TRACHEAL RESPIRATION 



125 



horizontal tube 2. The expirations are seen to be active, and 

 the equilibrium position of the whole system is the inspiratory. 

 The normal depth of respiration which varied but little is 

 about 40 mm 3 in an animal of 1 g weight. The vital capacity 



Fig. 70. Experimental arrangement to study respiration in air-breathing 

 aquatic insects. See text. (Krogh.) 



could be determined in different ways of which one was to 

 allow the animal to breathe pure oxygen from a slow current 

 of the gas through the tube 4 sealed on with plasticene to the 

 top of the funnel 3. When the tracheae were filled with oxygen 

 the normal stimulus for respiration would be much delayed, 

 the ventilation tracheae would collapse completely, and when 

 the animal arrived at the surface some ineffectual attempts 

 would precede the sudden opening up. The vital capacity 

 volume recorded by this was 60 mm 3 and was confirmed also 

 by a different method. The total capacity finally was de- 

 termined by letting the animal breathe from a bubble of 

 known size and containing an indifferent gas which could be 

 determined by analysis. From the dilution found after a 

 series of respirations the total capacity of the tracheal system 

 could be calculated and was found in the case considered to 

 be 90 mm 3 . By a short series of normal respirations the air 



