1214 PHYSIOLOGY 



regarding carbon dioxide as maintaining the activities of the various 

 nerve-centres at their norm ! level. But it is the hydrion concentra- 

 tion which appears to be the essential factor, and the acid substances 

 produced during oxygen lack are equally efficacious, but not so con- 

 venient. 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 produc- 

 tion 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 at first have a salutary effect on the total work of the 

 heart, but later leads 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 concentra- 

 tion at the cathode which is the seat of stimulation. If this were so, 

 all the activities of protoplasm might be regarded as determined by 

 the relative concentration of the H and OH ions within the cells or 

 in the medium surrounding the cells. 



THE REFLEX NERVOUS REGULATION OF RESPIRATION 

 Although the specific sensibility of the respiratory centre to C0 2 

 is the most important factor in determining the depth and rhythm 

 of the respiratory movements, these movements and the condition 

 of the respiratory centre itself are modified in a large degree by impulses 

 arriving at the centre along both vagi. Through other sensory 

 nerves of the body the respiratory movements can be altered reflexly, 

 but it is only through the vagi that a continuous stream of impulses 

 passes to the centre under normal circumstances, so that every respira- 

 tory movement is modified by these impulses. 



