36 A MANUAL OF VETERINARY PHYSIOLOGY 



resistance, the balance imparts to the fluid a certain velocity, 

 and the remaining pressure is known as the pressure velocity. 



The whole mechanics of the circulation can be worked out on 

 a model consisting of a syringe to represent the heart, elastic 

 tubes to represent the bloodvessels, and a few clamps to offer 

 the needful peripheral resistance. With such a model, if water 

 be forced into the arterial tubes, the clamps being open and the 

 peripheral resistance therefore very small, it is found, by means 

 of a manometer, that the pressure in the arterial tube rises with 

 each stroke of the syringe, and falls with the free pouring of the 

 contents into the tubes representing the veins. As the peripheral 

 resistance is small, the pulsation set up in the fluid readily passes 

 into the veins, and a manometer will here register nearly the 

 same rise and fall as was met with in the arteries. 



If, however, the vessels be clamped so as to produce a resist- 

 ance, the first stroke of the pump causes the arteries to become 

 distended ; they then recoil, and while undergoing this they 

 receive another stroke from the pump and become still more 

 distended. Once more they recoil on their contents, and are 

 once more distended by the action of the pump, and so on. If 

 during this time the arterial manometer be watched, it will be 

 observed that the mercury or water rises with each stroke of the 

 pump, but instead of falling at once to zero as it did in the un- 

 damped tube, it only has time to fall a short distance before a 

 second stroke of the pump sends it still higher than before ; this 

 is repeated at every stroke of the pump until the water or mercury 

 refuses to rise any higher in the tube, contenting itself by rising 

 to a certain height at each stroke of the pump, and falling to a 

 certain level during the interval between one stroke and another. 

 In other words, a mean pressure has been established in the tubes 

 representing the arteries, which has been brought about by the 

 peripheral resistance, the elastic recoil of the tube, and the 

 pumping of the syringe. So long as these factors remain the 

 same the mean pressure will not vary. If, however, the clamped 

 vessels be released, so as to allow fluid to flow more easily into 

 the tubes representing the veins, the manometer at once shows 

 a fall in the mean pressure owing to the removal of a certain 

 amount of resistance, and by removing this resistance completely 

 the mean pressure falls to zero. The mean pressure, then, repre- 

 sents the force which is necessary to cause as much fluid to pass 

 through the periphery as is being pumped into the system of 

 tubes by the syringe ; if the peripheral resistance is high the 

 pressure is high, and vice versa. 



A careful study of this experiment places us in complete pos- 

 session of the main facts of the circulation, but even up to this 

 point we have not learnt all the lessons it is capable of teaching. 



