374 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



strikingly seen, as the outer layers among the numerous corpuscles lag con- 

 spicuously. In the arterioles similar phenomena are seen if the normal swift- 

 ness of movement become sufficiently retarded for the individual corpuscles 

 to be visible. 



FIG. 98. To illustrate the forging ahead of a FIG. 99. The inert layer of plasma in the 



corpuscle at the centre of the blood-stream. small vessels. 



The arrow marks the direction of the blood. 



An appearance which also tells of friction is that of the so-called " inert 

 layer " of plasma. 1 In vessels, of whatever kind, which are wide enough for 

 several corpuscles to pass abreast, it is seen that all the red corpuscles are always 

 separated from the profile of their channel by a narrow clear and colorless 

 interval occupied, of course, by plasma. This is caused by the excess of the 

 friction in the layers nearest to the vascular wall (see Fig. 99). The friction 

 thus indicated, other things being equal, is less in a dilated than in a con- 

 tracted tube; and less in a sluggish than in a rapid stream. It probably 

 varies also with changes of an unknown kind in the condition of the cells of 

 the vascular wall. 



Behavior of the Leucocytes. If the behavior of the leucocytes be 

 watched, it will be seen to differ markedly from that of the red corpuscles, at 

 least when the blood-stream is somewhat retarded, as it so commonly is under 

 the microscope. Whereas the friction within the vessels causes the throng of 

 red corpuscles to occupy the core of the stream, the scantier leucocytes may 

 move mainly in contact with the wall, and thus be present freely in the inert 

 layer of plasma. Naturally their progression is then much slower and more 

 irregular than that of the red disks. Indeed, the leucocytes often adhere to 

 the wall for a while, in spite of shocks from the red cells which pass them. 

 Moreover, the spheroidal leucocyte rolls over and over as it moves along the 

 wall in a way very different from the progression of the red disk, which only 

 occasionally may revolve about one of its diameters. A leucocyte entangled 

 among the red cells near the middle of the stream is seen generally not only 

 to move onward but also to move outward toward the wall, and, before long, 

 to join the other leucocytes which are bathed by the inert layer of plasma. 

 It is due solely to the lighter specific gravity of the leucocytes that, under 

 the forces at work within the smaller vessels, they go to the wall, while the 

 denser disks go to the core of the current. This has been proved experimen- 

 tally by driving through artificial capillaries a fluid having in suspension par- 

 ticles of two kinds. Those of the lighter kind go to the wall, of the heavier 



1 Poiseuille: "Recherches sur les causes du mouvement du sang dans les vaisseaux capil- 

 laites," Academic des Sciences Savans etrangers, 1835. 



