72 HUMAN PHYSIOLOGY 



up energy is then gradually transformed into the energy of 

 the moving blood. This causes the flow to be continuous. 



The change of the intermittent into the continuous movement 

 occurs, according to the same principle, in the fire-engine. The 

 water which enters the engine periodically, leaves it in a contin- 

 uous stream, being pressed out continuously by the compressed 

 air in the air-chamber. 



The average velocity decreases as we proceed from the 

 arteries to the capillaries, and increases again from the capil- 

 laries to the veins. In the large arteries the rate is 200-400 

 mm per second; in capillaries 0.6-0.8 mm; in the large 

 veins it is but little less than in the arteries. The cause of 

 this difference is the difference in the total cross-section of 

 the various parts of the vessels. Through each total cross- 

 section of the vascular system there must pass in the same 

 unit of time the same quantity of fluid, in order that the flow 

 shall not become stationary and the blood collect in one 

 place. Now the cross-section of the aorta and the large 

 veins is much less than the total cross-section of all the 

 capillaries. As the rate is equal to the volume flowing 

 through in one second divided by the cross-section, it is 

 evident that the rate in the arteries and veins must be 

 greater than that in the capillaries. 



The rate in the larger vessels of animals is found in the follow- 

 ing manner. A blood vessel is cut and between the cut ends a 

 sufficiently wide tube, the contents of which have been accurately 

 determined, is inserted. The blood must now pass through the 

 tube. The time taken by the blood to pass from one end of the 

 tube to the other is then determined. For this experiment, the 

 tube is previously filled with some indifferent fluid, which is forced 

 from it into the vascular system by the incoming blood. Accord- 

 ing to this principle, the haemodromometer of Volkmann is built, 

 as is also, though more complicated, the Stromuhr of Ludwig. 



In the capillaries the distance traversed by a blood corpuscle is 

 measured directly with the aid of a microscope (e.g. in the web of 

 frog's foot). 



The pulsatory changes in the rate of the arterial blood can be 

 investigated by the plethysmograph, an apparatus registering the 

 changes in the pulse-volume of a limb. The changes in volume 

 are due to the periodic increase and decrease in the supply of 



