RESPIRATION 227 



Attempts have been made by experimental stimulation of the 

 vagus trunk to determine whether, as a matter of fact, it con- 

 tains both inspiratory and expiratory fibres. But the results 

 are neither so clear nor so constant that we can confidently 

 appeal to them in making a decision, and even some of the in- 

 vestigators who maintain the existence of but one anatomical 

 set of fibres believe that these are affected differently by different 

 kinds of stimulation momentary stimuli, for example, setting 

 up in them impulses which we may call inspiratory, and long- 

 lasting stimuli impulses which we may call expiratory. 



Excitation of the central end of the cut vagus below the origin 

 of its superior laryngeal branch, with induction shocks of mode- 

 rate strength, certainly causes quickening of respiration. If the 



FIG. 104. EFFECT OF STIMULATION OF CENTRAL END OF VAGUS ix A CAT. UPPER 



TRACE, RESPIRATION ; LOWER TRACE, BLOOD-PRESSURE. 



At the top are the time trace (seconds), and below it the signal line, the depres- 

 sion in which indicates the duration of the excitation. Practically no effect was 

 produced on the respiration, but a fall of blood-pressure with slowing of the heart. 



excitation be strong, there is arrest in the inspiratory phase. 

 A brief mechanical stimulus, or a series of such, has a similar 

 effect. But chemical stimulation (e.g., with a strong solution 

 of potassium chloride) or long-continued mechanical excitation 

 like that produced by stretching or compression of the nerve, 

 or certain kinds of electrical stimulation for instance, the very 

 weakest induction shocks, or the closure of an ascending voltaic 

 current* cause slowing of the respiratory movements or ex- 

 piratory standstill. This is also the usual, though not the 

 invariable, result of stimulating the superior laryngeal, even 

 when weak induction shocks are employed. With stronger 

 stimulation energetic contractions of the expiratory muscles 

 * I.e., a current passing towards the head in the nerve. 



152 



