THE CIRCULATION OF THE BLOOD AND LYMPH 107 



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 in- 

 crease as we pass towards the axis of the bloodvessel, and 

 reach its maximum there (p. 168). 



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 

 cornes 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 

 increases as we pass from the heart along the branching 

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

 gradually diminishes again 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. Although in strictness we are only at present con- 

 cerned 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 

 tnergy, and it is necessary to know what becomes of the 



