OF RESPIRATION. 25 



sufficient, unless the CO 2 tension were greater in the 

 pulmonary capillaries than anywhere else in the circulation. 

 The nature of the agency by which this is brought about 

 is indicated by the fact that the addition of oxygenated 

 haemoglobin to serum in vacuo decomposes its carbonates, 

 setting free CO 2 , so that venous blood yields more CO 2 to 

 oxygen than to the barometer vacuum. Consequently the 

 discharge of CO 2 in pulmonary respiration is directly pro- 

 moted by the absorption of oxygen. 



Respiration can be maintained without difficulty in an 

 atmosphere which contains much less than the normal pro- 

 portion of oxygen, so that an animal supplied with a limited 

 quantity of air continues to breathe in it until it has used 

 all but a fraction of the oxygen it contains. 



The process by which the circulating blood gives oxygen 

 to the living protoplasm with which it comes into relation 

 in the capillary blood-vessels, and receives CO 2 , is often 

 called " internal respiration." The existence of such an 

 exchange of gases in the tissues is proved by the obser- 

 vation that venous blood differs, in the proportion of oxy- 

 gen and CO 2 which it contains, according to the tissue 

 through which it has circulated. 



The separation of CO 2 by protoplasm, and the absorption 

 of oxygen, are distinct and independent processes, and do 

 not go on. pari passu. The former is variable ; its variations 

 are dependent on the functional activity of the tissue ; the 

 latter is constant and is associated with restitution. The 

 independence of the two processes is proved (i) as regards 

 muscular tissue, by the observation that muscle which has 

 been entirely deprived of oxygen can be thrown into 

 functional activity (i.e., contraction) without receiving any 

 supply, and that in contracting it gives off CO 2 (Hermann) ; 

 and (2) as regards the entire organism, by the observation 

 that a frog, if kept at a low temperature, continues to dis- 

 charge CO 2 at nearly the normal rate, in an atmosphere of 

 pure nitrogen (Pfluger). 



