RELATION OF BLOOD-PRESSURE TO PULSE-RATE. \2J 



sets of vessels. This experiment shows that, even when the difference of pressure 

 is almost entirely set aside, the passive blood presses upon the arterial walls, i.e., 

 on account of the overfilling of the blood-vessels a slight pressure is exerted upon 

 the walls, even when there is no circulation. [As already stated, the arterial 

 pressure depends on the condition of the central organ the heart and on the 

 peripheral organs the small arteries. If the action of the heart be arrested, then 

 the blood-pressure rapidly falls. Fig. 108 shows the effect on the blood-pressure 



Fig. 108. 

 Blood-pressure tracing taken with a mercurial kymograph from the carotid of a rabbit. 

 o - x, abscissa ; stimulation of vagus begun at a and stopped at b. 



of arresting the action of the heart by stimulation of the peripheral end of the 

 vagus. There is a sudden fall of the arterial pressure, as shown|by the rapid fall 

 of the curve from a.] 



[Variations in Animals. The pressure in the arterial system depends upon the balance 

 between the inflow and outflow, i.e., upon the heart and the state of the arterioles. But it is 

 to be noted that the central factor, the heart, varies in different animals. In the rabbit the 

 heart normally beats rapidly, so that section of the vagi does not cause any great increase in 

 the number of beats, nor is the blood-pressure much raised thereby. In the dog, on the other 

 hand, the beats are considerably increased by section of the vagi, while the blood-pressure 

 rises considerably. Atropin paralyses the cardiac terminations of the vagus, and thereby trebles 

 the number of heart-beats in the dog, while it only raises it 25 per cent, in the rabbit ; in man, 

 again, the number may be doubled. As Brunton has shown, this difference of the initial 

 number of heart-beats and the action of the vagus have important relations to the action of 

 drugs on the blood-pressure. For example, if an intact rabbit be caused to inhale amyl nitrite, 

 the blood-pressure falls at once and rapidly, while in the dog the fall may be slight. The pulse 

 of the dog, however, is greatly accelerated, so much so as to be nearly as rapid as that of the 

 rabbit. In both, the vessels are dilated, but in the dog, notwithstanding this dilatation, which 

 per se would cause the pressure to fall, the heart of the dog beats now so rapidly as to com- 

 pensate for this, and thus keeps the blood-pressure nearly normal ; while the increased rate of 

 beating in the rabbit is not sufficient for this purpose,* If the vagi in the dog be divided, the 

 subsequent inhalation of amyl nitrite causes a fall of blood-pressure like that in the rabbit 

 (Brunton).] 



[Relation of Blood-Pressure to Pulse-Rate. When the blood-pressure rises in 

 an intact animal, as a rule the pulse-rate falls, owing to stimulation of the vagus 

 centre increasing the cardio-inhibitory action, while a fall of blood-pressure is 

 accompanied by an increase of the number of pulse-beats for the opposite reason, 

 the action of ^ the medullary cardio-inhibitory centre being increased. But the 



