RESPIRATION 



429 



The results of these experiments would seem to indicate that the vagus 

 nerve contains two classes of nerve-fibers, one of which, when stimulated 

 with weak induced electric current, inhibits and regulates the discharge 

 of:,nerve energy from the inspiratory center, and thereby the extent and 

 frequency of the inspiratory movement; the other of which when stimulated 

 excites or augments the discharge of nerve energy from the inspirator^ 

 center and thereby leads to an increase in the depth or amplitude of the 

 inspiratory movement. According as the one or the other of these two 

 classes of fibers are excessively stimulated, will the inspiratory center be 

 inhibited or augmented in its activity to such an extent that the chest walls 



will come to rest in the first 

 instance in the state of expira- 

 tory standstill, in the second 

 instance in the state of inspira- 

 tory standstill. 



The stimulus adequate to 

 the excitation of the pulmonic 

 terminations of the vagus 

 FIG. 199. POSITIVE VENTILATION (Head), tfnder nerve-fibers in the physiologic 

 the influence of positive ventilation, the inspiratory 

 contractions of the diaphragm become less and less 

 till they disapoear completely. 



Diaphragm. 

 Seconds. 



condition was formerly be- 

 lieved to be the chemic action 

 of carbon dioxid; it is now 

 believed to be a mechanic action, the result of the alternate distention and 

 collapse of the walls of the pulmonic alveoli. Thus, it has been shown by 

 [Head that if the lungs are actively inflated (positive ventilation) there will 

 be produced an inhibition of the inspiratory and an augmentation of the 

 expiratory movement until the inspiratory muscles are completely relaxed 

 as indicated by the relaxation of the diaphragm, the movements of which are 

 simultaneously recorded (Fig. 200), a result similar in all respects to that 



Negative 

 ventilation. 



Diaphragm. 



Seconds. 



FIG. 200. NEGATIVE VENTILATION. (Head). At a negative ventilation was commenced. 

 Fhe expiratory relaxation of the diaphragm is seen to become more and more incomplete, until it 

 inally enters into continued contraction. 



broduced by stimulation of the superior larnygeal nerve. On the other hand, 

 if the lungs are collapsed by the artificial withdrawal of air (negative ventila- 

 tion) there will be produced an augmentation of the inspiratory and an 

 Inhibition of the expiratory movements until the inspiratory muscles are in 

 a, condition of tetanic contraction as indicated by the contraction of the 

 iiaphragm (Fig. 201) and by the state of the thorax which is that charac- 

 ;eristic of extreme inspiration, a result similar in all respects to that produced 

 by moderate stimulation of the central end of the divided vagus. 



