VELOCITY AND PRESSURE OF BLOOD-FLOW. 



449 



and the variations in vessels of different sizes experimental de- 

 terminations are necessary. While the general principle involved 

 in these determinations is simple, their actual execution in an 

 experiment is attended with 

 some difficulties, and various 

 devices have been adopted. 

 The most direct method .per- 

 haps is that used in the in- 

 strument devised by Ludwig, 

 namely, the stromuhr. The prin- 

 ciple used is to cut an artery 

 or vein of a known size and de- 

 termine how much blood flows 

 out in a given time. We may 

 define the velocity of the blood 

 at any point as the length of 

 the column of blood flowing by 

 that point in a second. If we 

 cut the artery there a cylindri- 

 cal column of blood of a defi- 

 nite length and with a cross-area 

 equal to that of the lumen of 

 the artery will flow out in a 

 second. The volume of the 

 outflow can be determined di- 

 rectly by catching the blood. 

 Knowing this volume and the 

 cross-area of the artery, we can 

 determine the length of the 

 column that is, the velocity 

 of the flow since in a cylinder 

 the volume, V, is equal to the 

 product of the length into the 

 cross-area. 



V = length X cross-area, or 

 croJarea 



We cannot, of course, make 

 the experiment in this simple 

 way upon a living animal; the 

 loss of so much blood would at 

 once change the physical and physiological conditions of the circula- 

 tion, and would give us a set of conditions at the end of the experi- 



Fig. 177. Ludwig's stromuhr: a and 6, 

 The glass bulbs; a is filled with oil to the 

 mark (5 c.c.), while b and the neck are filled 

 with salt solution or defibrinated blood; p, 

 the movable plate by means of which the 

 bulbs may be turned through 180 degrees, 

 c, c, for the cannulas inserted into the artery ; 

 , the thumb screw for turning the bulbs; 

 h, the holder. When in place the clamps 

 on the arteries are removed, blood flows 

 through c into a, driving out the oil and 

 forcing the salt solution in b into the head 

 end of the artery through c'. When the 

 blood entering a reaches the mark, the bulbs 

 are turned through 180 degrees so that 6 lies 

 over c. The blood flows into b and drives 

 the oil back into a. When it just fills this 

 bulb, they are again rotated through 180 

 degrees, and so on. The oil is driven out of 

 and into a a given number of times, each 

 movement being equal to an outflow of 5 c.c. 

 of blood. When the instrument has been 

 turned, say, ten times, 50 c.c. of blood have 

 flowed out. Knowing the time and the 

 caliber of the artery, the calculation is made 

 as described in the text. A modification of 

 the form of this instrument has been de- 

 vised by Tigerstedt.* 



29 



*"Skandinavisches Archiv f. Physiol.," 3, 152, 1891. 



