272 RESPIRATION 



arrival of regulating impulses acting like an escapement movement, 

 and allowing a certain amount of discharge. When the vagi are 

 cut, the inspirations are greatly prolonged and deepened, because 

 the check on the discharge of the centre has been removed. 



Attempts have been made by experimental stimulation of the 

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

 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 investigators who main- 

 tain 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 moderate 



Fig. 124. Respiratory Tracings: Dog. A, normal; B, effect of stimulation of the 

 central end of vagus; C, effect of section of both vagi. (Tracing taken as in 

 Fig. 135. P- 295.) Time-tracing, seconds. 



strength, certainly causes quickening of respiration. If the excita- 

 tion 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 elec- 

 trical stimulation for instance, the very weakest induction shocks, 

 or the closure of an ascending voltaic current* cause slowing of 

 the respiratory movements or expiratory 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 may occur. These facts undoubtedly suggest the existence 

 in the vagus of two kinds of afferent nerve- fibres that affect the 

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



