64 HUMAN PHYSIOLOGY 



done by a muscle and the amount of energy dissipated as 

 heat, it is possible to determine the relationship of these 

 to one another, and thus to compare muscle with other 

 machines as to proportion of energy which is utilised to 

 produce work. To do this it is necessary to be able to 

 convert "work units" into "heat units," or vice versa. It 

 has been found that 0-45 gramme-degrees is equivalent to 

 1 kilogram-metre. 



I The proportion of work to heat is not constant. Gradually 

 increasing the stimulus increases both work production and 

 heat production, but the latter is increased more rapidly, 

 and reaches its maximum sooner. Again, as muscle becomes 

 f exhausted its heat production declines more rapidly than its 

 \work production. Exhausted muscle, therefore, works more 

 economically. If an unloaded muscle is made to contract no 

 /work is done and all the energy is given off as heat, and the 

 J same thing happens where a muscle is so loaded that it 

 cannot contract when stimulated. 



But the point of practical importance to determine is 

 How much of the energy liberated by muscle in normal 

 conditions is usually used for mechanical work, and how 

 much is lost as heat? It will afterwards (p. 312) be shown 

 that all the energy of the body comes from the food, and 

 the amount of energy yielded by any food may be deter- 

 mined by burning it in a calorimeter. To determine the 

 energy used in mechanical work some form of work measurer 

 or ergograph may be used e.g. a wheel turned against a 

 measured resistance. By converting the work units of the 

 work thus done into heat units, and subtracting this from 

 the total energy of the food, the energy lost as heat may 

 be determined, and thus the relationship between work pro- 

 duction and heat production may be found. By experiments 

 on men, horses, and dogs, Zuntz has found that about Jrd 

 of the energy liberated may, under favourable conditions, be 

 available for mechanical work, while rds is lost as heat. 

 Compared with other machines, such as steam-engines, 

 muscle must be regarded as an economical worker, and it 

 has the advantage that the heat liberated is necessary to 

 maintain the temperature at which the chemical changes 

 which are the basis of life can go on. 



