METHODS OF INVESTIGATION. 73 
The third type of respiration apparatus is best known by the 
name of Zuntz,* from the extensive development given it by this 
investigator, although it has assumed various forms in the hands 
of different experimenters. This apparatus is radically different 
from the other two types in that it is intended simply for the deter- 
mination of the respiratory exchange in the lungs. For this pur- 
pose the expired air is collected, either by means of a mask or a 
tracheal cannula, its volume measured, and its content of carbon 
~.~ dioxide and of oxygen determined in an aliquot sample, the com- 
position of the inspired air being assumed to be that of the normal 
atmosphere. The fundamental principle is really that of the Petten- 
kofer apparatus, but, owing to the fact that the excretory gases 
are not diluted with many times their volume of air, the results are 
much sharper and it is possible to determine the amount of oxygen 
consumed as well as of the carbon dioxide given off. In addition 
to this advantage, it permits the experimenter to follow the varia- 
tions in the respiratory exchange in comparatively short periods. 
It is thus especially adapted for investigating such questions as the 
influence of muscular work upon metabolism, and it is in the study 
of this question that it has found its chief application. On the 
other hand, it is impracticable to continue its use through long 
periods—a day, e.g—and it takes no account of the excretion 
through the skin and the alimentary canal. Only by indirect 
methods, therefore, is it possible to compute the total income and 
outgo of carbon by its use. 
But while the Zuntz form of respiration apparatus is especially 
adapted for investigating the carbon metabolism during short 
periods, it is important that these periods be not made too short. 
What is actually determined by the use of any form of respiration 
apparatus is the excretion or absorption of carbon dioxide or oxy- 
gen. In an experiment extending over several hours, we may 
fairly assume that this is substantially a measure of the actual pro- 
duction or consumption of these gases going on in the tissues. In 
periods of a few minutes, however, there is always a possibility of 
an accumulation of oxygen or a partial retention of the products 
of metabolism in the tissues or the blood, while, on the other hand, 
* Rohrig & Zuntz, Arch. ges. Physiol., 4, 57; v. Mehring & Zuntz, ib., 32, 
173; Geppert & Zuntz, 7b., 42, 189. 
