RATE OF BLOOD-FLOW IN THE VESSELS. 



323 



ure in the veins is perhaps 5 mm. and in the great veins near the 

 heart negative, so that a cannula introduced into this part of the 

 circulatory system would show a depression of the mercury in the 

 distal limb of the manometer, indicating that there was a suction 

 from within the vein ; this constitutes negative pressure. It is the 

 pressure under which the blood is in the arteries that causes it to 

 spurt or jet when the vessel is cut, while the flow from a wounded 

 vein or capillary is continuous. 



Pick's spring myograph is another form of this instrument, 

 in which the movements of a hollow spring are communicated 

 to a writing lever. 



RATE OF BLOOD-FLOW IN THE VESSELS. 



The caliber of the blood-vessels is constantly changing, and 

 the rate of the flow of blood through them is subject to great 

 variations, so that any estimate of the velocity of the flow can 

 at best be but approximate. There are two factors which enter 

 into the problem : (1 ) the caliber of the vessel at the point where 

 the velocity is to be ascertained, and (2) the amount of blood 

 passing that point in a given time, 

 for the velocity is inversely pro- 

 portional to the sectional area. 



Rate of Flow in the Arte- 

 ries. It is in these vessels that 

 the velocity is the greatest, begin- 

 ning at the heart and gradually 

 diminishing along the course of 

 the arterial system. Its maximum 

 is at the time of the systole. 



The Stromuhr. One of the in- 

 struments used to determine the 

 rate of speed is the stromuhr of 

 Ludwig (Fig. 169). This consists 

 of a U-shaped tube, glass above 

 and metal below, expanded into 

 two bulbs, A and B, which can 

 be filled from the top. The lower 

 extremities of these tubes are con- 

 nected with the ends of a divided 

 artery, so that the channel between 

 the two is continuous through the 

 stromuhr. The instrument is so 

 constructed that while in place the upper portion, including the 

 bulbs, can be so rotated at c as to bring A in connection with 6, 

 and B with a, and this process repeated as often as desired, the 

 flow through the instrument not being interrupted. The tubes 

 and the bulb B are filled with defibrinated blood, which in A 



FIG. 169. Diagram of longitudi- 

 nal section of Ludwig's "stromuhr." 

 The arrows mark the direction of the 

 blood-stream. For further descrip- 

 tion see the text (Curtis). 



