THE CIRCULATION OF THE BLOOD AND LYMPH in 



coming the extra resistance ; the potential energy of the blood 

 is also drawn upon, and the lateral pressure falls sharply in the 

 capillary region, as well as the velocity. Where the capillaries 

 open into the veins, the lateral pressure again sinks abruptly, 

 while the velocity begins to increase, till in the largest veins it 

 is probably about half as great as in the aorta. 



Where does the extra kinetic energy of the blood in the veins 

 come from ? To say that the vascular channel again contracts 

 as the blood passes from the capillaries into the veins, and that, 

 since the 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 un- 

 available form, into heat, in the capillary region. The mean 

 velocity of the blood in the capillaries is not more than -J^ to 

 2^75- of the velocity in the aorta ; the kinetic energy of a given 

 mass of blood in the capillaries cannot therefore be more than 

 (2w) 2 > or Twirrr f i ts 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 potential 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 re- 

 sistance, 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 respiratory move- 

 ments 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. 121). 



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 (Volk- 

 mann). The tube is filled with salt solution, and the blood admitted 

 by means of a stopcock 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 



