392 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The metallic lower part of the instrument, which includes both limbs of 

 the tube, is completely divided horizontally at c. The two parts are so built, 

 however, as to be maintained in water-tight apposition. This arrangement 

 permits the whole upper part of the instrument, including the glass bulbs, to 

 be rotated suddenly upon the lower, so that the bulb B may correspond with 

 the entrance for the blood at a, and the bulb A with the exit for the blood at 

 b. If this rotation be effected at the instant when the space between the two 

 marks on A has been filled with blood, the bulb J5, now charged with oil, 

 will be filled by the blood which enters next, and the first charge of the ani- 

 mal's own blood will make its exit at 6. Oil will now pass over from B to 

 A; when the line between it and the blood which is leaving A has just 

 reached the lower mark on A, the bulbs are turned back to their original 

 position. Thus, by repeated rotations, each of which can be made to record 

 upon the kymograph the instant of its occurrence, a number of charges of 

 blood can be received and transmitted in succession ; it is always the same 

 space, between the marks on A, which is used for measuring the charge ; and 

 the time of the experiment can be much prolonged. By this procedure the 

 errors due to a single brief observation can be greatly reduced. Indeed, the 

 time of entrance of a single charge of blood would be quite too short to give 

 a satisfactory result. 



The use of the stromuhr not only affords necessary data for the calcu- 

 lation of the average speed of the blood, but seeks directly to measure the 

 volume of blood delivered in a given time by an artery to its capillary dis- 

 trict. It is evident that this volume is a quantity of fundamental importance 

 in the physiology of the circulation. Could we ascertain it, by direct meas- 

 urement or by calculation, for the aorta or pulmonary artery, we should know 

 at once the volume of blood delivered to the capillaries in one second, and 

 thus the time taken for the entire blood to enter either those of the lungs or 

 of the system at large. By this knowledge, many important problems would 

 be advanced toward solution. 



The Measurement of Rapid Fluctuations of Speed. The stromuhr 

 can give only the average speed of the blood during the experiment. To 

 study rapid fluctuations of speed, another method is needed. If, in a large 

 animal, a vessel, best an artery, be laid bare, a needle may be thrust into it at 

 right angles. If the needle be left to itself, the end which projects from the 

 artery will be deflected toward the heart, because the point will have been 

 deflected toward the capillaries by the blood-stream. The angle of deflection 

 might be read off, could a graduated semicircle be adjusted to the needle. If 

 the stream be arrested, the needle returns to its position at right angles to the 

 artery. The greater the velocity of the stream, the greater is the deflection of 

 the needle. If, later, the same needle be thrust into a tube of rubber through 

 which water flows at known rates of speed, the speed corresponding to each 

 angle of deflection of the needle may be determined. If the needle were 

 made to mark upon a kymograph, variations of the speed would be recorded 

 as a curve. 



