282 



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



blood has to pass probably does not exceed from -^ to ^V inch 

 (O'o mm.), and therefore the time required for each quantity of blood 

 to traverse its own appointed portion of the general capillary system 

 will scarcely amount to a second. It is during this time that the 

 blood does its duties in reference to nutrition. 



In the larger vessels direct observations of this kind are not 

 possible, and it is necessary to have recourse to some instrumental 

 method. 



Volkmann was the first to make more or less accurate measure- 

 ments by introducing a long (J -shaped glass tube into the course of 

 an artery. A diagram of this hcemodromometer, as it was termed, is 

 shown in the accompanying diagram (fig. 248); this is lilled with 



FIG. 248. Volkmann's 

 Haemodromom ter. 



FIG. 249. Ludwig's 

 Stromuhr. 



salt solution, and provided with a stop-cock a ; this tap is so arranged 

 that the blood can flow straight across from one section of the artery 

 to the other ; then at a given instant it is turned into the position 

 shown in the diagram, and the blood has to traverse the long (J-tube, 

 and the time that it takes to traverse the tube, the length of which 

 is known, is accurately observed. If the sectional area of the tube is 

 the same as that of the artery, the velocity is obtained without 

 further correction; but the difficulty of obtaining glass tubes with 

 the exact calibre of every blood-vessel which one desires to experi- 

 ment with led to the abandonment of this method, and Ludwig's 

 Stromuhr (literally stream-clock) took its place. This consists of a 



