70 THE MECHANISM OF LIFE 



a gas at a relatively high temperature into the kinetic energy of 

 a material structure (pistons or turbines) at a relatively low 

 temperature. 



It may be that the muscle of an animal is a heat mechanism of 

 this kind — a thermodynamic machine — but it is not likely, and 

 it is far more probable that the muscle contracts because the 

 chemical energy of the proteids, fats, and carbohydrates is trans- 

 formed into mechanical work without passing through the stage 

 of heat. There are physical mechanisms that do this. A 

 galvanic battery may produce a current of electricity, and the 

 latter may work an electric motor and no heat at all (or only a 

 trace of it) may be generated. Here chemical energy (that of the 

 zinc and acid in the battery) transforms into electricity, and the 

 latter does mechanical work in the motor. Now all that we can 

 state positively about the events that happen when a muscle 

 contracts are these: there are complex, chemical compounds in 

 the protoplasm of the muscle which break down (or " explode "). 

 Very complex molecules thus dissociate into relatively simple 

 ones, and the energy that held these complex molecules together 

 is liberated and is transformed into the mechanical work done 

 by the muscle when it contracts against a resistance. 



But it is quite certain that heat is also generated — that every- 

 one knows who becomeswarm when he does hard muscular work. 

 Now, is this generation of heat a necessary step in the doing of 

 bodily work ? In cold-blooded, wholly aquatic animals it is not 

 certain that heat is produced, or, if it is, it is only in very small 

 quantity. A warm-blooded animal preserves a constant tem- 

 perature, which is usually considerably higher than that of its 

 environment, and so it must generate heat. And it is probable 

 that this heat is produced by the oxidation of the relatively 

 simple molecules into which the complex protoplasmic substance 

 breaks down. These products of dissociation are oxidised into 

 water and carbonic acid. 



Protein- containing tissues break down into the same products, 

 and also into simple nitrogenous substances that later on become 

 urea in the liver and kidneys — that is, into such substances as 

 can easily be removed from the muscle in solution in the circu- 

 lating blood. 



The Removal of Waste — Glandular Activity. — In the heat 

 engine the working substance — steam in the steam engine, or the 

 products of the explosion in the internal combustion motor — 



