OF MOVEMENT IN ANIMALS. 



39 



what media does this force pass before it becomes mechanical 

 work ? 



In steam engines, heat is the necessary medium between 

 the oxidation of the fuel and the developed mechanical work. 

 It is very probable that the same 

 thing takes place in the muscles. 

 The chemical action produced by 

 the nerve within the fibre of the 

 muscle disengages heat from it : 

 this heat in its turn is itself 

 partially transformed into work. 

 We say partially, since accord- 

 ing to the second principle of 

 thermo-dynamics, heat cannot be 

 entirely transformed into me- 

 chanical work. 



Certain facts seem to justify 

 these views : thus, by warming 

 a muscle, we change the form of 

 it, and may see it contract in 

 length as it expands in breadth. 

 These effects disappear when the 

 muscle is cooled. 



Muscular fibre is not singular 

 in its power of transforming heat 

 into work. India-rubber, for in- 

 stance, has an analogous property, 

 and this substance may be made 

 to imitate the muscular phe- 

 nomena to a certain degree. If 

 we take a strip of india-rubber 

 (not vulcanised), and, drawing it 

 between the fingers, stretch it out 

 to ten or fifteen times its original 

 length, we see that it becomes 

 white, and of a pearly lustre. At the same time the strip 

 will become sensibly warm, and it will tend energetically to 

 return to its original condition, so that if we let go either of 

 its ends, it will instantly resume its former length, and fall to 



Fig. 9. — Transformation of heat into 

 work by a strip of india-rubber. 



