40 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



airtight when greased with lanoline, causing at the same time a 

 minimum of inconvenience. These masks are much used in Danish 

 laboratories for all experiments which have to last more than a few 

 minutes at a time. 



Valves such as Miiller's and other fluid valves, generally filled 

 with water or mercury, were formerly used extensively. They have 

 the advantage that leakage backward is impossible, but their resist- 

 ance is generally considerable. Zuntz uses the " Darmventile " in- 

 vented by Speck. These are certainly effective and the resistance 

 very slight, but the valves are large and cumbrous. Reliable metal 

 valves with a minimum resistance have been constructed by Chauveau 

 (Tissot, 1904) and by the firm of Siebe, Gorman (Douglas, 191 1 ). Bohr 

 constructed rubber valves which, slightly modified, have given entire 

 satisfaction in Danish laboratories (fig. 14). 



i. B. (a) PULMONARY VENTILATION APPARATUS OF THE CLOSED- 

 CIRCUIT TYPE. 



While closed-circuit respiration apparatus intended to measure the 

 total gas exchange of man and large animals are probably less ad- 

 vantageous than the air-current types, quite the reverse is the case 

 with corresponding instruments for measuring the pulmonary gas ex- 

 change only. On account of the small dimensions to which such an 

 instrument can be reduced it is easy to make it airtight, the more so 

 as the pressure can always be maintained absolutely equal to the 

 atmospheric. Another advantage obtained by the small dimensions is 

 that the variations in the composition of the air can be left out of ac- 

 count and the oxygen absorption can be read off directly on the meter 

 measuring the oxygen admission. Even a continuous graphic record 

 of the absorbed oxygen can easily be obtained. The instruments of 

 this type can give finally a quantitative graphic record of the volumes 

 of air inspired and expired. 



Apparatus of this type have been constructed by Ludwig 

 (Sanders-Ezn [1867]), Zuntz and Rohrig [1871], Regnard [1879] and 

 others, but these forms are now obsolete. Two forms are in use at 

 present. 



Krogh's apparatus (fig. 15) [1913] which is a modification of an 

 instrument constructed by Haldane and Douglas [1912], is furnished 

 with valves and the air is circulated by the respiratory movements of 



