THE CIRCULATION OF THE BLOOD. 327 



The pressure throughout the system is the result of the resistance to the flow of 

 the water 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 

 distention of the tube. This latter effect continues until the elastic recoil of the 

 walls of the tube exactly counterbalances the pressure of the water from within. 

 When this condition is established the tube becomes practically a tube with rigid 

 walls, and hence so long as the primary pressure is uniform, the velocity and lateral 

 pressure will obey the laws which hold true for rigid tubes. 



If, however, the primary pressure be intermittently applied or alternately 

 increased or decreased, and the water forced into the tube, previously filled with 

 water but under no particular pressure, it will be forced out of the peripheral end 

 of the tube more rapidly during the period of the increase of pressure and 

 less rapidly during the period of the decrease of pressure or it may cease entirely. 

 The extent to which the outflow becomes merely remittent, or entirely intermittent, 

 will depend on the amount of resistance, whether this be due to length of tube or 

 a narrowed outlet, and the degree of elasticity. 



When these factors are of such a nature that the resistance is very high and the 

 elasticity slight, the outflow will be intermittent. But if they are made to change 

 gradually, and this is especially the case with the resistance, from a slight to a 

 greater value, the outflow gradually changes from an intermittent to a remittent 

 and finally to a continuous outflow and for the following reasons: 



