EESPIEATION 



307 



the respiratory centre are under the control of higher nerve 

 centres, and through these they may be thrown into action at 

 any time, or even prevented from acting for the space of a 

 minute or so. But, after the lapse of this period, the re- 

 spiratory mechanism proceeds to act in spite of the most 

 powerful attempts to prevent it. 



The activity of the centre is chiefly regulated by the amount 

 of C0 2 in the blood going to it, and everything which leads to 

 an increase in the C0 2 increases the activity of respiration. 

 So perfect is this mechanism that the percentage of C0 2 in the 

 air in the lungs is kept very constant in different conditions. 



This is probably the explanation of a peculiar type of 

 breathing, known as Cheyne Stokes breathing, which sometimes 

 occurs in heart disease and in other conditions, when the patient 

 stops breathing for a time, then begins to breathe first quietly, 

 then more forcibly, and after several respirations again with 

 decreasing depth till the respirations stop. In these cases the 

 respiratory centre is less excitable than usual, and is called into 

 action only when C0 2 has accumulated in the blood. After this 

 accumulation has been got rid of by the forcible respirations the 

 activity of the centre again wanes. 



Decrease in the amount of oxygen in the blood acts in 

 the same way, but since the amount of oxygen in the arterial 

 blood varies much less than the amount of C0 2 , this factor 

 plays a less important part in regulating breathing. At high 

 altitudes the decrease in the pressure of oxygen in the air 

 breathed, leading to its decreased amount in the blood, pro- 

 duces a marked increase of respiratory movements. 



But the respiratory centre is also acted upon by various 

 ingoing nerves. 



Vagus. Since the vagus is the ingoing nerve of the re- 

 spiratory tract, we should expect it to have an important 

 influence on the centre (fig. 140). 



Section of one vagus causes the respiration to become slower 

 and deeper ; but after a time the effect wears off and the 

 previous rate and depth of respiration is regained (fig. 141). 



Section cf both vagi causes a very marked slowing and 

 deepening of the respiration, which persists for some time, 

 and passes off slowly and incompletely. But if after the vagi 



