292 



MANUAL OF PHYSIOLOGY. 



FIG. 129. 



R.H. 



L.H, 



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 

 the key to the most important 

 dynamic principles of the circula- 

 tion. 



The cause of the blood's motion 

 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 



Diagram of halation, showing represented by the large elastic 

 right (R.H) and left (r>. H.) hearts, and arteries, which can be more or less 



distended, according as (1) the 

 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. 



When a small artery is cut across and the orifice directed up- 

 ward, the blood is thrown two or three feet in jerks. When a 

 vein is cut, the blood only trickles gently from its orifice, the 



