THE EXPIRATORY CENTRE. 397 



traction. That is to say, the resistance to the discharges of 

 the centre being entirely taken away (which is equivalent 

 to the total removal of the spring in our example), the cen- 

 tre sends out uninterrupted and non-rhythmic stimuli to 

 the inspiratory muscles* 



The pneumogastric nerve gives two branches to the larynx; 

 known respectively as the superior and inferior (recurrent) 

 laryngeal nerves; the action of these on the respiratory 

 centre is opposite to that of the fibres from the lungs 

 coming up in the main pneumogastric trunk. If the 

 superior laryngeal branch be divided and its central end 

 stimulated, the respirations become less frequent but each 

 one more powerful; hence this nerve is supposed to increase 

 the resistance to the discharges of the respiratory centre. 

 'The same, but to a less degree, is true of the inferior laryn- 

 geal branch. 



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 pas- 

 sive. But in dyspnoea expiration is a muscular act, and 

 so there must be a section of the respiratory centre control- 

 ling the expiratory muscles. This 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 dysp- 

 noea the stimulus becomes sufficient to cause it also to 

 discharge, but only after the more irritable inspiratory 

 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 those 

 already referred to, which -diminish the resistance to dis- 

 charge of the inspiratory centre, and perhaps also increase 

 the resistance to the expiratory discharge. This set is ex- 

 cited 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 impulses which increase the resistance to in- 



