RESPIRATION 165 



however slowly setting free energy by settling down the steps 

 which lead to stability and ultimate, inanimate rest ; and the 

 only source of this energy is combination with oxygen. In the 

 case of a seed the oxygen need not come from without. Seeds 

 retain their power of germination after long occlusion in 

 nitrogen or other neutral gases. But all the time some change 

 is occurring, some internal oxidation which resolves their less 

 stable into more stable compounds. Otherwise they would not 

 be alive. A physiologist is willing to believe that this may 

 continue for ten years, fifteen years for any period that the 

 botanist tells him that he has, under verifiable conditions, 

 observed that it does occur ; but when he is told that peas 

 taken from the hand of an Egyptian mummy, or seeds set free 

 by the spades of navvies after a far longer burial, have been 

 found to retain their vitality, his credulity is stretched beyond 

 breaking-point. He cannot imagine a change so slow as to 

 be spread over a geological period, still without exhaustion of all 

 changeable compounds. 



The term " respiration " has been extended until it is 

 synonymous with " oxidation." At one time it was supposed 

 that the combination of oxygen with oxidizable substances 

 occurred in the lungs. The lungs were the hearth of the body, 

 to which the blood brought fuel which burned in the air drawn 

 into them. When it was understood that the actual com- 

 bination of combustible material with oxygen occurs, not in 

 the lungs, but in the tissues, a somewhat illogical distinction 

 was made between " external respiration " the combination 

 of oxygen and blood in the lungs and " internal respiration " 

 the combination of oxygen and tissue-substances. The 

 terms are not comparable. The taking up of oxygen by the 

 haemoglobin of blood is a different process to the union of 

 oxygen, after the haemoglobin has parted with it, with the 

 carbon, hydrogen, and nitrogen of the tissue-substances. 



The blood-stream carries both fuel and oxygen to the tissues, 

 but the fuel while in the blood is not in an oxidizable con- 

 dition. The foods are taken up by the tissues. They enter 

 into combination with their protoplasm. Oxygen also com- 

 bines with tissue-substances. In proportion as the tissues are 

 active oxidized compounds are split off. They fall into the 

 lymph, whence they are absorbed by the blood. If they are 



