NEGATIVE AFTER-ACTION. 



3°3 



cause the appearance of an inspiratory tone and to lengthen the 

 individual inspiratory contractions. This effect is still more marked if 

 the division of the vagi be carried out after the medulla oblongata has 

 been separated from the upper parts of the brain by a section through 

 the strife acusticae. In this latter case the inspiration which follows 

 division of the second vagus may last from forty-five seconds to 

 one and a-half minutes. At the end of this time there is a quick 

 complete relaxation of the diaphragm, followed by a slow expiratory 

 contraction of the abdominal muscles. When this active expiration 

 has reached a considerable height, the abdominal muscles suddenly 

 relax, and the inspiratory muscles contract violently, producing 

 a second prolonged inspiratory spasm. Gad l pointed out that these 

 effects indicate that the impulses previously ascending the vagi were 

 inhibitory in character, rather than inspiratory, as was supposed by 

 Eosenthal. Whereas Hering and Breuer had imagined that the vagus 

 is traversed alternately by expiratory and inspiratory impulses, Gad 

 concluded that the only impulses travelling up the vagi are expiratory 

 or inhibitory in nature. These impulses are excited by the condition of 

 distension of the lung, and are therefore present even at the end 

 of a normal expiration, being only abolished by total collapse of the 

 lung. 



It is evident that we can explain the results of distension or collapse 

 of the lungs just as well on this hypothesis as on that of Hering 

 and Breuer. When the lungs are forcibly distended, the normal 

 inhibitory influence of the vagus is increased, and an expiratory pause 

 is the result. Sudden collapse of the lungs causes a diminution in 

 the normal inhibitory stimuli, and therefore increased inspiratory 

 activity. 



Head, though agreeing with Gad in thinking that the collapse of the 

 lung in normal quiet expiration acts only by a removal of inhibitory 

 stimuli, does not accept his view that the only impulses travelling 

 up the vagi are inhibitory. If this were the case, sudden collapse of 

 both lungs should be exactly equivalent to section of both vagi ; whereas 

 Head, using a more perfect method of investigation than Gad, has 

 shown that the results of collapse are far more pronounced than those 

 of simple section of the vagi. Moreover, it is difficult to under- 

 stand the summation of effects produced by negative ventilation on the 

 assumption that each suction merely causes a diminution of previously 

 existing stimuli. 



Assuming that the inhibitory impulses which ascend the vagi 

 are the more important under normal conditions of respiration, how 

 can we explain the quickening effect of these stimuli ? In endeavour- 

 ing to answer this question, it must be remembered that every stimulus, 

 whether produced by inflation or collapse, is followed by a positive and 

 a negative after-reaction. After the division of the vagi, the breathing 

 finally acquires the type characterised by long powerful inspirations, 

 separated by complete expiratory pauses. If now we could suddenly 

 reinstate the vagi, it is evident that in the very next inspiration, before 

 the movement could be completed, the dilatation of the lungs would 

 send an inhibitory impulse up to the centre, checking the inspiratory 

 movement and causing a subsequent pause (positive after-action). As 

 soon as this positive after-action was sufficiently diminished, a second 

 1 Arch./. Physiol, Leipzig, 18S0, S. 1 ; ibid., 1881, S. 538. 



