REGULATION OF RESPIRATORY MOVEMENTS 1213 



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

 atmosphere, the concentration of the hydrogen ions rises to l-4x!0~ 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. We may 

 approach this delicacy of reaction by using a large molecule as our indi- 

 cator. Thus, as we have seen, the dissociation curve of haemoglobin is sensi- 

 tive to the change in reaction caused by raising the tension of carbon dioxide 

 in the haemoglobin solution by 10 mm. Hg. (cp. Fig. 495). 



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 1 of oxygen supply to the cells. The respiratory centre would then 

 not differ qualitatively from any other part of the central nervous system. Its 

 special function would be determined simply by the evolution 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, and are thrown into activity by excess of these 

 ions and inhibited, or put to rest, by relative deficiency of the ions. 

 In their relation to H and OH ions respectively the medullary centres 

 have a sensibility five times as great as the spinal centres. The 

 condition of apnoea, which is associated not only with cessation of 

 respiratory movements but also with fall of blood pressure, may be 

 ascribed to relative increase in the OH ions or diminution in the 

 H ions. 



Since the animal has developed a mechanism by means of which 

 changes in the reaction of the blood can be rapidly adjusted by varying 

 the excretion of carbon dioxide, whilst the excretion of other acids 

 is relatively slow, carbon dioxide may be regarded as the normal 

 respiratory hormone, and so far we may agree with Henderson in 



