REGULATION OF THE RESPIRATORY MOVEMENTS 1137 



when a larger percentage of oxygen is supplied to the animal, whereas that produced by 

 carbon dioxide excess dies away slowly on exposure to normal conditions. It would seem 

 that, when the exposure to low oxygen tensions is of short duration, no lactic acid is pro- 

 duced in the blood. If therefore we ascribe the hyperpncea to the production of lactic 

 acid, we must locate the production of this acid in the respiratory centre itself. There 

 are no inherent improbabilities in such an assumption, but it is difficult at present to 

 see how it can be put to the test of experiment. 



In dealing with the question of the blood alkalinity we defined neutrality as a con- 

 dition in which there were equivalent concentrations of H and OH ions. In the blood 

 the H ion concentration is about 0*3 X 10~ 7 N. The alkalinity is expressed by 



concentration OH ions 



. The acids and bases of the blood serum and of the tissue fluids 

 concentration H ions 



generally are in such proportions as to maintain the approximate neutrality of these 

 fluids even a lL er considerable additions of acid or alkali. Thus hydrochloric acid may 

 be added to the extent of '025 N, or NaOH to the extent of '005 N, without causing 

 any marked alteration in the reaction of the blood. Although the change produced 

 by the addition of acids or alkalies is so minute, it is appreciable by electrical 

 methods, and it may still more readily be appreciated' by and act as a stimulus for the 

 cells of the body themselves. Thus we have not yet succeeded in determining electric- 

 ally the change in hydrogen ion concentration caused by the change from arterial to 

 venous blood. If however blood serum be saturated with carbon dioxide at a full 

 atmosphere, the concentration of the hydrogen ions rises to T4 x 10 ~ 7 N, while after 

 removing the greater part of the carbon dioxide from the same serum by the passage of a 

 stream of air, the concentration of the hydrogen ions sinks to '008 X 10 ~ 7 N. As the 

 respiratory centre responds to such minute changes of concentration as would be 

 expressed by a difference of 0*2 per cent, of an atmosphere in the carbon dioxide tension 

 of the circulating blood, it must possess a sensitivity greater than any of our physical 

 means for measuring the concentration of hydrogen ions in a fluid. Wevnay approach 

 this delicacy of reaction by using a large molecule as our indicator. Thus, as we have 

 seen, the dissociation curve of haemoglobin is sensitive to the change in reaction caused 

 by raising the tension of carbon dioxide in the haemoglobin solution by 10 mm. Hg. 

 (cp. Fig. 509). 



The regulating factor in the blood is probably not carbon dioxide nor any special acid, 

 but the concentration of hydrogen ions in this fluid or in the cells of the centre itself. 

 Such a conclusion brings under one head all the several factors which we know to act 

 upon the respiratory centre, namely, tension of carbon dioxide, presence of acids in the 

 blood especially lactic and considerable diminution of oxygen supply to the cells. 

 The respiratory centre would then not differ qualitative^ from any other part of the 

 central nervous system. Its special function would be determined simply by the evolu- 

 tion to a marked degree of a sensibility to hydrogen ions which is already possessed by 

 the whole of the central nervous system and indeed by practically every tissue of the body. 



We may conclude that mere lack of oxygen is not to be regarded in 

 itself as an excitatory agent. Its influence will be rather to paralyse all 

 activity. On the other hand, excitation is caused by the products of 

 metabolism, which vary according as the oxygen supply is ample or 

 insufficient for the needs of the cells. In the former case activity results 

 in the production of carbon dioxide, in the latter of lactic acid, and perhaps 

 other substances. Both these are acid substances and their production 

 will therefore raise the concentration of the hydrogen ions in the cells 

 where they are produced as well as in the blood. The nerve centres are 

 extremely sensitive to minute changes in the hydrion concentration either 

 in themselves or in the fluids surrounding them, arid are thrown into activity 

 by excess of these ions and inhibited, or put to rest, by relative deficiency 

 72 



