THE NERVOUS MECHANISM OF RESPIRATION. 377 



such an experiment on a rabbit a curve be taken by means of the isolated 

 slip of the diaphragm, 271, it will be seen (Fig. 104, A) that the slip elon- 

 gates somewhat ; that is to say, previously in a state of slight tonic contrac- 

 tion, it changes in the direction of expiration. If, on the other hand, the 

 trachea be suddenly closed at the end of an expiration (Fig. 104, B\ when 

 the lungs have returned to their emptied condition, the result is an increase 

 of the sequent inspirations, that is to say, an augmentation of inspiratory 

 impulses. If the chest or if the lung only be gently inflated, a temporary 

 cessation of all inspiration may be produced, accompanied sometimes by an 

 attempt at expiration. If, on the other hand, air be sucked out of the 



FIG. 105. 



Effects of Repeated Inflations. Positive Ventilation. (Head.) The lower curve is described, 

 as in Fig. 104, by a lever attached to a slip of the diaphragm. The upper curve shows the infla- 

 tions from x to y, which were made without any attempt to draw the air out at each inflation ; 

 each rise on this curve denotes an inflation. It will be observed that as the inflations are con- 

 tinued the respiratory movements of the diaphragm are gradually " knocked down." 



chest, or if one lung be made to collapse by puncture of one pleural cham- 

 ber, a prolonged inspiration is the frequent result, the diaphragm being 

 thrown into a prolonged inspiratory tetanus. If the lungs are repeatedly 

 inflated, without any means being taken to draw out the air after each infla- 

 tion (Fig. 105), a procedure which we may speak of as positive ventilation, 

 the result is that the inspiratory efforts are diminished, and if the ventila- 

 tion is continued may cease altogether. If, on the other hand, air is repeat- 

 edly sucked out of the lungs without any corresponding inflations (negative 

 ventilation), the inspiratory efforts are increased (Fig. 106), and the increase 

 may be such as to bring the diaphragm to a state of tetanus. And in 

 general, though several complications occur which we cannot discuss here, 

 the results of inflation of the lungs on the one hand and of suction or col- 

 lapse of the lungs on the other hand, show that the mere inflation or per- 

 haps rather the mere distention of the lung tends to inhibit inspiratory and 

 usher in expiratory impulses, while collapse of the lung tends to inhibit 

 expiratory and to develop inspiratory impulses, the effect on the inspiratory 

 impulses, as might be expected from the dominance of the inspiratory por- 

 tion of the centre, being more marked than the effect on the expiratory 

 impulses. That the instrument by which these effects are produced is the 

 vagus nerve is shown by the fact that they are no longer distinctly recog- 

 nizable when both vagus nerves are divided. And that the results are due 

 to the mere mechanical expansion and collapse of the lung in insufflation 

 and collapse, and not to any chemical influences exerted by the larger 

 amount or smaller amount of air present in the lung in the two cases 

 increasing or diminishing the absorption of oxygen and escape of carbonic 

 acid, is shown by the fact that the results remain in their main features 

 the same when some indifferent gas, such as hydrogen, is used for inflation 

 instead of air or oxygen. We infer therefore that the expansion of the 

 pulmonary alveoli in* some way or other so stimulates the endings in the 



