n8 THE CIRCULATION OF THE BLOOD AND LYMPH 



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 varia- 

 tions 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 maxi- 

 mum there (p. 191). 



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 circula- 

 tion. 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 some- 

 what as we pass from the heart along the branching arteries, under- 

 goes an abrupt augmentation, 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, 

 capillaries, 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 

 any part of the vascular channel really implies. To say that when 

 the channel widens the velocity diminishes is not to explain the 

 meaning of this diminution. A diminution of velocity implies a 

 diminution of kinetic energy, and it is necessary to know what becomes 

 of the energy that disappears. The stock of energy imparted by the 

 contraction of the heart to a given mass of blood constantly diminishes 

 as it passes round from the aorta to the right side of the heart, for 

 friction is constantly being overcome and heat generated. This energy, 



