292 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXI. 



arterial pressure it is increased by plethora, diminished by anaemia. 

 It is increased by a decrease in the rate and force of the heart, and 

 by a dilatation of the arterioles. It is diminished by the opposites. 

 It is quite easy to understand how this is ; when the heart 

 beats with increased force, it naturally raises the pressure in 

 the arteries ; but an increase during systole in the force of 

 propulsion into the arteries means an increase also during dia- 

 stole in the force of suction upon the venous blood, that is, a 

 reduction of the pressure there ; it becomes more negative than 

 it usually is. 



A] 



Fig. 281. Effect of weak stimulation of the peripheral end of vagus on arterial blood- 

 pressure (carotid of rabbit). BP, blood-pressure ; A, abscissa or base line ; T, time in 

 seconds. Note fall of blood-pressure and slow heart-beats. 



With regard to the arterioles, contraction in the arterioles 

 means a rise in pressure in the arteries, just as narrowing the 

 doors of a theatre during the exit of the audience will increase 

 the pressure behind the doors ; but a contraction of the arterioles 

 causes a fall in pressure in the capillaries and veins beyond 

 them, just as the narrowing of the theatre doors will lessen 

 the congestion in the street outside of them. 



Capillary pressure is increased by 



1. Dilatation of the arterioles; the blood-pressure of the large 

 arteries is then more readily propagated into them. 



2. The size of the arterioles remaining the same, increase of 



