GENERAL METABOLISM. 621 



stuff, while at other times it is necessary, in order to draw a clear picture, 

 to measure the total intake and outgo. A simple problem, for ex- 

 ample, is to determine whether a definite substance acts as an albumin 

 sparer. Here, in most cases, it is sufficient to estimate the amount of 

 protein in the food without introducing any serious error, by merely de- 

 termining the amount of nitrogen contained therein. The amount of 

 nitrogen in the urine and in the faeces then shows clearly whether the 

 animal experimented upon is in nitrogen equilibrium or not. If the 

 animal is once found to be in such equilibrium, i.e. eliminates the same 

 quantity of nitrogen that it receives in the food, then by feeding the given 

 food we can easily determine whether the elimination of nitrogen is in- 

 creased, diminished, or remains the same. The experiment in this case is 

 so simple, because we know that nitrogen is given up by the kidneys and 

 not by the lungs or skin. Such an experiment, however, is not perfectly 

 satisfactory. A number of questions always arise concerning such results. 

 We are by no means justified in assuming that the appearance of nitrogen 

 in the urine is a sign that there has been a total consumption of the albu- 

 min. We know that in all cases only a part of the carbon appears com- 

 bined with nitrogen in urine. The rest of the carbon chains from the 

 cleavage-products of proteins are broken down in a different manner. 

 These chains may remain in the organism long after all of the nitrogen 

 has been eliminated, and take part in metabolism in a way which is not 

 yet clear to us. At all events, in an exact investigation it would be neces- 

 sary to take into account also the elimination of the sulphur. But even 

 here we cannot be entirely satisfied. Only by combining the examination 

 of the urine and faeces with that of the remaining elimination products, 

 especially the gaseous ones, shall we obtain an exact insight into the 

 influence upon the total metabolism. In many cases even with such 

 experiments the results are not entirely satisfactory. We should know 

 oftentimes more accurately to what extent kinetic energy has been changed 

 into potential energy, in order to judge correctly the physiological 

 nutritional value of the individual foodstuff. 



Before considering the details of an experiment in metabolism, we 

 must bring forward the fact that an exact insight into the questions 

 concerning metabolism can only be expected when influences which have 

 no bearing upon the problem at hand are excluded as completely as 

 possible. Comparative experiments in which the basal conditions are as 

 nearly alike as possible, should be carried out as a rule upon the 

 same animal. Individual peculiarities which sharply influence metabolism 

 should never be disregarded. One of the most important requirements to 

 be satisfied in a metabolism experiment is that the test should be carried 

 out for a considerable length of time. It is not possible to draw any 

 exact conclusions from observations made only during a period of twenty- 



