150 NUTRITION OF FARM ANIMALS 



act at a considerable distance from their point of origin, 

 while others normally produce their effect within the secret- 

 ing cell. 



210. Intracellular enzyms in the body. Still more recently 

 the presence of intracellular enzyms in all parts of the animal 

 body has been recognized. It has been shown that a very 

 considerable variety of reactions which are known to take place 

 in the body may also be brought about outside the body by the 

 action of extracts of various tissues and organs under conditions 

 apparently excluding the action of any living organisms. Con- 

 sequently, they have been ascribed to the action of enzyms 

 originally present in the cells, and the reactions in the body 

 have been regarded as due to these same enzyms. The 

 idea of intracellular enzyms has thus been extended to account 

 for the metabolic activities of the organism, and this explana- 

 tion has been very generally accepted by physiologists. Accord- 

 ing to this view, the body cells bring about metabolic changes 

 substantially in the same way as do the cells of yeast or of 

 the lactic acid bacillus, viz., by the formation of appropriate 

 enzyms which act upon the substances to be metabolized. 

 This phase of the subject is a comparatively new one 

 and unanimity as to individual cases has by no means been 

 reached, but of the value of the general conception as a working 

 hypothesis there can be little question. 



The word explanation is used above, of course, in a limited 

 sense. It is not known how the cell produces enzyms, nor with 

 any degree of certainty how an enzym acts. Nevertheless, 

 this hypothesis, if confirmed, is a real explanation as far as it 

 goes, in that it enables related phenomena to be grouped to- 

 gether from a broader standpoint, as will be apparent from the 

 following paragraphs. 



211. Enzym reactions reversible. A chemical reaction is 

 said to be reversible when it may progress in either direction 

 according to the conditions. For example, if a mixture of hydro- 

 gen and iodin in molecular proportions be heated to 448 C. 

 hydrogen iodid is produced. If, however, hydrogen iodid be 

 heated to the same temperature it yields hydrogen and iodin. 

 The reaction between these two elements, then, is represented 

 by the equation 



H 2 + I 2 ^ (HI) 2 



