318 



air is inspired, and the expired air directed into the collecting bag 

 (Fig. 179). 



The respiratory exchange is calculated by squeezing the contents 

 of the bag through a meter, and thus measuring the volume of the air 

 expired in the given time, and by determining the composition of 

 samples of the expired air. For example, if it be found that a man 

 at rest has breathed out on an average 7 litres per minute, and that 

 the expired air contains, say, 4 per cent, of CO 2 and 15-80 per cent, 

 of 0.,, then, taking the percentage of 2 in the inspired air under the 

 conditions of the observation* as 20-80, the percentage absorbed i> 



5 (20-80 - 15-80). the amount of O 2 absorbed is =350 c.c. per 



minute. Likewise, the amount of CO, given out is _ = 280 c.c. 



100 



per minute. 



In the case of small animals, another method of procedure is adopted. 

 The volume of C0 2 expired is estimated from the weight of C0 2 given 

 out by the animal, while the oxygen used is arrived at by subtracting 



M N A B C D 



FIG. 180. THE HALDANE-PEMBREY RESPIRATION APPARATUS. 



the loss in weight of the animal in a given time from the combined 

 weights of CO 2 and water given off by it. The apparatus suitable for 

 a mouse or rat is illustrated in Fig. 180. A beaker serves as animal 

 chamber ; this is closed by a cork and pierced by inlet and outlet tubes and 

 a thermoinfler. Th!%ork is soaked in melted paraffin before insertion 

 to secure air-tkyg: closure. The b^ker is generally placed in a water- 

 bath regulate^Bo the desired temperature. Air is drawn into the 

 chamber through S. meter by means of an aspirator or filter pump. 

 The incoming air isfreed from C0 2 and water by being drawn through 

 a bottle (M) contaMng soda lime, and another (N) containing pumice 

 and sulphuric *cid. The issuing air is led through a pair of tubes 

 (A, B) containing sulphuric acid and pumice to remove the water, 

 and another pair, C containing soda lime, D containing sulphuric 

 acid and pumice. Tube C removes the C0 2 , and D catches the water 

 liberalld from the soda lime. In actual practice, tubes C and D 

 are duplicated, as a control. The duplicates should not change in 

 weight during an experiment. From the increase in weight of tubes 

 A, B during a given time the weight of water given off is obtained; 



nWti 



For accj 



work these volumes must be reduced to C. and 760 mm. 



