198 PRINCIPLES OF PHYSIOLOGY 



movements. Two respiratory centres have 

 been by some assumed to exist in the bulb 

 an inspiratory and an expiratory centre. 

 Then carbonic acid may be supposed to 

 stimulate the inspiratory, while oxygen might 

 either stimulate the expiratory or produce no 

 effect. There can be no doubt, however we 

 may theoretically explain the facts, that the 

 quality of the blood affects the respiratory 

 centres. 



112. But the centres for respiration are also 

 influenced by nervous impulses coming from 

 the periphery. The lungs are supplied by 

 the vagi or pneumogastric nerves. These 

 contain both sensory fibres for carrying 

 nervous impulses upwards to the bulb, where 

 the vagi originate, and motor fibres which 

 supply the muscles of the larynx and the 

 muscular fibres in the walls of the bronchial 

 tubes. The larynx is highly sensitive through 

 the medium of a sensory nerve, the superior 

 laryngeal branch of the vagus, which leaves 

 the main trunk in the neck and is dis- 

 tributed to the lining membrane of the 

 larynx. Experiment has shown that if the 



