306 BIOLOGY 



we add a little of the secretion of the pancreas, the starch and 

 the water will chemically combine to produce sugar, a new 

 compound. The pancreas produces a substance which is called 

 amylopsin, which has the power of causing a chemical union 

 of the starch with the water. This substance we call an enzyme. 

 It is not alive nor does it need any living environment for its 

 action. If we separate a little of it from the pancreatic juice 

 and put it in a test tube with water and starch, it will cause 

 the union of the water and the starch exactly as it does in the 

 digestive tract. Now we do not know exactly the nature of this 

 enzyme, nor just how it brings this union about; therefore the 

 vital process of digestion is not entirely understood at present. 

 We do know, however, that digestion itself is only a chemical 

 change, and that the same chemical union of the starch with 

 the water can be brought about without the presence of this 

 enzyme. The fact that we do not exactly understand how 

 the pancreatic juice acts in this case is no stranger than the 

 fact that we do not understand exactly how a spark causes a 

 bit of gunpowder to explode. We do not doubt that the ex- 

 plosion of the powder is the result of chemical and physical 

 forces, and there is no more reason to doubt that the combina- 

 tion of the starch with the water, under the influence of amy- 

 lopsin, is also the result of chemical and physical forces. 



The same principle holds in regard to the digestion of all 

 other foods in the digestive tract of animals. Each of the di- 

 gestive juices contains special enzymes, each food is acted 

 upon by enzymes, and in all cases the food undergoes a chemi- 

 cal change. Apart from the fact that they are brought about 

 by these enzymes, there is little or nothing to distinguish be- 

 tween chemical changes taking place in the body and similar 

 changes taking place outside of the body. Digestion, in other 

 words, is a chemical process and controlled by chemical laws. 



The Absorption of Food. The digested food passes through 

 the intestine, being forced along by the muscular action of 

 the intestinal wall. As it passes through the intestine it is 



