160 THE ENGINES. OF THE HUMAN BODY 



work needs a daily allowance of fuel which may be re- 

 presented by 30 oz. (850 grammes) of sugar. Of that 

 amount ib\ oz. are dissipated in heat — in keeping the 

 machine warm and in maintaining it ; only about 3J oz. 

 (a little less than 12 per cent.) is converted into effective 

 mechanical work. With an equal amount of petrol 

 (30 oz.) a motor cycle will carry a man over nine miles 

 of ordinary undulating country road ; 25! oz. of the 

 fuel supplied is wasted as heat ; only \\ oz. (15 per cent.), 

 or much less, is transformed into effective work. The 

 human heart transforms 25 per cent, of the fuel with 

 which it is supplied into effective work ; the rest goes to 

 heat the body. We are justified, therefore, in speaking 

 of food as fuel. The fuel of an engine, however, needs 

 no flavouring or seasoning ; if the petrol has been dis- 

 tilled, if all grit or sediment has been filtered off before 

 the cylinder is reached, then all has been done that is 

 needed to satisfy the palate of 'an engine. It is otherwise 

 with the fuel supplied to the human machine ; certain 

 ingredients in minute quantities have to be mixed with 

 it to satisfy the palate and meet the needs of particular 

 requirements of the human machine. 



Nothing could be more simple than the alimentary 

 system of a motor cycle. The fuel is poured through a 

 mouth or opening into a stomach or tank (fig. 1 p. 10) ; 

 from the tank the petrol is carried by means of a feed-pipe 

 — the alimentary canal — to an apparatus — the carburettor 

 (fig. 1) — which absorbs the petrol and transforms it into a 

 combustion mixture. At the carburettor the alimentary 

 canal ceases, for there the fuel actually enters the engine. 

 The carburettor, we shall see, corresponds to an elaborate 

 machinery which has been built into the containing wall of 

 the alimentary system of the human body — a machinery 

 which selects and absorbs from the food such elements 

 as are to be used in the body as tissue fuel. Just before 

 the petrol enters the carburettor it passes through a float 

 chamber (fig. 1) which automatically regulates the rate 

 of flow according to the amount of absorption. We 

 shall see that there are corresponding mechanisms set along 



