52 RESPIRATION 



inspiration or expiration depend on the alveolar CO 2 percentage. 

 In apnoea a very slight amount of inflation or deflation is suffi- 

 cient to cause inhibition of inspiration or expiration. In conse- 

 quence of this the respiratory movements are nearly jammed in 

 a mean position during apnoea unless considerable force is ex- 

 erted, which is not the case with ordinary methods of artificial 

 respiration. With a normal stimulation of the respiratory center 

 by CO 2 and a normal respiratory frequency, the limits of inflation 

 or deflation at which the Hering-Breuer inhibition occurs are a 

 good deal wider, and with a diminished respiratory frequency, 

 or an increased percentage of CO 2 in the air inspired, the limits 

 are much wider still. Thus the respiratory center tends indirectly 

 to govern artificial respiration unless the latter is of a specially 

 vigorous kind. 



That the center responds, even during apnoea, with tonic con- 

 traction of the diaphragm to deflation of the lungs, and with re- 

 laxation to inflation, was clearly shown by Head's experiments; 

 and the inspiratory or expiratory pressures produced by the 

 diaphragm and other respiratory muscles can easily be demon- 

 strated in man. The continued control of respiratory movements 

 during apnoea or voluntary suspension of the breathing, or during 

 voluntary variations in the frequency of breathing, is thus readily 

 intelligible. In voluntary forced breathing or in forcible artificial 

 respiration, this control is broken down. It must not, however, be 

 assumed that because the ordinary gentle methods of human 

 artificial respiration have such a small effect during ordinary 

 apnoea, the effect will be equally small where the suspension of 

 breathing has been caused by asphyxiation or the action of an 

 anaesthetic or other poison. In these cases the excitability of the 

 respiratory center to the Hering-Breuer stimuli is possibly as 

 much depressed as its excitability to CO 2 , in which case the 

 artificial respiration will not be insufficient. 



The normal rate and depth of breathing in any individual is evi- 

 dently an expression of the normal balance between chemical and 

 nervous stimuli. The normal is fairly constant because the balance 

 is a stable one. It may, however, be greatly altered under abnormal 

 conditions, and it can easily be interfered with voluntarily. 



It is evident from the foregoing discussion that we cannot 

 separate the nervous from the "chemical" control of breathing, 

 since each determines the other at every point. From too exclusive 

 a consideration of the nervous side of the control it has been sup- 

 posed, on the one hand, that the center is essentially automatic in 



