102 



RESPIRATION 



The oxygen absorbed furnishes one of the elements of the combustive 

 katabolism by which life is manifested. Partial katabolism is apparently 

 always primary and oxidation secondary namely, in the combustion 

 of the first products of the katabolism. About the details of this union 

 of the oxygen and the tissue-molecules we know but very little. We are 

 sure what some of the end-products of these complicated chemical 

 reactions are, as will be seen later in the chapter on Nutrition; and 

 we know that carbon dioxide is the chief of these end-products, the most 

 universal in the tissues. It is the one representative of the actual com- 

 bustion of the cells, especially of the fats and carbohydrates of the tissues 

 and of the still circulating food as well. When wood or coal is burned 



in the air, oxygen is used up 



FlG - 50 and carbon dioxide, the union- 



product of the carbon and the 

 oxygen, is given off. Moreover, 

 heat is liberated, and, with the 

 proper mechanisms attached, 

 energy of several forms. In 

 these respects, at least, bio-meta- 



FIG. 51 



Jib 



Section through the gills of the lamprey (Am- 

 mocaetes): ks, gill-cavity; kb, anterior, and kb 1 , 

 posterior gill-plates; z, septum; ka, gill-artery; 

 uv, vein; vm, ventral body-muscle; e, epithe- 

 lium. (B. Haller.) 



Section through the nasal cavities of the 

 duck-bill (Ornithorhynchus). (Zuckerkandl.) 



FIG. 52 



>*/ 

 IJHQl Oj O I O ( 0_j 0\_0 I O I O jolojol O I I Q^O ;_Q | 



z i 



bfriep 



Diagrammatic section of the gill-plate of a fish: ep, epithelium; bl, erythrocytes; z, cells 

 of the bloodvessels; bm, basal membrane; z', bloodvessel. (Marianne Plehn.) 



bolism is like the oxidation in a steam-engine's furnace. Because of 

 these similarities it has long been customary to speak of the combustion 

 going on in the body, of the "life-giving oxygen," of the food as the 



