THE ANIMAL MACHINE 



work, is brought to the muscle through the streams 

 of blood which flow from the heart in the arterial 

 channels, and in part also through the lymphatic system. 

 The blood itself gains its supply from the digestive 

 system and from the lungs. The digestive system sup- 

 plies water, that all-essential diluent, and a great vari- 

 ety of compounds elaborated into the proper pabulum; 

 while the vital function of the lungs is to supply oxygen, 

 which must be incessantly present in order that the 

 combustion which attends muscular activity may take 

 place. What virtually happens is that fuel is sent from 

 the digestive system to be burned in the muscular 

 system, with the aid of oxygen brought from the lungs. 



In this view, the muscular apparatus is a species of 

 heat engine. In point of fact, it is a curiously delicate 

 one as regards the range of conditions within which 

 it is able to act. The temperature of any given organism 

 is almost invariable; the human body, for example, 

 maintains an average temperature of 98^ degrees, 

 Fahrenheit. The range of variation from this tem- 

 perature in conditions of health is rarely more than a 

 fraction of a degree ; and even under stress of the most 

 severe fever the temperature never rises more than 

 about eight degrees without a fatal result. That an 

 organism which is producing heat in such varying quan- 

 tities through its varying muscular activities should 

 maintain such an equilibrium of temperature, would 

 seem one of the most marvelous of facts, were it not 

 so familiar. 



The physical means by which the heat thus gener- 

 ated is rapidly given off, on occasion, to meet the varying 



