THE CIRCULATION OF THE BLOOD 363 



at branchings and to internal friction, it has to overcome in finish- 

 ing its round, the pressure on the blood diminishes as we follow 

 it from the aorta to the venae cavae. In the larger arteries the fall 

 of pressure is gradual and small, since the amount of resistance 

 met with in the flow through them is but little. In the small 

 arteries and capillaries the resistance overcome and left behind 

 is (on account of the great internal friction due to their small 

 caliber) very great, and consequently the fall of pressure. between 

 the medium-sized arteries and the veins is rapid and considerable. 

 Modifications of Arterial Pressure by Changes in the Heart-beat. 

 A little consideration will make it clear that the pressure prevail- 

 ing at any time in a given artery depends on two things the rate 

 at which the vessel is filled, i. e., upon the amount of work done 

 by the heart; and the ease or difficulty with which it is emptied, 

 that is, upon the resistance in front. A third factor has to be 

 taken into account in some cases; namely, that when the muscular 

 coats of the small arteries contract the local capacity of the vas- 

 cular system is diminished, and has to be compensated for by 

 greater distention elsewhere, and vice versa. This would of itself 

 of course bring about changes in the pressure exerted on the 

 contained liquid, but for the present it may be left out of con- 

 sideration. If we suppose a system such as represented in Fig. 110, 

 to be in equilibrium, with the pump injecting into B a certain 

 volume of liquid per minute, and the elastic tension of the tube B 

 just sufficient to force that volume through the resistance D' in 

 the same time, it is clear that the pressure indicated on the 

 gauge x will be very nearly constant. If, now, the volume of 

 liquid forced into B in a minute be increased, either by the pump 

 working faster or by its pumping more at each stroke, there will 

 evidently be an accumulation in B, since its tension is adjusted 

 to force out the less volume per minute, but this accumulation, 

 by stretching the tube still more, increases its elastic tension, so 

 that this is presently great enough to force out the added volume 

 as fast as it comes in. The pressure-gauge will now stand at a 

 higher point, showing that the contents of the tube are under 

 greater pressure than before. Similarly, a diminution in the in- 

 flux of liquid into B will be followed by a fall of pressure within 

 it as the walls of the tube adjust themselves to the smaller volume 

 to be forced out per minute. Precisely the same reasoning may 



