120 THE CIRCULATION OF THE BLOOD AND LYMPH 



same quantity must flow through every cross-section of the channel, 

 the velocity must necessarily be greater in the narrower than in the 

 wider part, does not answer the question. The greater portion of 

 the kinetic energy of the arterial blood is, as we have seen, destroyed, 

 or, rather, changed into an unavailable form, into heat, in the capil- 

 lary region. The mean velocity of the blood in the capillaries is not 

 more than ^g-^- to -^fa of the velocity in the aorta; the kinetic energy 

 of a given mass of blood in the capillaries cannot therefore be more 

 than (^) 2 , or -f^^ of its kinetic energy in the aorta. In the veins, 

 taking the velocity at half the arterial velocity, the kinetic energy 

 of the mass would be one-fourth of that in the aorta, or at least 

 10,000 times as great as in the capillary region. This extra kinetic 

 energy comes partly from the transformation of some of the poten- 

 tial energy of the blood. The resistance in the veins is very small 

 compared with that in the capillaries; less of the potential energy 

 represented by the lateral pressure at the end of the capillary tract 

 is required to overcome this resistance, and some of it is converted 

 into the kinetic energy of visible motion, the lateral pressure at the 

 same time falling somewhat abruptly. Contributory sources of 

 kinetic energy in the veins are the aspiration caused by the respira- 

 tory movements and the pressure caused by muscular contraction 

 in general, which, thanks to the valves, always aids the flow towards 

 the heart. From these two sources new energy is supplied, to rein- 

 force the remnant due to the cardiac systole (p. 133). 



Measurement of the Velocity of the Blood i. Direct Observation. 

 (a) This method can be applied to transparent parts by observing the 

 rate of flow of the corpuscles under the microscope. But it is only 

 where the blood moves slowly, as in the capillaries, that the method 

 is of use. (b) Part of the path of the blood through a large vessel may 

 be artificially rendered transparent by the introduction of a glass tube, 

 of approximately the same bore as the vessel (Volkmann). The tube 

 is filled with salt solution, and the blood admitted by means of a stop- 

 cock at the moment of observation. The time which the blood takes 

 to pass from one end of the tube to the other is noted, and the length 

 divided by the time gives the velocity of the blood in the tube. If the 

 calibre of the tube is the same as that of the artery, this is also the 

 velocity in the vessel; but if the calibre is different, a correction would 

 have to be made. The method is not a good one, for the reason, among 

 others, that the long tube introduces an extra resistance. 



2. Ludwig's Stromuhr. This instrument measures the quantity of 

 blood which passes in a given time through the vessel at the cross- 

 section where it is inserted. It consists of a U-shaped tube, with the 

 limbs widened into bulbs, but narrow at the free ends, which are con- 

 nected with a metal disc. By rotating the instrument, these ends 

 can be placed alternately in communication with a cannula in the 

 central, and another in the peripheral, portion of a divided artery; 

 or they can be placed so that none of the blood passes through the bulbs, 

 but all goes by a short-cut. One limb of the instrument is filled with 

 oil, and the other with defibrinated blood. The limb containing the 

 oil is first put into communication with the central end, and that con- 

 taining the blood with the peripheral end, of the artery. The blood 



