60 RESPIRATORY MECHANISMS 



oxygen pressures the diffusion of 2 and C0 2 in the air 

 will be ample, and nothing can be gained by respiratory 

 movements. 



Nevertheless such movements have been described again 

 and again (Maas, 1939). It is true that when the pneumo- 

 stome is closed the pressure in the pulmonary cavity may be 

 increased (Dahr, Ysseling, 1931), even up to 20-30 mm Hg in 

 Helix pomatia (Maas), but these reactions have absolutely no 

 respiratory significance. The closing of the pneumostome 

 may itself serve to reduce evaporation (Wit, 1933). 



Dahr found that at low oxygen pressures, from about 70 mm 

 down to 15 mm, Arion and Helix show peculiar reactions which 

 are taken to be "dyspnoeic." These were confirmed for Helix 

 by Ysseling, to whom Dahr's papers remained unknown. 

 While the pneumostome usually remains open in moist air 

 at normal 2 pressure, it is closed at frequent intervals and for 

 quite long aggregate times at low 2 pressure. Dahr found, 

 for instance, that at 15 mm 2 Arion will close its lung 15-30 

 times in 10 minutes and for about 7 minutes in all. During 

 the closed period the enclosed air is moved about by muscular 

 contractions. In this particular case it appears that the 

 oxygen lack causes a partial collapse of the lung, and the 

 movement may serve to bring the air in contact with the whole 

 of the respiratory surface. If the lung could be kept open and 

 inflated the diffusion would provide definitely better condi- 

 tions for oxygen uptake, the more so as the small amount of 

 oxygen present must be materially reduced by the metabolism 

 during the period of closure. 



Dahr has shown further (1927) that at low 2 pressure the 

 metabolism is reduced by the insufficient supply of 2 . This 

 is brought out very clearly in a series of comparative experi- 

 ments on the same animal in progressive inanition. On the 

 first day the 2 absorption from atmospheric air was 110 

 ml/kg/hour, while in 5% 2 (37 mm) it was only 33. On the 

 second day the figures were 88 and 32, between the 11th and 

 14th 63 and 23, and after about 35 days 39 and 31 respectively, 



