MUSCLE 



261 



f 01 runted 

 ^^ rubber 

 tubing 



Valve 



2 way tap 



Side tube for 

 sampling (with clip} 



Fig. 135.- 



-Douglas bag for collection 

 expired air. 



of 



bat it has been found that under normal conditions 

 during experiments of short duration, their combustion is 

 so small that it may be neglected. 



The composition of the 

 air taken in being known, 

 when (1) the quantity of 

 air expired and (2) its 

 composition is ascer- 

 tained, it is possible to 

 determine how much O.j 

 has been taken up and 

 how much COo has been 

 given otf in a definite 

 time. 



In experiments on man 

 this is done by collecting 

 the air in a special rubber 

 bag devised by Douglas, 

 and which can be carried 

 on the back of the subject. A mouthpiece is fitted 

 with two valves, through one of which air is inspired, and 

 through the other of which it is expired into the bag 

 (fig. 135). The amount of air breathed may be measured 

 by passing it through a gas meter, and it may be analysed 

 by means of Haldane's apparatus. 



In animals the exchange can be determined 

 a canula inserted into the trachea instead of a mouthpiece. 

 This method has been used by Zuntz and his co-workers on 

 experiments on horses at work. 



The indirect method has been tested against the direct 

 method, and has been found to give very concordant results. 



Zuntz also used another indirect method. He measured 

 the work done on some form of work-measurer or ergometer, 

 e.g. a wheel turned against a measured resistance. By 

 converting the work units of the work thus done into heat 

 units and subtracting this from the total energy of the food, 

 expressed in heat units, the energy lost as heat may be 

 determined, since ail energy not used for mechanical work 

 is dissipated as heat. Thus the relationship between work 



by using 



