204 



A TEXTBOOK OF PHYSIOLOGY 



peripheral end of the artery which is under investigation. The whole 

 instrument is washed out with oil to prevent clotting, and filled with 

 defibrinated blood. So soon as the blood is allowed to flow from the 

 artery, the metal ball (6) is driven over by the current till it reaches 

 the end of the cylinder (a). The instrument is then rapidly rotated 



FIG. 107. STROMUHR. (Tigerstedt.) 



on the drum (k), so that the position of the entering and exit 

 tubes is icversed. The metal ball is now once more driven by the 

 current to the opposite end of the cylinder. This procedure is 

 repeated several times, and the number of revolutions during 

 the period of observation is noted. The capacity of the cylinder 



FIG. 108. CHAUVEAU'S H.EMODROMOMETER. 



(a), minus the volume of the ball (b), multiplied by the number of 

 revolutions, gives the volume of blood which has passed during the 

 period of observation; and this volume, divided by the time and the 

 sectional area of the artery, gives the mean velocity per second. In 

 using the stromuhr, the mean velocity in an artery is found to vary 

 greatly. This, for the most part, is owing to the variations of resist- 



