232 THE CIRCULATION OF THE BLOOD 



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 cor- 

 puscles occupy the middle of the stream and move with comparative 

 rapidity; the colorless 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 



4-fFfWKr 



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

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

 D, small capillaries. (Bailey.) 



due to their having a tendency to adhere to the walls of the vessels. Some- 

 times, 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 intermit- 

 tent current of the ventricular systole is not always converted into a con- 

 tinuous stream by the elasticity of the arteries before the capillaries are 

 reached. The intermittency of the flow occurs both in capillaries and 

 veins and a venous pulse is produced. The same phenomenon may occur 

 when the arteries become rigid from disease, and when the beat of the 



