202 HAND-BOOK OF PHYSIOLOGY. 



be impossible to perform those voluntary respiratory acts which have been 

 just enumerated and explained, as speaking, singing, and the like. 



The respiratory movements and their rhythm, so far as they are invol- 

 untary and independent of consciousness (as on all ordinary occasions) are 

 under the governance of a nerve-centre in the medulla oblongata correspond- 

 ing with the origin of the pneumogastric nerves; that is to say, the motor 

 nerves, and through them the muscles concerned in the respiratory move- 

 ments, are excited by a stimulus which issues from this part of the nerv- 

 ous system. How far the medulla acts automatically, i.e., how far the 

 stimulus originates in it, or how far it is merely a nerve-centre for reflex 

 action, is not certainly known. Probably, as will be seen, both events 

 happen; and, in both cases, the stimulus is the result of the condition of 

 the blood. 



The respiratory centre is bilateral or double, since the respiratory 

 movements continue after the medulla at this point is divided in the mid- 

 dle line. 



As regards its supposed automatic action, it has been shown that if 

 the spinal cord be divided below the medulla, and both vagi be divided 

 so that no afferent impulses can reach it from below, the nasal and laryn- 

 geal respiration continues, and the only possible course of the afferent im- 

 pulses would be through the cranial nerves; and when the cord and me- 

 dulla are intact the division of these produces no effect upon respiration, 

 so that it appears evident that the afferent stimuli are not absolutely 

 necessary for maintaining the respiratory movements. But although au- 

 tomatic in its action the respiratory centre may be reflexly excited, and the 

 chief channel of this reflex influence is the vagus nerve; for when the 

 nerve of one side is divided, respiration is slowed, and if both vagi be cut 

 the respiratory action is still slower. 



The influence of the vagus trunk upon it is twofold, for if the nerve 

 be divided below the origin of the superior laryngeal branch and the cen- 

 tral end be stimulated, respiratory movements are increased in rapidity, 

 and indeed follow one another so quickly if the stimuli be increased in 

 number, that after a time cessation of respiration in inspiration follows 

 from a tetanus of the respiratory muscles (diaphragm). Whereas if the 

 superior laryngeal branch be divided, although no effect, or scarcely any, 

 follows the mere division, on stimulation of the central end respiration 

 is slowed, and after a time, if the stimulus be increased, stops, but not in 

 inspiration as in the other case, but in expiration. Thus the vagus trunk 

 contains fibres which slow and fibres which accelerate respiration. If we 

 adopt the theory of a doubly acting respiratory centre in the floor of the 

 medulla, one tending to produce inspiration and the other expiration, 

 and acting in antagonism as it were, so that there is a gradual increase in 

 the tendency to produce respiratory action, until it culminates in an in- 

 spiratory effort, which is followed by a similar action of the expiratory 



