THE CIRCULATION OF THE BLOOD AND LYMPH 109 



determine the actual velocity of any given particle of the water at any 

 given moment within a measured interval ; nor does it tell us whether 

 or not the average velocity of the current has itself undergone 

 variations within the period of observation. 



We have dwelt upon this point because the measurement of 

 the velocity of the blood, to which we must now turn, involves 

 the same considerations. Within the smaller arteries, as the 

 microscope shows us, and as we should in any case expect from 

 what we know of fluid motion, the blood-current, apart from 

 the periodical variations in its velocity, due to the action of the 

 heart, varies in speed from point to point of the same cross- 

 section. The layer next the periphery of the vessel, the so- 

 called peripheral plasma-layer or Poiseuille's space, moves more 

 slowly than the central portion, the axial stream. In fact, we 

 must suppose that in the large as well as in the small vessels 

 the layer just in contact with the vessel-wall is at rest, while the 

 stratum internal to this slides on it and has its velocity diminished 

 by the friction. The next layer again slides on the last, but 

 since this is already in motion, its velocity is not so much 

 diminished, and so on. The velocity must therefore increase 

 as we pass towards the axis of the bloodvessel, and reach its 

 maximum there (p. 177). 



Again, the velocity must be altered wherever an alteration 

 occurs in the width of the bed, that is, in the total cross-section 

 of the vascular system ; for since as much blood comes back 

 in a given time to the right side of the heart as leaves the left 

 side, the same quantity must pass in a given time through every 

 cross-section of the circulation. Wherever the total section of 

 the vascular tree increases, the blood-current must slacken ; 

 wherever it diminishes, the current must become more rapid. 

 Now, the total section, increasing somewhat as we pass from the 

 heart along the branching arteries, undergoes an abrupt augmen- 

 tation, and reaches its maximum in the capillary region. It 

 suddenly diminishes again at the venous end of the capillary 

 tract, and then more gradually as we pass heart wards along the 

 veins, but never becomes so small as in the arterial tract. We 

 must, therefore, expect the mean velocity to be greatest in the 

 large arteries, less in the veins, and least in the arterioles, capil- 

 laries, and venules. It must, of course, be remembered that the 

 total section varies from time to time at any given distance from 

 the heart. The capillary tract is especially variable in its area, 

 and capillaries full of blood at one moment may be collapsed and 

 empty at another, according to the changes of calibre and pressure 

 in the arteries which feed and the veins which drain them. 



Although in strictness we are only at present concerned with the 

 arteries, it will be well to consider here what a change of velocity at 



