PHYSICAL FORCES OF THE CIRCULATION. 285 



and valves to prevent its backward flow upon the 

 capillaries. 



PHYSICAL FORCES OF THE CIRCULATION. 



When liquid flows through a tube it does so as the result of a 

 difference of pressure in the different parts of the tube ; the liquid 

 moving from the part where the pressure is higher towards that 

 where it is lower. 



The energy of the flow corresponds with the amount of differ- 

 ence in the pressure, and varies exactly with the pressure-differ- 

 ence, being continuous so long as the pressure is unequal in dif- 

 ferent parts, and being interrupted when the pressure is equalized 

 throughout the tube. If liquid be forcibly pumped into one ex- 

 tremity of a long tube, such as a garden hose, a pressure-differ- 

 ence is of course established, the pressure becoming greater at 

 the end into which the liquid is pumped, consequently a current 

 takes place towards the open end. So long as the free or distal 

 end of the tube is quite open and on the same level as the rest, 

 no very great increase of pressure can be brought to bear on the 

 walls of the tube, no matter how forcibly the pumping may go 

 on, as the liquid easily escapes, and therefore flows out all the 

 more quickly as the pumping becomes more energetic. If, how- 

 ever, the outflow be impeded by raising the distal end of the 

 tube to any considerable height, or by partially closing the orifice 

 with a nozzle or rose, then the pressure within the tube can be 

 greatly increased by energetic pumping, and the tube being elastic 

 will swell when distended by the pressure. 



It can be further observed in this common operation that the 

 smaller the orifice of the nozzle be, the greater the pressure in 

 the tube with a given rate of working the pump ; and, the orifice 

 remaining the same, the pressure will increase in proportion as 

 the pump is more energetically worked. Or in other words, the 

 pressure within the tube will depend on (a) the force used at the 

 pump, and (6) the degree of impediment offered to the outflow. 



If the tube be resilient, and if the nozzle have a small orifice 

 so that a high pressure can be established within the tube, it will 

 be found that the liquid will flow from the nozzle in a continuous 



