IS THE BODY A MACHINE ? 29 



preserved almost indefinitely, retaining its power 

 to effect the change whenever put under proper 

 conditions. The change of starch into sugar is 

 thus a simple chemical change occurring under 

 the influence of chemical affinity under certain 

 conditions. One of the conditions is the presence 

 of this saliva ferment. If we can not exactly un- 

 derstand how the ferment produces this action, 

 neither do we exactly understand how a spark 

 causes a bit of gunpowder to explode. But we 

 can not doubt that the latter is a purely natural 

 result of the relation of chemical and physical 

 forces, and there is no more reason for doubting 

 it in the former case. 



What is true of the digestion of starch by sa- 

 liva is equally true of the digestion of other foods 

 in the stomach and intestine. Each of the diges- 

 tive juices contains a ferment which brings about 

 a chemical change in the food. The changes are 

 always chemical changes and are the result of 

 chemical forces. Apart from the presence of these 

 ferments there is really little difference between 

 laboratory chemistry and living chemistry. 



Absorption of food. The next function of this 

 machine to attract our attention is the absorption 

 of food from the intestine into the blood. The 

 digested food is carried down the alimentary ca- 

 nal in a purely mechanical fashion by muscular 

 action, and when it reaches the intestine it begins 

 to pass through its walls into the blood. In this 

 absorption we find engaged another set of forces, 

 the chief of which appears to be the physical 

 force of osmosis. The force of osmosis has no 

 special connection with life. If a membrane sepa- 

 rates two liquids of different composition (Fig. i), 

 a force is exerted on the liquids which cause them 



