62 VETEEINAKY 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 make this comparison it is necessary to be able to convert 

 " work units " into " heat units," and vice versa. It has been 

 found that 0'45 gram-degrees or small calories are equivalent 

 to 1 kilogram-metre. 



The proportion of work to heat is not constant. By 

 gradually increasing the stimulus both work production and 

 heat production are increased, but the latter is increased 

 more rapidly, and reaches its maximum sooner. Again, as 

 muscle becomes exhausted, its heat production declines more 

 rapidly than its work production. Exhausted muscle, there- 

 fore, 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 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. 339) be shown that 

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

 amount of energy yielded by any food may be determifted by 

 burning it in a calorimeter (see p. 341). To determine the 

 energy used in mechanical work some form of work measurer 

 or ergometer 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 produc- 

 tion and heat production may be found. By experiments on 

 men, horses, and dogs, Zuntz has found that about one-third of 

 the energy liberated may, under favourable conditions, be avail- 

 able for mechanical work, while two-thirds is lost as heat. The 

 proportion of energy evolved in mechanical work and heat in 

 normal men has been also studied by Atwater by means of the 

 Respiratory Calorimeter (see p. 408). In these experiments 

 only about six per cent, of the energy was used for mechanical 

 work and the rest was lost as heat. Compared with other 



