286 MANUAL OF PHYSIOLOGY. 



stream, and will not follow the jerks communicated by the pump. 

 That is to say, the interrupted energy of the pump is stored up 

 by the elastic tube and converted into a continuous pressure ex- 

 erted on the fluid. But if the tube be quite rigid, or the orifice 

 too wide to allow the pressure within the tube to be raised suffi- 

 ciently high, then the fluid will flow out of the end of the tube in 

 jets which correspond with the strokes of the pump ; that is to say, 

 the outflow will follow closely the pressure-difference caused by 

 the pump at the point of inflow. 



Now these simple facts (which can be verified experimentally 

 with an ordinary enema bag, a yard of elastic tubing, and a short 



glass tube drawn to a point) form 

 FIG. 129. the key to the most important 



dynamic principles of the circu- 

 lation. 



The cause of the blood's motion 

 L.H. is simply a difference in the pres- 

 sure within the various parts of 

 the vascular system, for the heart 

 acts as the pump filling the tube 



represented by the large elastic 

 Diagram ol Circulation, show- . , 



ing right (R.H.) and left (L.H.) artenes > whlch can be more or less 

 hearts, and the pulmonary (p) and distended, according as (1) the 

 systemic (s) sets of capillaries. outflow is impeded or facilitated 



by the contraction or relaxation of 



the muscular arterioles which form the outlet, or as (2) the inflow 

 is increased or diminished by the greater or less activity of the 

 heart's action. 



From the foregoing facts, and what has been said of the direc- 

 tion of the blood current, namely, that it flows from the arteries 

 through the capillaries into the veins, it would then appear that 

 the pressure in the arteries exceeds that in the capillaries, and 

 the pressure in the capillaries must in turn be greater than that 

 in the veins, the blood flowing in the direction in which the pres- 

 sure becomes less. 



The difference in the manner in which the blood flows from a 

 cut artery and a cut vein shows that a great difference exists in 

 the pressure within the two sets of vessels. 



