222 HAKDBOOK OP PHYSIOLOGY. 



The Capillary Flow. 



It is in the capillaries that the chief resistance is offered to the prog- 

 ress of the blood ; for in them the friction of the blood is greatly in- 

 creased by the enormous multiplication of the surface with which it is 

 brought in contact. 



When the capillary circulation is examined in any transparent part 

 of a full-grown living animal by means of the microscope (iig. 187), the 

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

 corpuscles moving along, mostly in single file, and bending in various 

 ways to accommodate 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 in the larger capillaries, but 

 sepecially well marked in the small arteries and veins, in contact with 



Fig. 187. Capillaries (C.) in the web of the frog's foot connecting a small artery (A) with a 

 small vein V (after Allen Thomson). 



the walls of the vessel, and adhering to them, there is a layer of liquor 

 sanguinis which appears to be motionless. The existence of this still 

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

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

 &een in watching the movements of the blood-corpuscles. The red 

 corpuscles occupy the middle of the stream and move with comparative 

 rapidity; the colorless corpuscles run much more slowly by the walls of 

 the vessel; while next to the wall there is often a transparent space in 

 which the fluid appears to be at rest; for if any of the corpuscles hap- 

 pen 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. Part 

 of this slow movement of the colorless corpuscles and their occasional 

 stoppage may be due to their having a natural tendency 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. 



