ANIMAL MOTORS 285 



movers. That is, a large per cent of the energy repre- 

 sented by the food eaten is converted into work, a larger 

 per cent than is possible to realize in most motors. Pro- 

 fessor Atwater in his recent experiments found the 

 average thermodynamic efficiency of ma-n to be 19.6 per 

 cent. Experiments conducted by the scientist Him have 

 shown the thermodynamic efficiency of the horse to 

 be about 0.2. The best steam engines give an efficiency 

 equal to this, but the average is much below. Internal- 

 combustion engines will give a thermal efficiency from 20 

 to 30 per cent. 



400. Muscular development. It is possible to consider 

 the animal as a motor, but the animal is made up of a 

 great number of systems of levers and joints, each sup- 

 plied with a system of muscles which are in reality the 

 motors. Muscles exert a force in only one way, and that 

 by shortening, giving a pull. For this reason muscles 

 are arranged in pairs, as illustrated by the biceps and tri- 

 ceps, which move the forearm. It is not clearly under- 

 stood just how muscles are able to exert forces as they 

 do when stimulated by nerve action. The theory has 

 been advanced that the shortening of the muscles is due 

 to a change of the form of the muscular cell from an 

 elongated form to one nearly round, produced by pressure 

 obtained in some way within the cell walls. There is no 

 doubt but there is a transformation of heat energy into 

 mechanical energy. While at work and producing mo- 

 tion there is but little change in the temperature of the 

 muscles, but when the muscles are held in rigid contrac- 

 tion, there is a rise in temperature. Another author* has 

 likened this to a steam plant, which while at work con- 

 verts a large portion of the heat generated in the fire 

 box into mechanical energy, but as soon as the engine is 

 *F. H. King, in "Physics of Agriculture." 



