452 THE HUMAN BODY. 



things is true. A small amount of heat is liberated when 

 haemoglobin combines with oxygen in the pulmonary capil- 

 laries, but the affinities thus satisfied are so feeble that the 

 energy liberated is trivial in amount when compared with 

 that set free when this oxygen subsequently forms stabler com- 

 pounds elsewhere. There is good reason to believe that 

 hardly any of this latter class of oxidations occurs in the 

 living circulating blood at all; its cells do, no doubt, use up 

 some oxygen and set free some carbon dioxide; but not 

 enough to be detected by ordinary methods of analysis. The 

 percentage of oxygen liberated in a vacuum by two specimens 

 of the blood of an animal, taken one from an artery near the 

 heart, and the other from a distant one, are practically the 

 same; showing that during the time occupied in flowing two 

 or three feet through an artery the blood uses up no appreci- 

 able quantity of its own oxygen; while in its brief capillary 

 transit it almost suddenly loses so much oxygen as to become 

 venous. The difference is explained by the fact that the 

 blood gives off oxygen gas through the thin capillary walls 

 to the surrounding tissues; and in the latter the oxidation 

 takes place. As we have already seen, a freshly excised 

 muscle deprived of blood can still be made to contract; and for 

 some considerable time if it be the muscle of a cold-blooded 

 animal. During its contraction it evolves large amounts 

 of carbon dioxide, although the resting fresh muscle contains 

 hardly any of that gas. Here we have direct evidence of 

 oxidation taking place in a living tissue and in connection 

 with its functional activity; and what is true of a muscle 

 is probably true of all tissues: the oxidations which supply 

 them with energy take place within the living cells themselves. 

 The statement frequently made that the oxygen in the cir- 

 culating blood exists as ozone, rests on no sufficient basis; 

 decomposing haemoglobin does ozonize some oxygen when 

 exposed to the air, but there is no ozone in fresh blood. Ex- 

 periments made by adding various combustible substances, as 

 sugar, to newly drawn blood, also fail to prove the occurrence 

 of any oxidation of such bodies in that liquid. 



Tissue-Building and Energy-Yielding Foods. The 

 Human Body, like that of other animals, is, on the whole, 

 chemically destructive; it takes in highly complex substances 

 as food, and eliminates their elements in much simpler 

 compounds, which can again be built up to their original 



