THE REGULATING MECHANISM 197 



blood ; but when it contains more carbonic 

 acid and relatively less oxygen, it is dark 

 purplish-hued venous blood. Again, if we 

 breathe deeply a number of times in succession 

 so as to introduce as much oxygen as possible, 

 we can then " hold the breath," that is to say, 

 we can for a time cease breathing. Divers 

 do this before they make their plunge into the 

 sea. In physiological language, blood con- 

 taining an excess of oxygen produces a state 

 called apnoea, during which respiration is sus- 

 pended. On the other hand, if the blood 

 contains more than a certain amount of 

 carbonic acid, so as to be highly venous, there 

 is a tendency to make rapid movements 

 of inspiration so as to get rid of the excess 

 of carbonic acid and introduce more oxygen. 

 This happens in asphyxia, produced from any 

 cause. These phenomena can be accounted 

 for if we consider the influence of the kind of 

 blood circulating through the respiratory 

 centre. When the blood is rich in oxygen, the 

 centre is not stimulated, but when deficient in 

 oxygen and rich in carbonic acid, the centre is 

 stimulated so as to produce inspiratory 



