THE RESPIRATORY MECHANISM. 421 



coming up in the main pneumogastric trunk. If the supe- 

 rior laryngeal branch be divided and its central end stimu- 

 lated, the respirations become less frequent but each one 

 more powerful; hence this nerve appears to contain fibres 

 which increase the resistance to inspiratory discharges from 

 the respiratory centre. The same, but to a less degree, is true 

 of the inferior laryngeal branch. Both are inhibitory fibres 

 so far as inspiration is concerned; whereas the main vagus 

 stem when its central end is electrically stimulated is acceler- 

 ator or augmentor. 



The Expiratory Centre. Hitherto we have considered 

 breathing as due to the rhythmically alternating activity and 

 rest of an inspiratory centre and such is the case in normal 

 quiet breathing, in which the expirations are passive. But 

 in dyspnoea expiration is a muscular act, and so there must 

 be a section of the respiratory centre controlling the expira- 

 tory muscles, and we may regard the whole centre as consist- 

 ing really of two; an inspiratory and expiratory. The latter 

 part of the respiratory centre, however, is less irritable than 

 the inspiratory part, and hence when the blood is in a normal 

 state of aeration never gets stimulated up to the discharging 

 point. In dyspncea the stimulus becomes sufficient to cause 

 it also to discharge, but only after the more irritable inspira- 

 tory centre; hence the expiration follows the inspiration. 

 This alternation of activity is, moreover, promoted by the fact 

 that the pneumogastric nerve-fibres coming up from the 

 lungs are of two kinds. The predominant sort are the 

 accelerator set already referred to, which favor discharge of 

 the inspiratory centre, and perhaps also increase the resist- 

 ance to the expiratory discharge. This set is excited when 

 the lungs diminish in bulk, as in expiration; and when the 

 whole nerve is stimulated electrically they usually get the 

 better of the other set, which carry up to the medulla im- 

 pulses which inhibit inspiratory discharges. This set is 

 stimulated by expansion of -the lungs, even in quiet breath- 

 ing: and they play a part in producing the phenomenon 

 of apncea. When the distention of the lungs is con- 

 siderable these fibres not only check inspiration but favor 

 expiratory movements. Hence, every expansion of the lungs 

 (inspiration) tends to promote an expiration, and every col- 

 lapse of the lungs (expiration) tends to produce an inspira- 



