3 8 2 TEXT-BOOK OF PHYSIOLOGY. 



-If the central end of the divided vagus be stimulated with weak 

 faradic currents, the respiratory movements are increased in fre- 

 quency and their depth diminished until the normal rate is restored. 

 With the cessation of the stimulation the former condition at once 

 returns. This would indicate that in the physiologic state afferent 

 impulses are ascending the vagus fibers which influence the rate of 

 discharge from the respiratory center. If, however, the stimulation 

 is increased in strength, the inspiratory movement gradually so ex- 

 ceeds the expiratory that the muscles pass into the tetanic state and 

 the ches^walls come to rest in the condition of forced inspiration. 



v The vagus evidently contains fibers which augment the activity of the 

 inspiratory center and inhibit the activity of the expiratory center. O*- 

 F, on the other hand, the central end of the divided superior laryngeal 

 nerve be stimulated with faradic currents, the opposite effect is pro- 

 duced: viz., an excess of the expiratory over the inspiratory move- 

 ment until the chest-wajb gome to rest in the condition of forced 

 expiration. VThe/Juperior laryngeal nerve .evidently contains fibers 

 which augment me activity of the expiratory , center and inhibit the 

 activity of the inspiratory center.^^ 



The same result not infrequently follows stimulation of the divided 

 vagus and always after the administration of large doses of chloral. 



\ The vagus contains two classes of fibers, one of which augments the 

 activity of the inspiratory while inhibiting the activity of the ex- 

 piratory; the other inhibiting the inspiratory while augmenting the 

 expiratory. VThe stimulus adequate to the excitation of the^nerve- 

 nbersTn the physiologic condition was formerly believed 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 pulmonary alveoli^ Thus, it has been shown by Head that 

 if the lungs are actively inflated there will be produced an inhibition 

 of the inspiratory and an augmentation of the expiratory movement 

 until the chest comes to rest, a result similar in all respects to that 

 produced by stimulation of the superior laryngeal nerve. On the 

 other hand, if the lungs are collapsed by the artificial withdrawal 

 of air, there will be produced an augmentation of the inspiratory 

 and an inhibition of the expiratory movements until the chest comes 

 to rest in extreme inspiration, a result similar in all respects to that 

 produced by powerful stimulation of the central end of the divided 

 vagus\\These facts indicate that the respiratory mechanism is reflex 

 and self-regulating in character, and the stimulus, the alternate 

 collapse and distention of the pulmonary alveofi^collapse augmenting 

 the inspiratory and inhibiting the expiratory, distention, inhibiting 

 the inspiratory and augmenting the expiratory centeuX 



