164 HYDRAULIC PRINCIPLES. [BOOK i. 



intermittent force (like that of the systole of the ventricle) working 

 in a closed circuit of branching tubes so arranged that, while the 

 individual tubes first diminish in calibre (from the heart to the 

 capillaries) and then increase (from the capillaries to the heart), 

 the area of the bed first increases and then diminishes, the tubes 

 together thus forming two cones placed base to base at the capil- 

 laries, with their apices converging to the heart, and presenting 

 at their conjoined bases a conspicuous peripheral resistance, the 

 tubing on one side, the arterial, being eminently elastic, and on 

 the other, the venous, affording a free and easy passage for the 

 blood. It is the peripheral resistance (for the resistance offered 

 by the friction in the larger vessels may, when compared with 

 this, be practically neglected), reacting through the elastic walls 

 of the arteries upon the intermittent force of the heart, which 

 gives the circulation of the blood its peculiar features. 



101. Circum stances determining the character of the flow. 

 When fluid is driven by an intermittent force, as by a pump, 

 through a perfectly rigid tube, such as a glass one (or a system of 

 such tubes), there escapes at each stroke of the pump from the 

 distal end of the tube (or system of tubes) just as much fluid as 

 enters it at the proximal end. What happens is very like what 

 would happen if, with a wide glass tube completely filled with 

 billiard balls lying in a row, an additional ball were pushed in at 

 one end ; each ball would be pushed on in turn a stage further, 

 and the last ball at the further end would tumble out. The 

 escape, moreover, takes place at the same time as the entrance. 



This result remains the same when any resistance to the flow is 

 introduced into the tube, as, for instance, when the end of the tube 

 is narrowed. The force of the pump remaining the same, the 

 introduction of the resistance undoubtedly lessens the quantity 

 of fluid issuing at the distal end at each stroke, but it at the 

 same time lessens the quantity entering at the proximal end ; 

 the inflow and outflow remain equal to each other, and still occur 

 at the same time. 



In an elastic tube, such as an india rubber one (or in a system 

 of such tubes), whose sectional area is sufficiently great to offer 

 but little resistance to the progress of the fluid, the flow caused 

 by an intermittent force is also intermittent. The outflow being 

 nearly as easy as the inflow, the elasticity of the walls of 

 the tube is scarcely at all called into play. The tube behaves 

 practically like a rigid tube. When, however, sufficient resistance 

 is introduced into any part of the course, the fluid, being unable 

 to pass by the resistance as rapidly as it enters the tube from 

 the pump, tends to accumulate on the proximal side of the re- 

 sistance. This it is able to do by expanding the elastic walls of 

 the tube. At each stroke of the pump a certain quantity of fluid 

 enters the tube at the proximal end. Of this only a fraction can 

 pass through the resistance during the stroke. At the moment when 



