ON THE METHOD OF METABOLISM EXPERIMENTS 85 



diet. But in all animals, as in man, the intestine is always more or less filled 

 and one cannot tell without special means whether a given mass of fasces comes 

 from the diet which is being studied. To do so it is necessary to separate the 

 fasces corresponding to the diet in question from preceding and subsequent 

 fasces. The subject is allowed to fast for some twenty hours, then the particular 

 diet is begun, and with the first meal some substance is given which, like finely 

 powdered charcoal, will impart a characteristic color to the fasces. After the 

 last meal the subject is permitted to fast again for twenty hours, and with the 

 first food eaten after this period charcoal is once more given. With herbivorous 

 animals it is impossible to get a satisfactory separation; but this difliculty may 



A 



FIG. 39. Face mask for respiration experiments, after Love"n. Longitudinal section. A, the 

 mouthpiece. B and C are thin membranes which act as valves and serve to separate in- 

 spired from expired air. Inspiration takes place through B, expiration through C. 



be overcome by giving the particular diet under investigation for several days 

 before beginning the experiment. 



In order to collect the solid constituents contained in the sweat, the sub- 

 ject is required to wear thoroughly washed woolen clothes which will absorb and 

 retain all such solids. 



Rather complicated methods must be employed to collect and analyze the 

 gaseous products of metabolism (carbon dioxide and water vapor), and to 

 determine the amount of oxygen absorbed. Methods for all these purposes 

 were used by Lavoisier (1780), but they have been developed and improved 

 in many ways since his day. 



The simplest, if not the most satisfactory method, is to collect the respira- 

 tory products by the use of a face mask. The mask is connected by means of 

 tubes with apparatus for the measurement and collection of the inspired and 

 expired air, the two currents of air being separated by means of automatic valves 

 (Fig. 39). Instead of the face mask a gutta-percha plate, so arranged as to fit 

 between the lips and teeth and provided with a tube through which the air 

 passes, may also be used. A much more nearly air-tight closure of the mouth 

 is possible with this apparatus. 



By this method which has recently been improved, especially by Zuntz, the 

 cutaneous respiration is of course not determined, but it is of no particular 

 importance (cf. Chapter XIII). A more serious objection is, that with this 

 method it is very difficult, if not impossible, to continue the experiment for 

 more than a quarter of an hour to an hour. Where it is necessary to determine 



