254 THE BODY AT WORK 



mechanical work done and the heat set free by the oxidation of 

 this food. Food is the petrol the combustion of which causes 

 the movement of the car. The external force which stimulates 

 a receptor is too insignificant in amount to be carried to account. 

 Physiologists neglect it, just as engineers neglect the energy 

 liberated by the sparking-plug which ignites the petrol, when 

 they are estimating the efficiency of a motor. 



Compared with the amount of energy actually received from 

 the environment when a sensory cell of the eye or ear is 

 excited, the energy needed to start an artificial impulse in a 

 nerve is relatively enormous ; yet a well-known comparison of 

 the energy conveyed to the nerve in a certain experiment with 

 a nerve-muscle preparation from a frog, and the energy ex- 

 pended by the muscle in contracting, brings home to our minds 

 the fact that it is impossible to carry even this item to account. 

 The energy furnished to the nerve from an electric condenser 

 measured 0-001 erg ; the energy expended by the muscle 

 reached 100,000 ergs. 



It is easy to determine the amount of mechanical work which 

 results from a given expenditure of energy. By alternately 

 flexing and extending the joints of his legs, a man lifts his 

 own weight up a hill of a certain height. The work can be 

 measured in foot-pounds or in kilogrammetres. But this by 

 no means accounts for all the energy potential in his food. A 

 still larger amount is expended for the purpose of keeping the 

 body warm, or, not improbably, making it too warm ; in either 

 case generating heat which is dissipated into the atmosphere. 

 When a machine is being planned, attention is concentrated 

 upon the problem of how to get the largest result in work for 

 a given quantity of fuel. Fuel costs money. All energy dis- 

 sipated as heat is wasted. Every ounce saved makes for 

 economy. Engineers therefore speak of the " efficiency " of an 

 engine as the relation between the work actually done and the 

 work which would have been done if no energy had been wasted. 

 In the best steam-engines it stands at about 1 to 10. Since 

 the chief function of muscle is to do mechanical work, physio- 

 logists are apt to adopt the engineer's point of view. But in 

 the case of muscle this is justifiable only in a limited degree. 

 The body of a warm-blooded animal is maintained at a tempera- 

 ture higher than that of the surrounding air. Muscles are the 



