PROTOPLASMIC FUNCTION 35 



body. In all cases, whether in infusorium or in man, the basis of the 

 process is an attraction of the living matter for oxygen. This attraction 

 is apparently inherent in the protoplasm, and leads to the same general 

 chemical changes which oxidation would lead to outside of an organism. 

 Here as elsewhere it is only because of the unparalleled complexity of the 

 chemical reactions that the process is not explainable in all its details. 

 Of all the many interchanges between protoplasm and its environment, 

 this interchange of oxygen and carbon dioxide is by far the most uni- 

 versal, and as a source of energy the most important. 



Nutrition. The nutrition of a mass of protoplasm is the sum of those 

 chemical changes within it by which it is supplied with the means of 

 liberating energy and of building tissue. These changes are almost 

 infinitely complex, and are summarized under the name metabolism. 



The nutrition triangle. Anabolism versus katabolism. 



Metabolism has two phases: One of them, the upbuilding process, is 

 called anabolism, from the Greek ana, up. The other phase is katab- 

 olism, or the downtearing of organic tissues, from the Greek kata, 

 down. These words are of constant and ever-increasing use in physiology, 

 for they indicate the chemism as the basis of the material life of all animals 

 and plants. 



Nutrition is divided into several parts namely, digestion, absorption, 

 assimilation, and excretion. In highly developed animals, and in many 

 of the simpler, there are, besides these four, other nutritional processes, 

 such as prehension, mastication, insalivation, deglutition, circulation, 

 absorptive selection, urination, and defecation. The last two are strictly 

 processes of nutrition only in a broad sense of the term. At the same 

 time they are indispensable parts logically of the different processes of 

 nutrition, and there are therefore treated in that connection. The 



