CHAPTER IV. 



KATABOLISM. 

 SECTION I. 

 Introductory. 



Thus far we have treated of food preparation and dis- 

 tribution. Now we take up tissue decomposition and waste 

 elimination. Thus far the processes are ascensive and 

 distributive; those now to be discussed are descensive 

 and eliminative. In a word, thus far we have had to do 

 with anabolism. Now we take up katabolism. 



Introductory. — Exchange of matter with the exter- 

 nal world can take place only through an external sur- 

 face or an infolding of an external surface. Foreign 

 commerce can take place only through a coast line or an 

 infolded coast line or bay. Many of the so-called inte- 

 rior surfaces, such as the stomach, the intestines, the 

 lungs, the bladder, etc., are examples of such infoldings 

 of the exterior surfaces. Real interior surfaces — i.e., 

 surfaces which have no connection with the external 

 world — are found in the cavities of the blood system and 

 of the nervous system (brain), and also in the pleural 

 and peritoneal cavities. We repeat, then, that exchange 

 with the external world can take place only through an 

 external surface or an infolding of the same. 



In plants it takes place on a directly external surface 

 — on an exposed coast line. Air containing food bathes 

 the surface of the leaves, and water containing food the 

 28 415 



