614 GENERAL METABOLISM [CH. XLL 



Here the body was in nitrogenous equilibrium, and it eliminated 

 more water than it took in by 174 grammes, this being derived from 

 oxidation of hydrogen. It stored 39 '7 grammes of carbon, which is 

 equivalent to 52 grammes of fat. 



The next table gives the results of an experiment on the same man 

 on the same diet, but who did active muscular work during the day : 



Expenditure. Nitrogen. Carbon. Water. 



Urine 17'4 12'6 1194 



Faeces 2'1 14*5 94 



Expired air ..... 309'2 1412 



19-5 336-3 2700 



It is important to notice that the discharge of nitrogen was 

 unaltered, while that of both carbon and hydrogen was increased. 

 At one time protein was considered to be the great source of 

 muscular energy ; this was first disproved by an historical experiment 

 made by Tick and Wislicenus on themselves in their ascent of the 

 Faulhorn. Nature works in a most economical way in reference to 

 protein waste, and any increase in nitrogenous katabolism which 

 occurs during muscular work is insignificant. 



The balance-sheet method of investigation, though one of great 

 usefulness, tells us very little of the details which lead to the end 

 results. We must therefore now proceed to study the details, and 

 although there is a good deal of guesswork, and even ignorance upon 

 some essential points, we may most conveniently consider the 

 question under the three headings of our principal food materials, 

 namely, carbohydrates, fats, and proteins. 



Metabolism of Carbohydrates. 



In plants, carbohydrates are synthesised by the agency of chloro- 

 phyll from the simple materials carbonic acid and water, which form 

 their chief foods. The first substance formed is probably formic alde- 

 hyde, H.COH (which is the simplest carbohydrate known), and 

 this by condensation is converted into sugar, and finally, into 

 starch. We have no clear evidence that a synthesis of this kind 

 ever takes place in animals, but the main source of animal carbo- 

 hydrate is vegetable carbohydrate. This is taken in the food and 

 converted into glucose ; the glucose is taken to the liver and stored 

 as glycogen, and in the liver is once more liberated as glucose, and 

 distributed to the body in this form. The change from glycogen to 

 sugar is the work of an enzyme. Is the change from sugar to 

 glycogen also an enzyme action ? And if so, is another enzyme 

 responsible for it, or have we to deal with a case of reversible 

 zymolysis ? This is one of many unanswered questions. The other 

 important animal carbohydrate is lactose, a compound of dextrose 

 and galactose. If the food contains galactose as well as dextrose, the 



