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EXPERIMENTAL PHYSIOLOGY 



from which they have sprung. In examining the flow of a fluid 

 along a tube, it is found that the resistance offered to the flow 

 becomes progressively greater as the diameter of the tube along which 

 it is forced is diminished. The resistance of the system of capillaries 

 and minute arteries must therefore be very great, and this high peri- 

 pheral resistance explains many of the important facts that we know 

 of the distribution and flow of the blood in the various parts of the 

 body. 



Other important points for us to note at the commencement of 

 our study of the physical characters of the circulation are : (a) that 

 the arterial walls are highly elastic, and (b) that the source of the 

 energy required to propel the blood through the vessels is the 

 rhythmic contraction of the heart. 



By imitating these three features of the circulation in an arti- 

 ficial schema, we can reproduce and study many of the important 



Fig. 124. — Schema of the Circulation. 



phenomena presented to us by the circulation. Thus in the schema 

 shown in fig. 124 the heart is represented by the enema syringe s, 

 which is provided with two valves, v 1 and v 2 , which permit the flow 

 of the fluid from v 2 to v 1 , but not in the reverse direction. The 

 arterial system is represented by the rubber-tubing v 1 t, upon the 

 course of which two manometers, m 1 and m 2 , are inserted. The mano- 

 meter consists essentially of a U-tube held in a vertical position. 

 The bend of the U is filled with mercury, and one limb which remains 

 open to the air carries a light float which rests on the upper surface 

 of the mercury and accurately follows its movements. The upper 



