284 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



of the cut artery as before ; there is also a turntable arrangement at 

 F, by means of which the two upper tubes C and D may be connected 

 as in the figure ; or by twisting it through two right angles, D can bo 



( ^ made to communicate with A, and 



C with B. In place of the two 

 bulbs of Ludwig's instrument 

 there is a glass cylinder H which 

 contains a metal ball E. The 

 whole instrument is washed out 

 with oil to delay clotting, and 

 filled with defibrinated blood. As 

 soon as blood is allowed to flow 

 from the artery, the ball E is 

 driven over by the current till it 

 reaches the other end of the 

 cylinder; the instrument is then 

 FIG. 250. Tigerstedt's stromuhr. rapidly rotated through two right 



angles, and once more the ball is 



driven to the opposite end. This is repeated several times, and the 

 number of revolutions during a given period is noted. The capacity 

 of the cylinder minus that of the ball is ascertained, and the velocity 

 is calculated by the same formula as that already given. 



The Stromuhr has one advantage over the hasmodromometer, in 

 that it enables one to note changes in mean velocity during the 

 course of an experiment. The mean velocity varies very greatly 

 even during a short experiment. Thus, in the carotid artery of a 

 dog, the velocity of the stream varied from 350 to *730 mm. per 

 second in the course of eighty seconds ; in the same artery of the 

 rabbit the variations were still more extensive (94 to 226 mm. per 

 second Dogiel). 



Other instruments have been devised which give the variations in 

 the velocity during the phases of the heart-beat ; and some of these 

 lend themselves to the graphic method, and give tracings of what is 

 called the velocity pulse. Before we can understand these, it is neces- 

 sary first to study the relationship of velocity to blood-pressure. Mere 

 records of blood-pressure give us no indication of the velocity of the 

 blood-stream; the latter depends, not on the absolute amount of 

 pressure, but on the differences of pressure between successive points 

 of the vascular system. When a fluid is in movement along a tube 

 the forces maintaining the flow are two in number, the one hydro- 

 kinetic, the other hydrostatic. Thus, if we consider the flow from 

 one point in the tube to another (say, for example, at 1 cm. dis- 

 tance), the forces producing the flow are (1) the kinetic energy pos- 



9 



by the blood when it enters the first spot (i.e. - - dynes, 



