58 THE ACTIVE FORCES OF LIVING ORGANISMS 



as a whole ; indeed, it has been stated that it affects 

 other parts of the nervous system, and especially the 

 brain, more markedly than the respiratory centre.* 

 The bare fact that arterial blood contains twice as 

 much carbonic acid as oxygen is enough to convince 

 one that it is an important factor in metabolism ; and 

 when one reflects that with every inspiration the 

 oxygen pressure around the nerve-cells is increased, 

 and that with every inspiration also the carbonic acid 

 pressure round the nerve-cells is diminished by the 

 sucking action of that movement on the great 

 veins, surely this conviction must be still further 

 strengthened. 



All metabolism may be said to take place, as we 

 have seen in the case of muscular contraction, between 

 certain limits of oxidation and of deoxidation, or, 

 if the expression be permitted, carbonic acidification. 

 It is impossible, in view of the composition of arterial 

 blood, to regard carbonic acid merely as a waste 

 product without influence on the nervous system, 

 for the proper functioning of which, indeed, a certain 

 balance between the two gases is probably essential. 

 The effect An abrupt but continued alteration in the proportions 



of varia- . „ . 



tions in in which they figure in the blood, if excessive, results 



the rela- . . 



tive in a disorganization of the nervous system. The 



of these offect of carbonic acid on the nervous system is 



gases. naturally very variable, depending as it does not 



only on the degree in which it is present, but on 



* Foster, loc. cit., p. 601. 



