ORGANS OF THE ENERGIZING SYSTEM 8i 



2Sb. The Digestive Processes in Animals. First there may 

 be the mechanical processes of mastication and deglutination : 

 the food materials are disintegrated, mixed with water, and 

 brought into the mouth, stomach, intestinal canal, etc., where they 

 become mixed with the digestive enzymes. 



zSc. Enzymes, Very many of the chemical processes that 

 occur in animals are initiated, controlled and accelerated by 

 enzymes. These we conceive to be chemical substances that are 

 elaborated in the glands and other bodily tissues. They act in 

 much the same ways as the catalysts that have long been known 

 in inorganic chemistry. Thus the inversion of cane sugar 

 proceeds rapidly when its solution contains a little mineral acid ; 

 oxygen and coal gas can be made to combine when they are made 

 to impinge on a tiny piece of platinum black ; unsaturated fats and 

 oils take up hydrogen, in certain conditions and in the presence 

 of some nickel oxide ; and so on. Analogous effects are such 

 as this : perfectly dry chlorine and hydrogen do not combine, 

 but do so quickly when the gases contain a trace of water vapour. 

 In most cases a catalyst accelerates a chemical reaction that is 

 possible in certain conditions but, in those conditions proceeds 

 slowly, or " infinitely slowly." 



By analogy enzymes are chemical substances, but no enzyme 

 has ever been prepared in a pure state : it is always an extract 

 from the tissue that has enzymatic qualities that is called an 

 enzyme. It is usually a system rather than a single substance. 

 Thus the pancreas secretes pancreatic juice and one active 

 principle of this is trypsinogen which is the precursor of trypsin. 

 But trypsin has no enzymatic qualities until it is " activated " 

 by a substance secreted in the intestine called enterokinase. And 

 there are analogous phenomena in the cases of other enzymes. 



The digestive juices are active because they contain enzymes. 

 The food materials ingested are complex in composition and 

 usually they contain parts that are incapable of serving for nutri- 

 tion. The various enzymes in the alimentary canal act on the 

 raw food materials, some dealing with the proteins, others with 

 the carbohydrates and others again with the fats. The undigested 

 residues may be further disintegrated by bacterial action in the 

 intestine and the final inutilizable products are expelled from the 

 latter. 



It is to be noted that the following account refers chiefly 



G 



