216 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. 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 consider- 

 able 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 re- 

 peated 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 diastole 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. 31, 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 



