200 



THE CIRCULATION OF THE BLOOD 



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 tend- 

 ency 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 

 into the capillaries at each stroke of the heart that there is not sufficient 

 remaining in the arteries to distend them. Thus, the intermittent current 

 of the ventricular systole is not always converted into a continuous stream 

 by the elasticity of the arteries before the capillaries are reached; and so 

 intermittency of the flow occurs both in capillaries and veins and a venous 

 pulse is produced. The same 'phenomenon may occur when the arteries 



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

 Section "; and a Small Vein. X 35. A, Vein; B, arteriole; C, large capillary; D, small capillaries. 

 (Bailey.) 



become rigid from disease, and when the beat of the heart is so slow or so 

 feeble that the blood at each cardiac systole has time to pass on to the capil- 

 laries before the next stroke occurs; the amount of blood sent at each stroke 

 being insufficient properly to distend the elastic arteries. 



It was formerly supposed that the occurrence of any transudation from 

 the interior of the capillaries into the midst of the surrounding tissues was 

 confined, in the absence of injury, strictly to the fluid part of the blood; in 



