THE CIRCULATION OF THE BLOOD 337 



increase in the resistance due to the progressive decrease in the size of the individual 

 tubes preponderates considerably over the decrease in the resistance due to the 

 widening of the stream bed, there must be an increase in resistance in the area 

 B-C and therefore a more rapid fall of pressure than inA-B. This fall, however, 

 will not be as steep as it might be for the reason that the decrease in the velocity 

 is attended by a decrease in the resistance and hence a lessened consumption of 

 the propelling power. In the section C-D the two factors, viz.: the narrowing of 

 the stream bed which increases the resistance, and the enlarging of the individual 

 tubes which decreases the resistance, exert an opposing influence on the pressure, 

 hence the fall of pressure will be proportional to the ratio between these two factors. 

 As the decrease in the resistance due to the progressive enlargement of the individual 

 tubes preponderates considerably over the increase in the resistance due to the 

 narrowing of the stream bed, there should theoretically be a rapid fall of pressure 

 from c to E. This rapid fall, however, will be to some extent prevented for the 

 reason that the increase in velocity due to the narrowing of the stream bed in- 

 creases the resistance to a high value and hence the pressure falls less rapidly than 

 it otherwise would. 



The pressure throughout the system is the result of (i) the downward pressure 

 of the water in the reservoir, and (2) the resistance to its flow, due to the cohesion 

 and friction of its molecules and its adhesion to the sides of the tubes, and its 

 extent in any one section will be proportional to the resistance yet to be overcome. 

 It will naturally be higher in the section A-B than in the section D-E, though the 

 difference in the level of the pressure between these two points will not be as great 

 as might theoretically be supposed from the small size of the tubes in c for the 

 decrease in velocity counterbalances in part the resistance which they offer. 



The general curve of the fall of pressure in this system is indicted by the 

 curved line extending from the pressure vessel to the outlet of the horizontal tube. 



The value of the pressures in these two sections and their relation to each other 

 could be varied either temporarily or permanently by the insertion of a series of stop- 

 cocks a, x, along the course of the tubes between B and c in the neighborhood of 

 their ultimate branchings by which an additional resistance could be superposed 

 on the system from A to the stopcocks. If the lumen of each stopcock has a certain 

 average value, so as to permit of a certain outflow of water, the pressure will 

 have a certain value in both A-B and D-E. But if the lumen of each stopcock is 

 decreased, there will be an increase in the resistance and hence a rise of pressure in 

 A-B and a fall of pressure in D-E. If, on the contrary, the lumen of each stopcock is 

 increased, there will be a decrease in the resistance and hence a fall of pressure 

 in A-B and a rise of pressure in D-E. The stopcocks may be spoken of as a 

 variable peripheral resistance. 



In the foregoing exposition it has been assumed that in all instances the pressure 

 in the pressure vessel was steadily acting. If, however, the pressure be made to 

 act intermittently as it can be by alternately opening and closing the stopcock, at A 

 both the velocity and the pressure will be alternately increased and decreased. 

 The outflow of the fluid during the moment the pressure is acting will be rapid, 

 and during the moment the pressure is not acting the outflow will cease. It 

 becomes therefore intermittent. Coincidently there is an alternate temporary 

 increase and decrease of the lateral pressure. 



THE FLOW OF A LIQUID THROUGH A TUBE WITH ELASTIC WALLS 

 UNDER AN INTERMITTENTLY ACTING PRESSURE 



When a tube with elastic walls is connected with a pressure vessel, the con- 

 ditions which are established on opening the stopcock and the consequent flow of 

 water, will soon approximate those observed in a tube with rigid walls. As the 

 water moves forward, it encounters friction, exerts a lateral pressure and causes a 



22 



