CH. XXI.] 



THE CAPILLARY FLOW. 



273 



The Capillary Flow. 



When the capillary circulation is examined in any transparent 

 part of a living animal by means of the microscope (fig. 268), 

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

 blood-corpuscles move along, mostly in single file, and bend in 

 various ways to accommodate themselves to the tortuous course 

 of the capillary, but instantly recover their normal outline on 

 reaching a wider vessel. 



At the circumference of the stream in the larger capillaries, but 

 especially well marked in the small 

 arteries and veins, there is a layer 

 of liquor sanguinis in contact with 

 the walls of the vessel, and adher- 

 ing to them, which moves more 

 slowly than the blood in the centre. 

 The existence of this still layor, 

 as it is termed, is inferred both 

 from the general fact that such an 

 one exists in all tubes traversed by 

 fluid, and from what can be seen 

 in watching the movements of the 

 blood-corpuscles. Anyone who has 

 rowed on a river will know that 

 the swiftest current is in the 

 middle of the stream. 



Fig. 200. (Japillunea (<Jj in tile web 

 of the frog's foot connecting a 

 small artery (A) with a small 

 vein (V) . (After Allen Thomson .) 



is in 



The red corpuscles occupy the middle of 

 the stream and move with comparative rapidity ; the colourless 

 corpuscles run much more slowly by the walls of the vessel; while 

 next to the wall there is a transparent space in which the fluid 

 is at comparative 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. Part 

 of this slow movement of the colourless corpuscles and their occa- 

 sional 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. 



When the peripheral resistance is greatly diminished by the 

 dilatation of the small arteries, so much blood passes on 

 from the arteries into the capillaries at each stroke of the 

 heart, that there is not sufficient remaining in the arteries to 

 Intend them. Thus, the intermittent current of the ventricular 



K.I'. T 



