146 THE VASCULAK MECHANISM. 



tinually increasing distention of the tube on the near side of the resistance, 

 will be sufficient to drive through the resistance, between each two strokes, 

 just as much fluid as enters the near end of the system as each stroke. In 

 other words, the elastic reaction of the walls of the tube will have converted 

 the intermittent into a continuous flow. The flow on the far side of the 

 resistance is in this case not the direct result of the strokes of the pump. All 

 the force of the pump is spent, first in getting up, and afterward in keeping 

 up, the distention of the tube on the near side of the resistance ; the imme- 

 diate cause of the continuous flow lies in the distention of the tube which 

 leads it to empty itself into the far side of the resistance at such a rate that 

 it discharges through the resistance during a stroke and in the succeeding 

 interval just as much as it receives from the pump by the stroke itself. 



This is exactly what takes place in the vascular system. The friction in 

 the minute arteries and capillaries presents a considerable resistance to the 

 flow of blood through them into the small veins. In consequence of this 

 resistance the force of the heart's beat is spent in maintaining the whole of 

 the arterial system in a state of great distention ; the arterial walls are put 

 greatly on the stretch by the pressure of the blood thrust into them by the 

 repeated strokes of the heart ; this is the pressure which we spoke of above 

 as blood-pressure. The greatly distended arterial system is, by the elastic 

 reaction of its elastic walls, continually tending to empty itself by overflow- 

 ing through the capillaries into the venous system ; and it overflows at such 

 a rate that just as much blood passes from the arteries to the veins during 

 each systole and its succeeding diastole as enters the aorta at each systole. 



109. Indeed, the important facts of the circulation which we have not 

 as yet studied may be roughly but successfully imitated on an artificial model, 

 Fig. 42, in which an elastic syringe represents the heart, a long piece of 

 elastic India-rubber tubing the arteries, another piece of tubing the veins, 

 and a number of smaller connecting pieces the minute arteries and capil- 

 laries. If these connecting pieces be made at first somewhat wide, so as to 

 offer no great resistance to the flow from the artificial arteries to the artificial 

 veins, but be so arranged that they may be made narrow by the screwing-up 

 of clamps or otherwise, it is possible to illustrate the behavior of the vascular 

 mechanism when the peripheral resistance is less than usual (and as we shall 

 see later on it is possible in the living organism either to reduce or to increase 

 what may be considered as the normal peripheral resistance), and to compare 

 that behavior with the behavior of the mechanism when the peripheral 

 resistance is increased. 



The whole apparatus being placed flat on a table, so as to avoid differences 

 in level in different parts of it, and filled with water, but so as not to distend 

 the tubing, the two manometers attached, one (A) to the arterial side of the 

 tubing and the other (V) to the venous side, ought to show the mercury 

 standing at equal heights in both limbs of both instruments, since nothing 

 but the pressure of the atmosphere is bearing on the fluid in the tubes, and 

 that equally all over. 



If, now, the connecting pieces being freely open, that is to say, the periph- 

 eral resistance being very little, we imitate a ventricular beat by the stroke of 

 the pump, we shall observe the following : Almost immediately after the stroke 

 the mercury in the arterial manometer will rise, but will at once fall again, 

 and very shortly afterward the mercury in the venous tube will in a similar 

 manner rise and fall. If we repeat the strokes with a not too rapid rhythm, 

 each stroke having the same force, and make, as may by a simple contrivance 

 be effected, the two manometers write on the same recording surface, We shall 

 obtain curves like those of Fig. 43, A and V. At each stroke of the pump 

 the mercury in the arterial manometers rises, but forthwith falls again to or 



