214 A MANUAL OF PHYSIOLOGY 



discharges of the respiratory centre are being continually 

 controlled and modified by impulses passing up the vagus or 

 clown from the brain, but especially up the vagus. When 

 the vagus is severed, the control of the higher paths becomes 

 more complete, and is sufficient still to keep the breathing 

 regular. When the higher paths are cut off, the vagus of 

 itself is able to regulate the discharge. But when both are 

 gone, the respiratory centre, freed from control, passes into 

 a condition of alternate spasm and exhaustion.* 



The rhythmical excitation of the regulating vagus fibres 

 must be brought about either by mechanical stimulation of 

 the nerve-endings in the lungs, due to the alternate stretching 

 and shrinking, or by chemical stimulation depending on the 



FIG. 81. RESPIRATORY TRACINGS (Doc). 



A, normal; B, effect of stimulation of the central end of the vagus; C, effect of 

 section of both vagi. (Tracing taken as in Fig. 100, p. 273.) Time-tracing, seconds. 



state of the blood. t Both views have found advocates, but 

 neither has been definitely proved. Nor are the results of 

 experimental stimulation of the nerve-trunk so clear or so 

 constant that we can confidently appeal to them in making 

 a decision. Excitation, with induction shocks, of the central 

 end of the cut vagus below the origin of its superior laryn- 

 geal branch certainly causes quickening of respiration, or, 

 if the excitation be strong, arrest in the inspiratory phase. 

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



* We may, perhaps, think of the respiratory centre as being ' wound 

 up ' like a clock, the periodic arrival of regulating impulses acting like 

 an escapement movement, and allowing a certain amount of discharge. 



f That the vagus is really excited is shown by the recent observations 

 of Lewandowsky, who finds that a negative variation (see p. 630) is set up 

 in the nerve when the lungs are inflated, though not when they collapse. 



