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 which 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 which 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 regulafion of breathing. 
When these nerves are cut or/ frozen across the 
breathing immediately becomes less frequent, but 
deeper, and acquirés-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- 
