62 THE HUMAN MECHANISM 



understand why long-continued, vigorous muscular action 

 produces marked fatigue in nerve cells and gland cells, 

 while the activity of the latter produces only inappreciable 

 fatigue in the muscles; for the amount of chemical change 

 and the production of wastes are far greater in the case of mus- 

 cular work than in that of nervous or glandular activity. 



6. The analogy of the engine. In previous chapters we 

 have compared the living body with a machine or locomotive 

 engine ; both do work, and both obtain the power for work 

 from the chemical changes in food or fuel. What we have 

 now learned about fatigue suggests an extension of the 

 same comparison. Every locomotive suffers impairment of 

 its working power with use, and special measures are taken 

 to limit this impairment as much as possible; the gases and 

 smoke are carried away at once by the chimney or smoke- 

 stack ; the furnace is provided with a grate so that the ashes 

 shall not accumulate and shut off the draft ; the bearings are 

 oiled and foreign matters removed; finally, as the consump- 

 tion of fuel goes on, the loss is made good by stoking. 



The continuance of the work of the engine requires two 

 things fresh supplies of fuel and the removal of wastes. 

 Obviously the blood performs these same offices for the cell. 

 It supplies to the cell fuel (food) from the alimentary canal 

 and oxygen from the lungs and it carries away the waste. 

 Provision is thus made to maintain the human machine in 

 working order and good condition during its activity. If the 

 blood flows too slowly through the muscle, the same thing 

 happens as in the locomotive when the fireman neglects to 

 rake the fire or to put on new fuel ; the efficiency both of the 

 human engine and of the locomotive may be impaired either 

 by the undue accumulation of the waste products of its own 

 activity or by the neglect to supply proper food or fuel. 



