358 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



katabolic processes, as the most important of all of them. 

 All that is necessary now in regard to the respiratory 

 function is only to remember that the general object of 

 respiration is the aeration of the blood, and thereby the 

 exchange of its C0 2 for oxygen of air. 



SECTION II. 

 Respiratory Organs among Vertebrates. 



Respiratory organs are of two general kinds, viz., 

 lungs and gills. The one kind is adapted for air breath- 

 ing, the other for water breath- 

 ing. The general plan of the 

 one is a complexly z'«-folded epi- 

 thelial surface ; of the other of 

 a complexly w^-folded epithe- 

 lial surface (Fig. 239). In both 

 cases alike the purpose is to ex- 

 pose as large a surface as pos- 

 sible to the oxygen of the air or 

 the water. The two kinds are 

 not homologous ; the one is not 

 transformed into, but, as already 

 shown (page 242), substituted for 

 the other. We take first lungs, and, of course, first of 

 all the lungs of man. 



Fig. 239. — General plan of 

 structure, A, of lungs; 

 B, of gills. 



I. LUNG RESPIRATION. 



The Lungs of Man. — The lungs are by far the 



largest organ in the body, although in weight they are 

 greatly exceeded by the liver. Their great volume is, 

 of course, due to the contained air. Their grayish-pur- 

 ple color and soft, elastic feel is well known. They are 

 a double organ, being divided into a right and left lung 

 by the mediastinum. 



