CABDIAC INHIBITION. 251 



This is shown by the fact that if both pneumogastric nerves 

 be cut in the neck the heart at once begins to beat a little 

 faster than before; the brake, so to speak, has been taken 

 off it. 



The Influence upon Arterial Pressure of Inhibiting 

 the Heart. If the heart be entirely stopped arterial pres- 

 sure will of course fall very rapidly, since the distended 

 arterial system will go on emptying itself through the capil- 

 laries into the veins, without receiving any fresh supply at 

 its cardiac end. So too if the heart be made to beat slower, 

 but with the same force in each stroke, it follows from the 

 facts pointed out in the last chapter (p. 238) that arterial 

 pressure will fall to a new and lower level, at which the 

 elastic arteries are only stretched enough to squeeze out in 

 a minute as much as they receive. As a matter of fact, 

 when the heart is made to beat slower by weak pneumo- 

 gastric stimulation each beat is usually a little more power- 

 ful than before. However, this extra force is usually not 

 sufficient to compensate entirely for the slower rate and so 

 the general arterial pressure falls. 



Use of the Cardio-Inhibitory Mechanism.. Although 

 the cardio-inhibitory centre is automatic and always in a 

 state of slight activity it is also greatly under the control of 

 afferent nerve-fibres reaching it which can arouse it to a 

 much greater activity, and so reflexly control the heart's 

 beat. If a frog be rendered insensible and its abdominal 

 cavity opened, it will be found that one or two smart taps on 

 the intestine will cause the heart to stop in diastole. If, 

 however, the pneumogastric nerves, or the spinal cord, or 

 the posterior roots of the spinal nerves, or the communicating 

 branches between the sympathetic nerves of the abdomen 

 and the spinal nerves, be previously cut, then striking the 

 intestine has no influence upon the heart; nor has it if the 

 cardio-inhibitory centre in the medulla oblongata be pre- 

 viously destroyed. We thus get evidence that the mechani- 

 cal stimulation of the intestinal nerves stops the heart re- 

 flexly through the < pnumogflg^kis r -t^ie afferent impulses 

 traveling from the sympathetic into the spinal nerves and 

 passing then up the spinal cord to the cardio-inhibitory 



