48 RESPIRATION 



Hering-Breuer phenomena with those relating to the governing 

 of the lung ventilation by the charge of CO2 in the alveolar air 

 and arterial blood. It seems very clear that the immediate cause of 

 the arrest of inspiration during ordinary breathing is the disten- 

 tion of the lungs to a certain point, and a consequent inhibitory 

 stimulus transmitted up the vagi. The experiments of Head/ in 

 which the movements of a slip of the diaphragm, the most promi- 

 nent inspiratory muscle, were recorded, show that this inhibition 

 produced an instant relaxation of the diaphragm. If the vagi 

 have been frozen the relaxation is greatly delayed, and even 

 after the delay is at first very imperfect. The inhibition of inspira- 

 tion initiates an expiratory phase, which continues until, in its 

 turn, it also is cut short by deflation to a certain point, at which 

 the vagi transmit an influence which inhibits expiration and 

 initiates the inspiratory phase. It appears from Head's experi- 

 ments that if the vagi are frozen after the inspiratory or expira- 

 tory phase has been initiated, this phase still continues. If with 

 vagi intact the breathing is partially obstructed, inspiration or 

 expiration is continued till either act is complete. The influence 

 transmitted through the vagi initiates inspiration or expiration, 

 therefore; and the center persists in the inspiratory or expiratory 

 phase till the vagus gives the signal which terminates the phase 

 and initiates the complementary phase. The center behaves as if 

 it always remembered the last signal; and the analogy between 

 any act dependent on memory and the duration of the inspiratory 

 or expiratory phases of breathing is evident. We are equally 

 reminded of the "refractory period" in the phases of cardiac and 

 other muscular activity. 



Where the "chemical" regulation of the respiratory center 

 exerts its preponderating influence is in determining the extent 

 to which inflation or deflation of the lungs must extend in order 

 that the Hering-Breuer stimuli should be effective, and also the 

 vigor and consequently the rapidity of the inspiratory and expira- 

 tory movements. Thus an increased CO2 stimulus causes increased 

 depth of breathing, since a greater inflation or deflation of the 

 lungs is required before the stimulus of inflation or deflation 

 becomes effective. At the same time the movements of the chest 

 wall become more rapid, so that the frequency of breathing is 

 not diminished in consequence of the greater distances traveled 

 by the chest walls. The net result is thus ordinarily increase 

 in depth without diminution in frequency. But if the frequency 



' Head, Journ. of Physiol., X, pp. i and 279, 1889. 



