THE LIBERATION OF ENERGY 165 



without, and the mean temperature of the 

 body must gradually rise. 



90. Energy is also liberated as motion by the 

 contractions of the muscles. The muscles may 

 either perform internal work, as the beating 

 of the heart, the movements of respiration, the 

 movements of the limbs on the trunk, the 

 movements of the involuntary muscles, as of 

 the bladder and bowel, or external work, as 

 in locomotion or in mechanical labour. All 

 internal movement is ultimately resolved 

 into heat. The external work can also be 

 measured and expressed as heat, and thus the 

 total energy of the body in twenty-four hours 

 liberated by a man doing say eight or ten hours 

 of hard work can be calculated. Further, 

 the energy represented by the complete com- 

 bustion of a diet sufficient for a man doing 

 this work and producing heat during twenty- 

 four hours can also be calculated. It can be 

 shown that there is a balance struck between 

 income and expenditure, and this balance, 

 in ordinary circumstances, is fairly constant. 

 It becomes interesting to consider man as a 

 transformer of energy with special reference 



