RESPIRATION 



47 



center with the respiratory movements is annulled, so that infla- 

 tion or deflation of the lungs has no immediate influence on the 

 respiratory rhythm. Hence the afferent impulses through which 

 the discharges of the center are coordinated with the movements 

 of the lungs are conveyed by the vagi. After section, or better (so 

 as to avoid excitatory effects produced by actual section), freezing 

 of the vagi, the breathing, as has been known since early last 

 century, becomes deeper and less frequent, the inspirations in 

 particular taking on a dragging character which, until the work 

 of Schafer, referred to below, was entirely attributed to the ab- 

 sence of the normal inhibitory effect conveyed through the vagi 

 on distention of the lungs to a certain point. Nevertheless the 

 respirations continue to be rhythmic, and to respond in their depth 

 to the stimulus dependent on varying percentages of COg in the 

 alveolar air. It was shown by Scott* however, that the control of 

 the alveolar CO2 percentage when excess of CO2 is present in the 

 air breathed becomes much less perfect, as the frequency of the 

 breathing cannot increase. 



The analogy between the Hering-Breuer stimuli transmitted 

 through the vagi and what Sherrington has named the **proprio- 

 ceptive" stimuli participating in reflex or voluntary movements 

 of the limbs is evident; though the rhythmic discharges of the 

 respiratory center are dependent on stimuli, not from the surface 

 of the body, but from the blood acting on the center. 



When, in addition to section of the vagi, the respiratory center 

 is also severed from its connections above the medulla oblongata, 

 the rhythmic discharges of the center become still less frequent, 

 and may be inadequate to prevent death from asphyxia. The 

 influence on the center of afferent stimuli from the respiratory 

 muscles has not yet been demonstrated directly; but the fact, 

 observed by Boothby and Shamoff,^ that an animal in which the 

 pulmonary branches of the vagi have been severed without injury 

 to the recurrent laryngeal nerve recovers after a sufficient time 

 a normal control over respiration seems to point to the existence 

 of such stimuli. The same conclusion has been still more clearly 

 reached in a quite recent paper by Schafer,^ who shows that the 

 slowed breathing after section of the vagi is largely due to ob- 

 struction caused by laryngeal paralysis. 



We must now endeavor to correlate the facts relating to the 



* Scott, Journ. of Physiol., XXXVII, p. 301, 1908. 



"Boothby and Shamoff, Amer. Journ. of Physiol., XXXVII, p. 418, 1915. 



'Schafer, Quart. Journ. of Exper. Physiol., XII, p. 231, 1919. 



