46 



THE ANIMAL AS A MACHINE. 



is based upon chemical action involving the slow com- 

 bustion of the carbon and hydrogen of their food and 

 muscle ; respiration furnishing the needed oxygen — an 

 element discovered by him three years earlier. Rum- 

 ford, in 1797, stated that animals, considered as ma- 

 chines, were efificient prime motors, and in 1799 pub- 

 lished his discovery of the identity of heat with mechani- 

 cal energy and his approximation to the value of the 

 mechanical equivalent ; and Scoresby and Joule, and 

 especially Hirn, later completely confirmed his views 

 on these points.* 



Modern research shows that the evolution of heat 

 is at least an invariable accompaniment of all action of 

 the animal system — also even of the brain and spinal 

 cord. Thought and manual labor, in the case of the 

 human machine, are alike productive of increased tem- 

 perature and of accelerated exhalation of carbon diox- 

 ide from the lungs and of salts from other organs. 

 Whether these chemical actions are direct results or 

 simply incidental to intermediate transformations of 

 matter and energy is as yet not fully determined. 

 The effect of exercise is invariably to increase greatly 

 the consumption of oxygen, and the elimination of 

 carbon dioxide, and the temperature of the body, within 

 narrow limits ; regulation being effected by exudation 

 of perspiration and its evaporation. Other effects of 

 increased exertion of the muscular system are the pro- 

 motion of digestion, the consumption of accumulated 

 combustible matter, as the fats, and increased rapidity 

 and thoroughness of blood circulation and of nutrition, 



* On these points, see Seguin and Lavoisier (Premier Memoir), Rum- 

 ford's Essays, and Philosophical MagazinCy 1846. 



