CENTRES OF MEDULLA OBLONGATA. 631 



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

 centre. 



Although we take the respiratory 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 

 restrain 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 

 the centre may be such that these remoter influences may become 

 quite powerless. This uncontrollable condition of the centre is 

 established when the blood flowing through it is abnormally 

 venous and the cells become over-stimulated. We all know how 

 short a time we can hold our breath by voluntary checking of the 

 centre, and most people have had occasion to observe the inordi- 



