RESPIRATION 433 



the inspiratory movement, as to lead to the condition of complete relaxation 

 of the inspiratory muscles with a consequent expiratory standstill. 



The same result, viz.: a complete relaxation of the inspiratory muscles 

 leading to an expiratory standstill, not infrequently follows strong stimulation 

 of the central ends of divided vagi, and always after the administration 

 of large doses of chloral. 



If however the central end of the divided vagus be stimulated with 

 induced electric currents of moderate intensity, the opposite effect is pro- 

 duced, viz.: an increase in the extent of the inspiratory movement and a 

 decrease in the extent of the expiratory movement until the inspiratory 

 muscles pass into the tetanic state and the thoracic walls come to rest in the 

 condition of a forced inspiration. This would indicate that the vagus nerve 

 also contains nerve-fibers which, with the proper degree of stimulation 

 will transmit nerve impulses that excite or augment the activity of the 

 inspiratory center, and therefore the extent of the inspiratory movement, 

 as to lead to the condition of tetanus of the inspiratory muscles. 



Inhibitor and Excitator Nerves. These experiments indicate that the 

 vagus nerve contains two classes of nerve-fibers, one of which, when stimu- 

 lated with weak induced electric currents, inhibits and regulates the dis- 

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

 and frequency of the inspiratory movement; the other of which when stimu- 

 lated, excites or augments the discharge of nerve energy from the inspira- 

 tory 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 



,-, ., . r ,, Diaphragm. 



the excitation of the pulmomc Seconds 



terminations of the vagus 

 nerve-fibers in the physiologic FIG. 196. POSITIVE VENTILATION (Head). Under 



Condition was formerly be- the influence of positive ventilation, the inspiratory 



i j , i , ! T ... contractions of the diaphragm become less and less 



heved to be the chemic action till they disa pp e ar completely. 

 of carbon dioxid; it is now 



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

 recoil 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. 196), a result similar in all respects to that 

 produced 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 

 28 



