THE MUSCULAR SYSTEM 405 



ments on the dog. He calculated that one-third of the energy 

 liberated appeared as work, while by experiments on men it was 

 found that the proportion was 25 per cent, as external work for 

 the muscles of the arms (turning a wheel), and 35 to 40 per cent, 

 for the legs (in mountaineering) , from which it would appear that 

 the muscles of locomotion are superior as work producers. Ex- 

 periments carried out on the horse by Zuntz* show that 34*6 per 

 cent, of the total energy contained in the food were converted into 

 muscular work. This shows that the animal body possesses an 

 efficiency of one-third in the case of the horse, and somewhat 

 higher than this in man, which far surpasses the best heat- 

 engine. 



Interesting as the comparison may be, a word of caution is 

 necessary. It is true that an engine and a muscle each take in 

 energy and utilise a part of it to do external work, but they work 

 in different ways and along different lines to produce the same 

 results. Thus the steam-engine receives its energy as heat; 

 originated by the combustion of fuel in the boiler-furnace, 

 converts a varying fraction of this into work, and discharges the 

 remainder, degraded in temperature, but otherwise unaltered. 

 A muscle, on the other hand, receives its energy in the form of the 

 food it takes from the blood. This it metabolises by chemical 

 processes, which are ultimately oxidational, converting the 

 potential energy of the food partly into work and, unlike the 

 engine, partly into heat (see p. 411), and giving off degraded 

 products of its metabolism, of which one is the same as that from 

 the furnace of an engine — viz., carbon dioxide. These few 

 remarks must suffice to emphasise the fact that a muscle is a 

 chemical-engine, and not a heat-engine. As Fick was careful to 

 point out, if an attempt is made to explain the working of a 

 muscle on the thermodynamic principles which govern the 

 working of a heat-engine, one is landed in the absurd result that a 

 muscle only converts into work T -^ part of the potential energy 

 it receives, the remaining <$/$ necessarily being converted into 

 heat ; we have seen that the efficiency of a muscle may be \. In 

 the case of insects, with their astounding locomotive activities, 

 if the efficiency of their muscles could be determined it would 

 probably be found to exceed that of mammalian muscle. 



Muscle Currents. — Great controversy has taken place as to 

 whether currents of electricity exist naturally in uninjured muscle. 

 It is found, for instance, that a piece of muscle isolated from the 

 body, and placed in connection with a galvanometer, may be 

 made to demonstrate the presence of electric currents which 



* ' Metabolism of Nutrients in the Animal Body and the Source of 

 Muscular Energy,' United States Department of Agriculture, vol. vii., 

 No. 7, 1895. 



