the body cavity. In the 

 amphibians the organ becomes paired, consist- 

 ing of two elliptical pouches of about equal 

 length which communicate with the pharyngeal 

 cavity through the bronchi and the trachea (Fig. 

 230). Furthermore, the breathing surface of 

 these sacs is increased enormously by mem- 

 branous partitions which project far into the 

 lumen of the main cavity. Thus, a beginning 

 is made of a differentiation of the lung into nu- 

 merous smaller compartments which are com- 

 monly designated as air-cells or alveoli. While some of the reptiles 

 retain this type of lung, many of them show a much higher state 

 of development of this organ, because the individual alveoli are en- 

 tirely separated from the main cavity and communicate with the 

 latter only through small orifices. In these we recognize the be- 

 ginnings of the bronchiolar tubules. With the increasing alveoliza- 

 tion of the lung, the bronchi and bronchioles are really separated 

 into an "extra pulmonary" system of tubes which are generally pro- 

 vided with solid cartilaginous rings and eventually also with muscular 

 tissue. The capillary networks which at first are restricted to the very 



FIG. 230. DIAGRAM 

 ILLUSTRATING THE FUNC- 

 TION OF THE AMPHIBIAN 

 LUNG. 



T, trachea; B, bron- 

 chi; L, lung of one side; 

 P, septa dividing the 

 main cavity into smaller 

 air-spaces or alveoli (A). 



