270 THE HUMAN BODY. 



the cardio-inhibitory centre through sensory nerves, and serve 

 to illustrate the many afferent fibres from different regions of 

 the Body which can influence the heart-beat. 



The cardio-inhibitory centre may also be stimulated 

 directly (as by piercing it with a needle) and stop the heart. 

 But a more interesting instance is its excitation by high 

 arterial pressure. Nearly always a very high pressure in the 

 aorta is accompanied by a slow pulse due to cardio-inhibitory 

 nerve-impulses, for if the vagi be cut under such circum- 

 stances the heart-rate immediately increases. The slower 

 beat, of course, by lessening the work of the heart tends to 

 bring back the high arterial pressure to a more normal level, 

 providing an adjustment of the heart's work to the condition 

 of the arterial system at the time. The brain, enclosed in 

 the rigid skull-cavity, is especially likely to be affected by 

 increased arterial tension, for distension of the intra-cranial 

 arteries must bring about greater pressure on all the other 

 contents of the skull; and the cardio-inhibitory centre is very 

 sensitive to increased pressure. If a small hole be bored 

 through the skull of a dog and a little innocuous fluid in- 

 jected so as to cause pressure on the brain, the beat of the 

 heart is promptly slowed and weakened, but if the pneumo- 

 gastrics have been previously cut the heart-beat is not 

 influenced. In man similar stimulation of the cardio-inhibi- 

 tory centre is shown in apoplexy, which is due to the bursting 

 of some vessel inside the skull and the effusion of blood, which 

 by pressure on the brain causes the unconsciousness and pa- 

 ralysis which characterize the stroke. During such a fit the 

 pulse is almost invariably very slow from the action of the 

 increased pressure on the cardio-inhibitory cells. This is 

 clearly a preservative action, for the resulting lower arterial 

 pressure makes the haemorrhage less, and more likely to come 

 to an end. Among conditions of the blood which stimulate 

 the cardio-inhibitory apparatus may be mentioned deficient 

 oxygenation, which will be referred to again when the phe- 

 nomena of suffocation are described. 



The Cardio-accelerator or Augmentor Nerves. The 

 influence of these on the heart is to quicken or strengthen 

 its beat or both: but only for a time, their final action being 

 to hasten exhaustion; they are essentially katabolic in their 

 influence on the nutrition of the organ. 



Both in frog and mammal they pass to the heart from 



