20 THE STORY OF THE LIVING MACHINE. 



has demanded attention for its own sake. The 

 practical utility of human physiology has stimu- 

 lated its study for centuries; and in the last fifty 

 years of scientific progress it has been human 

 physiology and that of allied animal that has at- 

 tracted the chief attention of physiologists. The 

 result is that while the physiology of man is toler- 

 ably well known, that of other animals is less un- 

 derstood the farther we get away from man and 

 his allies. For this reason most of our knowledge 

 of the living body as a machine must be derived 

 from the study of man. This is, however, fortu- 

 nate rather than otherwise. In the first place, it 

 enables us to proceed from the known to the un- 

 known ; and in the second place, more interest at- 

 taches to the problem as connected with human 

 physiology than along any other line. In our dis- 

 cussion, therefore, we shall refer chiefly to the 

 physiology of man. If we find that the functions 

 of human life are amenable to a mechanical ex- 

 planation we cannot hesitate to believe that this 

 will be equally true of the lower orders of nature. 

 For similar reasons little reference will be made 

 to the mechanism of plant life. The structure of 

 the plant is simpler and its activities are much 

 more easilly referable to mechanical principles 

 than are those of animals. For these reasons it 

 will only be necessary for us to turn our attention 

 to the life activities of the higher animals. 



What is a Machine ? Turning now to our more 

 immediate subject of the accuracy of the state- 

 ment that the body is a machine, we must first ask 

 what is meant by a machine ? A brief definition 

 of a machine might be as follows : A machine is a 

 piece of apparatus so designed that it can change one 

 kind of energy into another for a definite purpose. 



