CEI. XXI.] THE VELOCITY PULSE 281 



and the velocity is calculated by the same formula as that already 

 given. 



The Stromuhr has one advantage over the haemodromometer, 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 necessary 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 hydro - Static. FIG. 2S1. Tigerstedt's Stromuhr. 



Thus, if we consider the flow from 



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

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

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



a 



/vtt n^2, 



or - gramme-centimetres) ; and (2) the difference between the two 



lateral pressures at the two points in question. The important 

 point to remember with respect to the part the pressure plays, is 

 that the actual value of the lateral pressure does not matter, but 

 that the resulting velocity, so far as pressure is concerned, depends 

 only upon the fall of pressure between the two points. Therefore, 

 the measurement to be determined is the rate of fall of pressure, 

 or, as it is usually expressed, the pressure gradient. The steeper 

 this gradient is, the more rapid is the flow. Thus, if an artery 

 is suddenly cut across, the blood will spurt out at a far greater 

 velocity than it possessed when flowing along the intact artery, 



