22 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



are specially adapted to the study of the pulmonary gas exchange of 

 man. 



In both groups of methods we have two essentially different types 

 of apparatus : 



(a) The closed space or Regnault type, and 



(b) The air current or Pettenkofer type. 



A Regnault apparatus consists of a closed vessel or system of 

 vessels which must be absolutely airtight and in which the animal 

 breathes the same air over and over again. Arrangements are pro- 

 vided for absorbing the carbon dioxide liberated and generally also 

 for adding oxygen to replace that which is absorbed by the animal. 

 The carbon dioxide is determined generally in the absorbing system, 

 and the oxygen added is measured. For both gases a correction 

 must in most cases be introduced to account for changes in composi- 

 tion or quantity of the air enclosed in the apparatus. The influence 

 of this correction depends upon the volume of the apparatus which is 

 therefore made as small as possible. 



In the air-current apparatus atmospheric air is conducted in a 

 uniform current through the animal chamber, and the changes produced 

 in the air by the respiration of the animal are determined either 



a in the outgoing air as a whole by absorption of the total quantity 

 of carbon dioxide produced, or 



ft in a certain fraction of the outgoing air, in which case the total 

 air current must be measured. 



In most apparatus of the air-current type it is not essential that 

 the animal chamber is absolutely airtight. 



It follows from the above that each of the two types a and b has 

 its own special sphere of applicability, and that they cannot be used 

 indiscriminately without sacrificing the special advantages. 



A closed-space apparatus is the only one which can be used with 

 advantage for experiments involving a composition of the air breathed 

 differing from the atmospheric, e.g. for experiments on the influence 

 of varying oxygen percentages, and it must be used further for experi- 

 ments in which it is desired to measure the exhalation of small 

 quantities of gases other than carbon dioxide from the body. 



In the closed-space apparatus a very accurate determination of 

 the oxygen absorbed can be obtained with comparative ease and over 

 short periods, and it is therefore indispensable, especially for the 

 determination of the oxygen intake of very small animals. 



The difficulty of making a large Regnault apparatus absolutely 



