RESPIRATION 193 



the Alps, he takes all that he can get, for fresh air is one of the 

 few good things of which one can never have enough. 



Tissue Respiration. A frog will live for seventeen hours in an 

 atmosphere of nitrogen. Under these circumstances it is clearly 

 impossible for it to take up oxygen, yet for several hours it gives 

 off as much carbonic acid as it would do if it were living in air. 

 Such an observation as this proves that oxidation does not occur 

 in the lungs, but deeper in the body. At one time the blood 

 was regarded as the seat of oxidation ; the products formed by 

 the splitting up of proteins in the tissues were supposed to be 

 passed into the blood, where they came in contact with the 

 oxygen carried by haemoglobin. A certain amount of oxidation 

 does take place in the blood, as in all other tissues, for blood is a 

 living tissue and needs to respire. But the oxidation which 

 occurs in the blood is small in amount as compared with that in 

 the organs which the vessels traverse. Muscle and other tissues 

 detached from the body and free from blood give off carbonic 

 acid. It is possible to wash the blood out of the vessels of 

 a frog and to replace it with a solution of salt. In an atmo- 

 sphere of oxygen such a " saline frog " lives for a day or two, 

 taking in the same quantity of oxygen and giving off the same 

 quantity of carbonic acid as a normal frog. The oxygen is 

 chiefly absorbed through the skin, the carbonic acid discharged 

 from the lung. This experiment shows that blood is not 

 essential for oxidation. Oxidations do not occur in the salt 

 solution with which blood is replaced. Taking all the evidence 

 together, it seems to be safe to conclude that the tissues absorb 

 the oxygen which the oxyhsemoglobin brings into their neigh- 

 bourhood, and that they have some capacity of storing it. A 

 piece of detached muscle which gives off carbonic acid in an 

 atmosphere of nitrogen would appear to be holding a store of 

 oxygen, much as haemoglobin holds it. The proof is not quite 

 so definite as might be desired ; but we are probably justified in 

 holding the belief that the main part of the respiratory exchange 

 occurs in the tissues. Lymph dissolves oxygen which it obtains 

 from the blood. The tissues take it from lymph. Tissues set 

 free carbonic acid which lymph dissolves. Its tension being 

 higher than in blood, carbonic acid diffuses from lymph, through 

 the walls of the capillary vessels, into blood, from which it 

 passes into the air in the lungs. 



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