SKELETAL MUSCLE 79 



their weapons. The spectator would see the flight of 

 the arrows and might not pause to reflect that the force 

 applied by the marksmen is opposite in direction and 

 imparted in the intervals between the volleys. The 

 period of relaxation is not marked by a suspension of 

 activity on the part of a muscle but is occupied by a 

 most important readjustment. 



The Economy of the Contractile Process. Calcu- 

 lations are often made with reference to steam engines 

 with the object of discovering how great a part of the 

 total energy represented by the coal can be converted 

 to actual horse-power or work. It has not been possible 

 to raise the proportion thus convertible much above 15 

 per cent. As a rule something like seven-eighths of the 

 energy liberated when the fuel is burned escapes the 

 application which the engineer would like to make of 

 it. Most of it goes up the chimney as heat. Similar 

 computations can be made for the skeletal muscle for 

 here, too, we have a mechanism in which fuel is con- 

 sumed and heat and work produced. 



Under the best conditions a muscle can make a much 

 better showing in this respect than the engine. A 

 record of about 50 per cent, has been attained with 

 simple contractions. With tetanic contractions the 

 efficiency is lower and often of the same order as that of 

 the engine. It is not practicable in a work like this to 

 detail the ingenious methods by which these facts have 

 been established. A man going up a mountain is prob- 

 ably forced to radiate to his environment and make 

 latent by evaporation as much as three times the heat 

 which would be equivalent to the measurable work 

 accomplished. 



Skeletal Muscles as Heat-producing Organs. The 

 production of heat we have been discussing is not to be 

 thought of as an absolute and unfortunate waste. 

 Human beings and warm-blooded animals usually have 

 to maintain body temperatures many degrees higher 

 than those of their surroundings. Such a condition can 



