CENTRES OF MEDULLA OBLONG ATA. 631 



This centre is a good example of a so-called automatic centre ; 

 that is to say, the blood flowing through the medulla and nour- 

 ishing the cells suffices to supply them with the necessary energy 

 for their periodic activity, and we know that the quality of the 

 blood reaching this part modifies the activity of the cells ; for 

 the less oxygen and the more carbonic acid contained in the 

 blood, the more powerfully does it act as a stimulant to the 

 centre. 



Although we take the re-spiratory centre as an example of an 

 automatic centre, its daily work is arranged by means of afferent 

 impulses, so that the normal rhythm of breathing is regulated 

 and maintained by reflex action. The mechanical states of the 

 lungs whether distended as in inspiration or contracted as in 

 expiration seem to excite the terminals of certain fibres of the 

 vagus, which carry impulses to the centre, and thus excite or re- 

 strain the inspiratory movements. 



But this automatic centre can also be influenced by the higher 

 centres of the brain, for by our will we can obviously regulate 

 our breathing movements or stop breathing altogether for a time. 

 And further, the action of the respiratory centre can be much 

 altered by impulses arriving from the surface, as may" be seen by 

 the gasping inspirations which involuntarily follow the sudden 

 application of cold to the surface. 



Again, the activity of the centre may be quite altered by stimu- 

 lations of certain parts of the air-passages; so much so, that con- 

 vulsive actions of the respiratory muscles are brought about, 

 which induced some to speak of a sneezing centre and a coughing 

 centre in the medulla. But sneezing and coughing may be equally 

 well explained as a peculiar form of activity of the respiratory 

 centre, or a reflex alteration in the respiratory rhythm, caused by 

 irritation of the nasal or laryngeal mucous membranes, as by 

 supposing that special reflex centres exist for the purpose of sneez- 

 ing or coughing. 



Though the action of the respiratory centre can be modified 

 by (1) the will and by (2) various peripheral stimulations, and 

 is habitually regulated from the periphery through the (3) vagi 

 by the state of the lungs, the condition of the blood supplied to 



