68 



COMPARATIVE PHYSIOLOGY 



to about a quarter of its normal dimensions. The remaining 

 twenty-five per cent, might be effected through the mouth or 

 anus, or by the skin. Buddenbrock and Rohr investigated this 

 point and found that occlusion of the mouth and anus does not 

 reduce the total exchange of gas. The alimentary tract does 

 not therefore contribute materially to the respiratory process. 

 The oxygen content of the tissues was found by Buddenbrock 

 to be in equilibrium with a partial pressure of about 76 mm. of 



50 



30 



20 



0;:GOj. 



GOjv. 



5^.. ••■ 10 15 20 25 30 



Fig. 15. — (After Buddenbrock and Rohr.) 



mercury. It might be surmised, therefore, that no effects 

 would accompany reduction of the oxygen content of the air 

 so long as there remained at least ten per cent. This proved 

 to be the case. Below this point physical diffusion does not 

 compensate for the rate at which oxygen can be used up, and 

 the respiratory exchange falls. 



Very remarkable results accompany variation of the CO2 

 tension of the inspired air (Fig. 15). The intake of oxygen 



