REGULATION OF RESPIRATION 117 



to the tension of oxygen leads to the phenomenon known as 

 Cheyne-Stokes respiration, in which periods of breath- 

 ing alternate with periods of respiratory rest. It occurs 

 after the period of apnoea which follows excessive breathing. 

 It is explained in this way. Owing to the excessive breath- 

 ing which has just occurred CO.^ is to a great extent washed 

 out of the body. The centre is then stimulated to activity 

 by oxygen- want. By the respiratory movement thus 

 caused the need for oxygen *s immediately satisfied. The 

 tension of CO2, being meanwhile still below normal, respira- 

 tion ceases. This process is repeated until the tension 

 of CO2 regains its normal level and resumes the function of 

 regulating the centre. 



Cheyne-Stokes respiration is also found in certain toxic 

 states. It is then attributed to the influence of the toxin 

 upon the centre. 



But the chemical constitution of the blood is not the only 

 factor influencing lung ventilation. It is a matter of 

 everyday knowledge that the orderly sequence of the 

 respiratory movements may be interrupted by reflex 

 stimulation. The stimulus may be emotional or it may be 

 sensory, in the latter case originating from the surface 

 of the body or from the respiratory passages. Since the 

 most potent stimuli arise from the respiratory passages 

 themselves, it would be natural to seek an effect upon 

 respiration from the pulmonary nerve endings in the vagus. 



\Vhat happens when the vagi are cut ? On section of 

 one vagus, respiration becomes slower and deeper; on 

 section of both it becomes slower and deeper still, but the 

 alternation of movements is undisturbed. But another 

 change also occurs ; for increased CO2 tension now increases 

 the depth of respiration still further, but the rate of 

 respiration is unaltered. This would seem to show that 

 one function of the vagus is to regulate the rate and to 

 limit the extent of the respiratory excursion. What is 

 the cause of the stimulation of the vagus ? The answer to 

 this is given by Head's experiment, in which it was shown 



