THE CIRCULATION OF THE BLOOD. 



341 



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 stimu- 

 lation of the cardio-inhibitor center, and not to a direct action on the heart- 



FIG. 158. 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-MOTOR 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. 



muscle, inasmuch as the effect is not observed after division of the vagi. 

 When both the force of the heart and the peripheral resistance are simul- 

 taneously increased there is a rapid increase in pressure; the former factor 

 tends to increase, the latter factor, to 

 decrease, the velocity of the outflow. 

 According as the one or the other pre- 

 ponderates, will there be an increase 

 or decrease in velocity. If they balance 

 each other, there will be no change. A 

 rise of pressure from a combination of 

 these factors is rather a pathologic than 

 a physiologic condition and is observed in 

 certain diseases of the vascular apparatus. 

 The converse of these statements also 

 holds true. If when the general arterial 

 pressure is in a condition of equilibrium 

 the heart ejects into the arteries in a given 

 period of time a lessened quantity of 

 blood, either as a result of a decrease in 

 the rate or force, there will soon be a 

 diminution of the arterial distention and 

 a consequent fall in pressure (Fig. 159). 

 The velocity at the same time diminishes. 

 This continues until the outflow no longer 

 exceeds the inflow. Equilibrium will 

 again be established, but the pressure 

 will be at a lower level. 



If the peripheral resistance is 1 dimin- 

 ished by a dilatation of the arterioles, the 

 heart's contractions remaining the same, 

 the existing pressure soon diminishes, 

 increases. 



FIG. 159. A TRACING OF THE BLOOD- 

 PRESSURE IN THE CAROTID ARTERY OF A 

 RABBIT, showing a sudden decrease in 

 the pressure due to an arrest in the rate 

 and force of the heart-beat the result of 

 stimulating the vagus nerve from " on " 

 to "off." With the cessation of the stimu- 

 lation the pressure began to rise as the 

 rate and the force of the heart-beat re- 

 turned. (The abscissa should be 20 mm. 

 lower.) 



The outflow of blood "at once 



