THE REGULATING MECHANISM 201 



sensory filaments, and also by the quality of 

 the blood flowing through them. They are 

 not automatic. 



113. The same is true of the other centres 

 in the bulb. Thus the cardiac centres, acting 

 downwards through the vagi, tend to inhibit or 

 restrain the contractions of the heart, as already 

 explained, while fibres that find their way from 

 the cord into the sympathetic have an accele- 

 rating action (p. 187). We may therefore 

 assume the existence in the bulb of inhibitory 

 and accelerating cardiac centres. These again 

 are influenced by peripheral impulses. From 

 the heart impulses may pass to the bulb centres 

 and then to the cerebral centres and from them 

 again downwards to the centres in the bulb. 

 Impulses may also reach the cardiac centres 

 along any sensory nerve, and in this way there 

 may be a degree of inhibition or acceleration, 

 or impulses may come from the seat of con- 

 scious emotion in the cerebral centres and 

 terror may cause the heart momentarily to 

 miss its beats. 



114. In a similar way the vaso-motor centres 

 in the bulb may be influenced. The action of 



