REGULATION OF BREATHING 21 



Hering and Breuer discovered that if expiration is 

 prevented by blocking the outlet of air at the end of 

 an inspiration, particularly if the lungs are well dis- 

 tended, rhythmic breathing efforts are interrupted. 

 There is a long pause, during w^hich there is nothing 

 but expiratory effort ; and only after this long pause 

 is there an effort at inspiration. Similarly if inspira- 

 tion is blocked at the end of expiration there is a long 

 interval in v^hich only inspiratory effort is observed. 

 The rhythmic activity of the respiratory centre is 

 interrupted in either case. 



They also discovered that if the vagus nerves, which 

 proceed from the medulla oblongata in the brain, and 

 supply branches to the lungs, are cut, these effects are 

 no longer produced. Rhythmic inspiratory and expi- 

 ratory efforts continue, quite regardless of whether 

 the lungs are inflated or deflated. Clearly, therefore, 

 impulses proceeding up the vagus nerves from the 

 lungs are concerned in the regulation of breathing. 

 When these nerves are cut or frozen across the 

 breathing immediately becomes less frequent, but 

 deeper, and acquires a well-marked dragging char- 

 acter. 



Hering and Breuer interpreted their observations as 

 signifying that with the vagus nerves intact disten- 

 tion of the lungs excites the nerve-endings with the 

 result that impulses which stop or inhibit inspiration, 

 and excite expiration, pass up the nerves. On de- 

 flation of the lungs to a certain point during expira- 

 tion a corresponding process occurs which inhibits 

 expiration and excites inspiration. Thus the disten- 



