THE FLOW OF BLOOD IN THE ARTERIES 



209 



driving power is not consumed rapidly until the smallest arteries are reached, 

 and hence the fall in pressure in the large and medium sized arteries is only 

 relatively small (cf. page 222). 



D. VELOCITY OF THE BLOOD IN THE ARTERIES 



There are two ways of determining the velocity of the blood in the arteries, 

 according as one wishes to obtain the mean velocity in unit time, or the varia- 

 tions in speed which take place during a single heart beat. 



The velocity can only be determined by placing the necessary apparatus 

 directly in the path of the blood without interrupting its flow. Several forms 



m 



FIG. 80. FIG. 81. 



FIG. 80. Ludwig's Stromuhr or "current clock." 



FIG. 81. Chauveau's haemadromograph with air transmission, n, the needle; pi, small plate 

 on the pendulum; k, a tambour influenced by the strokes of the pendulum. The blood 

 flows in the direction of the arrow, m, a hard-rubber membrane which serves as a lid for 

 the vertical tube and as a fulcrum for the pendulum. 



of apparatus have been constructed for this purpose, the best known of which 

 is Ludwig's Stromuhr or "current clock" (Fig. 80). It consists of two glass 

 bulbs of equal size (A, B) which communicate directly with each other by means 

 of the U-shaped tube above. By means of the opening at the top oil is placed 

 in one bulb, A, and salt solution in the other, B, and the opening is then closed. 

 The two fluids are in contact with each other above. The tube A is now con- 

 nected with the central end and the tube B with the peripheral end of an artery. 

 When the arteries are undamped the blood flows into A and drives the contained 

 oil over into 5, the salt solution meantime being forced into the peripheral end 

 of the artery. When the blood has completely filled the bulb A, the two bulbs 

 are reversed, so that the blood flows now into B, displacing the oil once more 

 and driving the blood from A into the peripheral end of the artery. The capacity 

 of the bulbs being known, by counting the number of reversals necessary one 



