THE CIRCULATION OF THE BLOOD 35I 



experimentally brought about by an increase in the activity of the general 

 vaso-motor center due to the arrival of nerve impulses transmitted to it through 

 certain afferent nerves as when after division of the sciatic nerve in a curarized 

 animal the central end is stimulated with induced electric currents (Fig. 160), 

 or by injecting into the blood, adrenalin, which it is generally believed acts 

 on the muscle fibers of the vessel walls. Fig. 161 Physiologic causes may act 



FIG. 1 60. A TRACING SHOWING AN INCREASE IN THE BLOOD-PRESSURE IN THE CAROTID 

 ARTERY OF A RABBIT DUE TO AN INCREASE IN THE PERIPHERAL RESISTANCE FROM A CONTRACTION 

 OF THE ARTERIOLES CAUSED BY REFLEX STIMULATION OF THE VASO-MOTQR CENTER. The nerve 

 stimulated was the sciatic. Stimulation began at s. The rate of the heart-beat is unchanged. 

 With the cessation of the stimulation the blood-pressure falls for the reverse reasons. 



in the same way and reflexly cause arteriole contraction. A rise of 

 pressure from this cause much beyond the normal is to a large extent 

 prevented under physiologic conditions by a simultaneous decrease in the 

 rate and force of the heart-beat. This is due to a stimulation of the peripheral 

 ends of the depressor nerve, and a consequent reflex stimulation of the cardio- 

 inhibitor center, and not to a direct action on the heart-muscle, inasmuch 

 as the effect is not observed after division of the vagi. 



FIG. 1 6 1. TRACING SHOWING THE RISE OF BLOOD PRESSURE IN A CAT after the intra- 

 venous injection of minute doses of adrenalin injection at x. The abs 

 millimeters lower. 



3. When both the force of the heart and the peripheral resistance are sim- 

 ultaneously increased there is a rapid increase in pressure, the former fact* 

 tends to increase, the latter factor, to decrease, the velocity of the outflow. 

 According as the one or the other preponderates, will there be 

 crease or decrease in velocity. If they balance each other, ther 



