ACTION OF THE VAGUS ON RESPIRATION 295 



be closed at the end of an inspiration, the result is an immediate 

 relaxation of the diaphragm, and the expiration thus produced is enor- 

 mously prolonged (Fig. 170). In those cases where tonic contraction 

 of the diaphragm was present during the expiratory period, the muscle 

 continues to relax, and its fibres finally reach a condition of complete 

 elongation. After a varying interval, the pause is broken by an 

 inspiratory contraction considerably stronger than the normal inspira- 

 tions. The elongation which follows is again complete and a second 

 long pause follows, which is, however, shorter than the one which preceded 

 it, though longer than the normal pauses. If the tracheal cannula be 

 now opened, the respiratory movements become very rapid. 



If, on the other hand, the trachea is closed during the expiratory 

 pause, the breathing suffers the converse change. The pause, during 

 which the trachea is closed, lasts the usual time, and is followed by 

 an inspiratory contraction of the normal rapidity. But when the 

 inspiration has reached the normal strength, instead of ceasing 

 abruptly, it goes on increasing until the curve reaches a point far 

 above the crests of the normal inspiratory contractions (Fig. 171). Thus 

 closure of the trachea in the expiratory phase of respiration causes an 

 increase in the strength and duration of the inspiratory contractions. 



Fig. 171. — Effect on respiratory movements of closing the trachea during the 

 expiratory pause. The trachea was closed at + and opened at o. — Head. 



Still more marked results were produced by inflating the lungs, or by 

 allowing them to collapse entirely by making an opening in the thoracic 

 wall. The effect of blowing up the lungs was invariably to cut short an 

 inspiratory movement which might be proceeding, and to cause a long 

 expiratory pause. On allowing the lungs to collapse, a strong inspiratory 

 spasm of the diaphragm was produced, which lasted in the rabbit from 

 eight to ten seconds, giving place to gradually increasing oscillations of 

 the diaphragm, and finally, with the onset of dyspncea, to complete re- 

 laxations of the diaphragm between the spasmodic inspirations. 



Here we have evidently the mechanism for an automatic regulation 

 (Sclbststeuerujig) of the respiration, each expansion of the lungs in inspira- 

 tion tending to stop the inspiratory movement which caused it, and so 

 bring on the following expiration. The collapse of the lungs during 

 expiration would, on the other hand, send inspiratory impulses up the 

 vagi, which would excite again the activity of the inspiratory centres. 

 We see at once that the influence of the vagi must therefore be to 

 quicken the respiratory movements, and, by cutting them short, to make 

 them shallower, so that the results of cutting both vagi, by withdrawing 

 these quickening impulses, must be a slowing and deepening of the 

 respirations. 



