THE ACTIVE FORCES OF LIVING ORGANISMS 65 



If the intensity of these be diminished, a proportional 

 accumulation of chemical matter might doubtless be 

 looked for. 



But if an increase of iron, of hsemoglobin, or of the 

 red corpuscles be due, as seems likely, to diminished 

 intensity of metabolism, it would disappear as soon 

 as a return to a higher degree of tissue change took 

 place ; and it is generally recognised that the effects 

 of residence at high altitudes are somewhat evanescent, 

 more so in some cases, less in others, according to the 

 special conditions of life and the idiosyncrasies of 

 the individual. A relatively greater accumulation of 

 oxygen-carrying or oxidizable material in the blood 

 does not necessarily imply more rapid oxidation. 

 The tissues, not the blood, are the chief seat of oxida- 

 tion ; and this is so partly because they are directly 

 connected by nerve fibres with the nervous system 

 which controls metabolism, and partly on account of 

 their chemical composition. The connection of the 

 blood with the nervous system is in this sense, so far 

 as we know, only an indirect one. Trade may be said 

 to be good when the demand is equal to the supply. 

 So also in the present case, though a plentiful amount 

 of oxygen in the blood may act as a slight stimulus to 

 metabolism, yet the latter depends equally on such 

 factors as exercise and the perfect functioning of the 

 nervous system. The amount of oxygen in the blood, 

 as also the amount of blood circulating in the nervous 

 system and the blood-pressure, and the amount of 

 carbonic acid in the blood, are all, however, important 



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