42 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



movements, and governs the admission of oxygen by closing an electric 

 circuit at (5). The oxygen from the cylinder is measured by the meter 

 which records electrically by closing a circuit each time a certain quantity 

 has been admitted. Whenever an experiment has to be extended 

 over a long period, or if the absorption of oxygen is very rapid as dur- 

 ing heavy muscular work, the oxygen admitted must be nearly pure 

 to prevent the oxygen percentage in the small apparatus from falling. 1 



The apparatus in its present form does not allow the direct deter- 

 mination of carbon dioxide. When such determinations are desired 

 samples of expired and inspired air are drawn from the vessels (2) 

 and.(io). The respiratory quotient is determined by analysing these 

 samples for carbon dioxide and oxygen. The total respiratory ex- 

 change can also be measured over short periods by multiplying the 

 analytical results by the ventilation as measured from the graphic 

 record. 



The apparatus of Benedict (fig. 16) [1909, 191 2] is arranged to 

 measure both carbon dioxide and oxygen, and the recording spirometer 

 has an attachment (a " work adder") which automatically adds the ex- 

 cursions together and so records the rate of ventilation. The instrument 

 has no valves, but a rapid circulation of air is maintained by the 

 blower. This is necessitated by the great resistance in the water- 

 vapour absorbers. If this resistance were avoided the apparatus could 

 be simplified considerably. 



i. B. () AIR-CURRENT INSTRUMENTS. 



In the simplest forms of air-current instruments the inspired air 

 is separated from the expired by means of valves, and the whole of the 

 expired air is collected over a certain period. 



Speck [1892], Fredericq [1882], and Tissot [1904] have used 

 spirometers equilibrated in various ways, and in Tissot's apparatus 

 they are arranged to record graphically the expired volume of air. A 

 sample of air from the spirometer is afterwards analysed. The method 

 is simple, but the large spirometers necessary for experiments on man 

 are costly and cumbrous, and doubts may be entertained about their 

 contents being always completely mixed. For small animals (rabbits, 

 guinea-pigs, etc.) the spirometer method can sometimes be used to 

 advantage. 



Regnard [1879] collected the expired air in a rubber bag from which 



1 When the O 2 admitted shall be an accurate measure of the O 2 used the volume of the 

 apparatus must be reduced as far as possible. The apparatus shown in fig. 15 has for 

 special reasons been enlarged by the addition of an inspiration cylinder. 



