VOLUNTARY AND AUTOiMATIC MOVEMENTS. 273 



first and second vertebrae, the result is instant death 

 by the so-called dislocation of the neck. It has been 

 shown that the cause which induces this ceaseless 

 activity in the nerve-cells of the respiratory centre 

 lies in the character of the blood. When the blood 

 is quite saturated with oxygen, then the activity of 

 the respiratory centre commences.^ When the blood 

 becomes freer from oxygen, the respiratory motions 

 become stronger. 



Far from being necessarily active, independently and 

 without external incentive, the nerve-cells of the respi- 

 ratory centre are also rendered active by external cir- 

 cumstances. But they are much more sensitive than the 

 nerve-fibres, so that they are influenced even by slight 

 changes in the gaseous contents of the blood which 

 plays over them. And the other automatic nerve-cells 

 behave exactly as do the cells of the respiratory centre. 

 Yet small differences in sensitiveness occur among 

 them, so that some are excited even when only the 

 average amount of oxygen is contained in the blood, 

 others when a point lower than this average has been 

 reached, as happens only occasionally during life. 



It would take too long to apply this theory, now 



' Experimental proof of this may always be tried by anyone 

 on himself. Attention must be given for a time to the respiratory 

 movements, their depth and number being noted. From eight 

 to ten inspirations and expirations are then drawn slowly one 

 after the other. By this means much more air is introduced into 

 the lungs than by ordinary respiration, and the blood can therefore 

 thoroughly saturate itself with oxygen. If, after this, voluntary 

 respiration is ceased, it will be found that twenty seconds or more 

 elapse before a respiration again occurs, long enough that is for the 

 consumption of the introduced oxygen. Only after this do respira- 

 tions begin, at first weakly, but always increasing in strength, until 

 the former regular respiration again prevails. 

 13 



