METHODS OF MEASURING RESPIRATORY EXCHANGE 39 



place in the lungs, their application for determinations of the total 

 respiratory exchange is completely justified. 



All forms of apparatus for measuring the pulmonary gas exchange 

 possess some arrangement for connecting the measuring instruments 

 with the air passages of the man or animal experimented on, and 

 almost all possess valves which determine the direction of the air 

 current to and from the lungs. Before describing the different types 

 of apparatus in detail, it will therefore be useful to discuss the different 

 methods of connecting with the air passages and the different forms of 

 respiration valves. 



In animals tracheal cannulas are almost exclusively employed. It 

 is extremely difficult to put a mask or anything like it airtight on to an 

 animal, and the results which have been obtained by means of such 

 devices can only be utilized with great 

 caution, as pointed out by Tigerstedt 

 [T.M.]. On man mouthpieces of rubber 

 which fit between the teeth and the lips 

 are extensively used (Zuntz, Geppert 

 [1887], Haldane and Douglas and others), 

 and they are sometimes supplemented by 

 an extra piece of rubber tied round the 

 head and pressing against the lips from 

 outside. This precaution appears, how- 

 ever, to be unnecessary. To close the FlG " -><*'> nosepiece. 

 nose suitable nose clips are universally 



employed. Several persons feel it as a considerable inconvenience 

 to breathe through the mouth, and in such cases the nosepieces con- 

 structed by Benedict [1909] may be very useful (fig. 12). They can 

 easily be fitted tightly by inflating the rubber cushion. The diameter 

 of the tubes must necessarily be so narrow, however (7 mm.), that a 

 considerable resistance is inevitable when the breathing is increased. 



Masks of rubber which can be fitted on the face and enclose the 

 mouth and nose are far more convenient than mouthpieces or nose- 

 pieces, but it is extremely difficult to avoid leakage. Bohr constructed 

 masks which were specially fitted to each person on whom it was in- 

 tended to experiment. These masks (fig. 13) consist of a funnel- 

 shaped piece of tinplate coated on the edge with a substance used by 

 dentists and known commercially as Stent's composition. This sub- 

 stance becomes soft at a temperature about 50, and can then easily 

 be moulded on the face of a person and can be made to fit absolutely 



