THE CAPILLARY FLOW 



221 



blood is seen to flow with a constant equable motion; the red blood corpus- 

 cles moving along, mostly in single file, and bending in various ways to ac- 

 commodate themselves to the tortuous course of the capillary, but instantly 

 recovering their normal outline on reaching a wider vessel. 



At the circumference of the stream and adhering to the walls of the larger 

 capillaries, but especially well marked in the small arteries and veins, there 

 is a layer of plasma which appears to be motionless. The existence of this 

 still layer, as it is termed, is inferred both from the general fact that such a 

 one exists in all fine tubes traversed by fluid, and from what can be seen in 

 watching the movements of the blood corpuscles. The red corpuscles occupy 



FIG. 193. Capillary Network from Human Pia Mater, Showing also an Arteriole in 

 "Optical Section"; and "a Small Vein. X 350. .4, Vein; B, arteriole; C, large capillary; 

 D, small capillaries. (Bailey.) 



the middle of the stream and move with comparative rapidity; the color- 

 less corpuscles run much more slowly by the walls of the vessels; while next 

 to the wall there is a transparent space in which the fluid appears to be at 

 rest; for if any of the corpuscles happen to be forced within it, they move 

 more slowly than before, rolling lazily along the side of the vessel and often 

 adhering to its wall, figure 194. Part of this slow movement of the colorless 

 corpuscles and their occasional stoppage may be due to their having a ten- 

 dency to adhere to the walls of the vessels. Sometimes, indeed, when the 

 motion of the blood is not strong, many of the white corpuscles collect in 

 a capillary vessel, and for a time entirely prevent the passage of the red 

 corpuscles. 



When the peripheral resistance is greatly diminished by the dilatation of 

 the small arteries and capillaries, so much blood passes on from the arteries 



