352 



TEXT-BOOK OF PHYSIOLOGY 



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. 



i. If when the general arterial pres- 

 sure 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. 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. 



The decrease in the rate and force of 



the heart may be brought about suddenly 



, j 11 t_ j- , 



or more or less gradually by direct stimu- 



lation of the inhibitor fibers in the 1 trunk 



FIG. 162. A TRACING OF THE BLOOD- 

 PRESSURE IN THE CAROTID ARTERY OF A 

 RABBIT, showing a sudden decrease in 



the pressure due to-an^arres^m the rate of the yagus ner ve with Strong Or With 

 - & & 



- . , . ,. , /T ^. 



stimulating the vagus nerve from "on" weak electric- currents respectively (Fig. 



to " off. 



162) ; or, by a stimulation of the cardio- 

 inhibitor center by nerve impulses trans- 

 mitted to it through certain afferent 

 nerves, e.g., the afferent fibers in the 

 vagus,. when stimulated with induced electric currents (Fig. 163). 



2. If the peripheral resistance is diminished by a dilatation of the arteri- 



with, the cessation of .the stknu- 



ofTe LtUVi! 

 turned. (The abscissa should be 20 mm. 

 l wer -) 



i me ~ StfCt 



Abscissa 



FIG. 163. TRACING SHOWING REFLEX INHIBITION OF THE HEART AND FALL OF BLOOD-PRESS- 

 URE IN THE CAT, following stimulation of the central end of the vagus nerve. 



oles, the heart's contractions remaining the same, the outflow of blood at 

 once increases and the existing pressure soon diminishes. 



As a rule a diminution in peripheral resistance is attended by an increase 



