214 ARTIFICIAL MODEL. [BOOK i. 



just as much fluid as enters the near end of the system at 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 afterwards in keeping up, 

 the distension of the tube on the near side of the resistance ; the 

 immediate cause of the continuous flow lies in the distension 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 distension ; 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 

 overflowing 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 dia- 

 stole as enters the aorta at each systole. 



120. Indeed the important facts of the circulation which 

 we have as yet studied may be roughly but successfully imitated 

 on an artificial model, Fig. 30, in which an elastic syringe repre- 

 sents the heart, a long piece of elastic indiarubber tubing the 

 arteries, another piece of tubing the veins, and a number of 

 smaller connecting pieces the minute arteries and capillaries. 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 behaviour of the vascular mechanism when the peri- 

 pheral 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 behaviour with the behaviour 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 shew the mercury standing at equal heights in both limbs 

 of both instruments, since nothing but the pressure of the atmo- 



