RESPIRATION IN AIR 65 



movements and in which the pressure is lower than the 

 atmospheric, at least in the inspiratory position. In mammals 

 the thoracic cavity containing the heart and lungs is com- 

 pletely closed, and the pressure in it is "negative" in all 

 positions. 



The type and rate of ventilation varies greatly. In the 

 Amphibia and many reptiles (all snakes excepted) oscillations 

 ventilating only the bucco-pharyngeal cavity normally present 

 a frequent rhythm, which is at intervals interrupted by a 

 pulmonary expiration and inspiration. There is reason to 

 believe (Liidicke, 1936; Vos, 1936) that the bucco-pharyngeal 

 ventilation causes no significant uptake of 2 and only a slight 

 elimination of C0 2 , and Vos adduces some evidence (scarcely 

 conclusive) to show that the main function is olfactory. He 

 shows further that the complicated triphasic respiratory 

 rhythms, assumed by older authors (Babak, 1912) for the 

 pulmonary ventilation, were due to faulty technical procedures 

 and that in all the reptiles studied a rapid expiration and 

 inspiration is most often followed by an inspiratory pause of 

 variable length. In the warm-blooded animals a regular 

 alternation of inspiration and expiration is the rule. 



The regulation of the mechanical respiration is a very complicated 

 affair. Both the rate and the depth of respiratory movements 

 are subject to a large number of reflex stimuli and inhibitions 

 and vary accordingly, but given constant conditions of total 

 metabolism the ventilation, which is the product of rate and 

 depth, is far less variable than each of the two factors, because 

 the depth decreases when the rate increases, and vice versa. 

 The rightly famous experiments of Haldane and Priestley 

 (1905) showed for man that the ventilation is governed mainly 

 by the alveolar CO2 pressure which remains constant at a 

 constant rate of metabolism and increases slightly with large 

 increases in metabolism. Since the arterial blood is prac- 

 tically in C0 2 -tension equilibrium with the alveolar air, the 

 results of Haldane and Priestley could be most easily explained 

 by assuming an effect of the C0 2 -tension in the arterial blood 



