THE CIRCULATION. 177 



Tlic Pneumogastric or Cardio -inhibitory Nerves. 



The larger nerves, ending probably in the intrinsic 

 ganglia themselves, exercise a peculiar and interesting con- 

 trol over the beat of the hear,t. When they are stimulated 

 they cause the beat to be slowed, and if sufficiently stimu- 

 lated may cause the heart to stop beating altogether. For 

 this reason they are called inhibitory nerves. When these 

 inhibitory nerves by sufficient stimulation cause the heart 

 to stand still the heart stands still in diastole, perfectly re- 

 laxed, and not contracted in a systole. These cardio-inhib- 

 itory nerves not only slow the rate of the beat, but they 

 make the individual beats weaker. The result of this ac- 

 tion is, of course, to reduce the blood pressure in the arter- 

 ies, because the heart no longer forces the blood into these 

 arteries at the accustomed rate. These nerves seem to be 

 acting all the time in our bodies and thus continually keep- 

 ing the rate of the heart beat in check. For when both 

 nerves are cut the heart at once begins to beat faster, be- 

 cause the inhibitory action of the vagi has been removed. 



Inhibitory Center. 



It is, of course, not these nerves themselves that are in- 

 hibitory, for nerves are but the avenues of impulses trans- 

 mitted to them. These inhibitory impulses originate in a 

 center which lies in the medulla, called the cardio-inhibitory 

 center. From this center the inhibitory fibres of the vagi 

 run to the heart, and it is of course this center which is in 

 a continual state of excitation and so continually exercising 

 a checking control of the heart. 



This cardio-inhibitory center may be stimulated to in- 

 creased action in several ways. First, increased blood 

 pressure in the cranium causes an increased inhibitory 

 action. This may be shown by boring a little hole in the 

 cranium of an animal and then forcing into the cranium 

 some non-irritating liquid. A slowing of the heart beat at 

 once follows. This also explains why in apoplexy the pulse 

 sinks so rapidly. (Apoplexy is due to the bursting of blood 

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