THE REGULATING MECHANISM 199 



vagus be divided in the neck then irritation 

 of the upper end of the nerve causes deep 

 inspirations, and that strong stimulation 

 may stop breathing with a kind of spasm at 

 the end of an inspiration. This shows that the 

 impulses coming from the lungs are carried 

 upwards to the respiratory centre in the bulb, 

 and that they more especially stimulate 

 inspiration, possibly by acting on the in- 

 spiratory centre. On the other hand, stimu- 

 lation of the superior laryngeal branch of the 

 vagus causes expirations, and strong stimu- 

 lation may stop respiration at the close of an 

 expiratory spasm. Impulses therefore coming 

 sometimes from the larynx excite expirations. 

 Now one can imagine the terminal fibres of 

 the vagus in the lungs to be stimulated by 

 venous blood ; impulses would then be sent 

 to the inspiratory centre in the bulb. This 

 would be stimulated, with an inspiration as 

 the result. But inspiration is a muscular act 

 involving the diaphragm and the muscles that 

 raise the ribs, while expiration is an act mainly 

 due to an elastic recoil of the lungs and of the 

 walls of the chest. Consequently there is no 



