86 HUMAN PHYSIOLOGY 



The most important reflex influence upon breathing is 

 brought about by the vagus. Section of both vagi causes 

 deeper but slower inspiration, so that the total quantity of 

 air expired during a long period of time is not altered. 

 Stimulation of a central end of a cut vagus produces no 

 typical alteration in respiration. Sometimes the effect is 

 predominantly inspiratory, sometimes expiratory. If the 

 lungs of an animal are artificially inflated, an expiratory 

 movement is produced ; by artificial expiration (sucking the 

 air out of the lung) an inspiratory movement is called forth. 

 It is therefore supposed that the vagus supplies the lungs 

 with two kinds of sensory fibres, of which the one stimulates 

 expiration, the other inspiration (inspiratory inhibiting and 

 expiratory inhibiting). Of these the first is stimulated by 

 inflation of the lung during inspiration, the second by the 

 collapse of the lung during expiration. By means of this 

 mechanism the inspirations are decreased or increased. 



This action of the vagus seems to have for its object to 

 prevent the overfatigue of the muscles of respiration, for by 

 shallow breathing the muscles are less exerted. 



