196 PRINCIPLES OF PHYSIOLOGY 



such a thing as automatic action in the nervous 

 system ? One can imagine a nerve centre 

 acting automatically. Suppose that in some 

 way nervous energy is stored up in the centre 

 until there is such a state of tension as to 

 cause a discharge along certain nervous 

 paths. This discharge would lower the tension 

 and there would be an interval during which 

 energy would be again stored until the next 

 discharge, and so on. This would be an 

 automatic mechanism. Research has shown, 

 however, that this is not the way in which the 

 nervous centre works. It is not automatic, 

 but it is influenced, first, by the quality of the 

 blood flowing through it, and it is, in the 

 second place, controlled or regulated by 

 nervous impulses coming by sensory paths 

 from the periphery. Consider this with refer- 

 ence to the respiratory mechanism. In- 

 spiration mainly introduces fresh oxygen 

 into the blood, and both during inspiration 

 and expiration there is the removal of carbonic 

 acid. When the blood contains a certain 

 proportion of oxygen, even although carbonic 

 acid is also present, it is bright red arterial 



