226 A MANUAL OF PHYSIOLOGY 



at the end of expiration, a series of deep and long-drawn inspira- 

 tions occurs, the first of which begins at the moment when the 

 next normal inspiration ought to have taken place had the wind- 

 pipe been left free. The most obvious explanation of these 

 results is that the expansion of the lungs sets up impulses in 

 the vagi which cut short the inspiratory activity of the respira- 

 tory centre (inspiration-inhibiting fibres), while in collapse im- 

 pulses are set up which excite it to renewed inspiratory discharge 

 (inspiration-exciting fibres). Since ordinary expiration is in the 

 main not associated with active muscular contraction, the 

 inspiration-inhibiting fibres would be at the same time expira- 

 tion-exciting. Clearly this would constitute a so-called ' self- 

 steering ' arrangement, each inspiration leading inevitably to the 

 succeeding expiration, and each expiration providing the neces- 



FIG. 103. 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. 129, p. 288.) Time -tracing, seconds. 



sary stimulus for the succeeding inspiration. On this hypothesis 

 section of the vagi must necessarily be followed by slowing of the 

 respiratory movements, and we have seen that this is the case. 



A rival hypothesis is that the automatic activity of the re- 

 spiratory centre leads normally to the discharge of motor im- 

 pulses to the inspiratory muscles, which are cut short at each 

 expansion of the lungs by the inhibitory action of the vagus, 

 the nerve not being excited during pulmonary collapse, and 

 therefore carrying no inspiratory impulses to the centre. On 

 this assumption, we may think of the 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. When the vagi are cut the inspirations are greatly 

 prolonged and deepened, because the check on the discharge of 

 the centre has been removed. 



