THE HUMAN BOD f. 



But if this were so the lungs ought to be the hottest party 

 of the Body, and the blood leaving them by the pulmonary 

 veins much hotter than that brought to them by the pulmo- 

 nary artery after it had been cooled by warming all the tis- 

 sues; and neither of these things is true. A small amount 

 of heat is liberated when haemoglobin combines with oxygen 

 in the pulmonary capillaries, 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 subse- 

 quently forms stabler compounds elsewhere. It is now, more- 

 over, tolerably certain 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 car- 

 bon 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 of her 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 appreciable quantity of its own oxygen;, 

 while in the short time occupied in its brief capillary transit 

 it loses so much oxygen as to become venous. The differ- 

 ence is explained by the fact that the blood gives off oxygen, 

 gas through the thin capillary walls to the surrounding 

 tissues; and in them 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 consider- 

 able 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 prob- 

 ably 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 seem to form ozone when 

 exposed to the air, but fresh blood yields no sign of it.. 



