1088 PHYSIOLOGY 



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

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

 sufficient 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 excre- 

 tion 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 regarding carbon dioxide as maintain- 

 ing the activities of the various nerve-centres at their normal level. But it is 

 the hydrion concentration which appears to be the essential factor, and the 

 acid substances produced during oxygen lack are equally efficacious, but not 

 so convenient. Thus their production is not a steady process like that of 

 carbon dioxide, but, as Mathison points out, commences suddenly at a time 

 when the executive side of the nerve-cell is feeling the effect of oxygen 

 starvation, so that the cell may be too much disorganised to respond to 

 stimulation. " The broad margin of safety protecting the organism against 

 paralysis of its cells by oxygen starvation is assured by the sensitiveness of the 

 .medullary centres to hydrogen ion concentration and therefore to carbon 

 dioxide in common with other acids." 



On the other hand, it must be remembered that excessive production of 

 hydrogen ions may finally result in a condition of paralysis, which in the 

 nervous centres is expressed by narcosis. These effects can only be removed 

 by a free supply of oxygen. The concentration at which these results occur 

 varies, as we have seen, in different parts of the nervous system and also in 

 different tissues. Thus on the heart a slight increase in H ion concentration 

 causes diminished tone, which may lead to dilatation and failure of this organ. 

 The same effect is produced on the unstriated muscle fibre of the blood- 

 vessels. Since in the heart and blood-vessels the reverse effect is produced 

 by increasing the OH ion concentration, it is evident that the line of 

 ' physiological ' neutrality at which neither stimulation nor paralysis is 

 produced must vary in different tissues. 



It is an interesting question whether the electrical excitation of nerves 

 may not be due to a similar alteration in the hydrion concentration at the 



