MECHANICS OF THE LIVING MACHINE 305 



To follow out this question 'in detail would take more space 

 than could be devoted to it here. A few of the more important 

 functions of life may be considered, and will serve to show 

 how modern biological science endeavors to explain life phe- 

 nomena in terms of chemical and physical forces. In this 

 discussion we shall confine our attention wholly to the life of 

 animals. The life of plants is far simpler than that of animals, 

 and if it can be shown that the animal organism works in a 

 mechanical fashion, we may safely assume that the same prin- 

 ciple will hold for the vegetable kingdom. In following out 

 this thought we will consider in succession several of the im- 

 portant functions of animal life. 



Digestion. Digestion is simply a chemical change in the 

 nature of the food, and involves nothing mysterious, nor any 

 special forces. The foods when taken into the body are mostly 

 insoluble. In order to pass through the walls of the intestine, 

 they must first be dissolved in the liquids of ^the digestive 

 tract, and before they are dissolved they must be changed into 

 a soluble condition. The changes which make them soluble 

 are not peculiar to the living body, since they will take place 

 equally well in a chemist's laboratory. One of the most impor- 

 tant steps in digestion is the change of starch into sugar; and 

 starch, by proper chemical methods, can be changed into sugar 

 just as readily in the test tube of a laboratory as in the digestive 

 organs of an animal. The digestion of starch has nothing mys- 

 terious in it, and is only an instance of the application of the well- 

 known chemical forces. The same thing is true of all the other 

 changes in the food which we call digestion. They are all 

 chemical changes, resulting from the laws of chemical affinity. 

 The only feature concerning the process that is not intelligible 

 in terms of chemical law is the nature of the digestive juices. 

 The digestive juices contain substances that have the power 

 to bring about chemical changes. If we mix starch and water 

 together they will not combine to make sugar, but will remain 

 a mixture of starch and water. If, however, to this mixture 



