THE CIRCULATION. 295 



Except in the very smallest capillaries, it is noticed that the red 

 corpuscles always move in the axis of the current, while on either 

 side of this thread of moving cells there is noticed a transparent 

 layer of liquor sanguinis which is almost perfectly still or possesses 

 only slight motion. This layer, "Poiseuille's still space," where it 

 is plainly discernible, occupies about one-fifth of the space on each 

 side of the axial current, which occupies the remaining three-fifths 

 of the lumen of the vessel. 



Within the smaller blood-vessels the red corpuscles occupy the 

 middle of the stream, where, in single file, they glide along with 

 comparative rapidity; in larger vessels two or three may flow along 

 abreast. Along the outer edge of the central thread of red blood- 

 corpuscles move the white ones, many even getting into the space 

 of Poiseuille. The motion of the white corpuscles is one of rolling, 

 particularly when they are in the clear space next the vessel-wall or 

 in direct contact with the latter, since they are sticky by nature. 

 The contact of the rapidly moving axis current also assists in giv- 

 ing to the white corpuscles their rolling motion. Their motion is 

 so slow at times that they adhere to the vessel-wall. 



It has been demonstrated by physical experiments that particles 

 of least specific gravity (white corpuscles) in all capillaries are 

 pressed toward the wall, while those of greater specific gravity (red 

 corpuscles) remain in the middle of the stream. 



One of the characteristics of the capillary circulation is the dis- 

 appearance of the pulse. Ordinarily this has been accomplished by 

 the resistance which is offered to the current on its way to the 

 periphery. When, for any reason, the arterioles are greatly relaxed, 

 and there exists at the same time high blood-pressure, so much blood 

 flows into the capillaries from the lessened resistance to its current 

 that a distinct pulse passes along the capillaries to the veins. This 

 pulse is characteristic of aortic insufficiency or in cases of atheroma 

 of the arteries. In the latter condition the vessels become calcified 

 and rigid and so behave physically as inelastic tubes. 



Cause of Movement of Blood. The force of the heart transformed 

 into arterial tension is the real cause of the movement of the blood 

 in the capillaries. This is not the only influence, for gravity can 

 exert influences that are either favorable or opposed to the current 

 of the blood. 



Swiftness of Blood in the Capillary System. Since very many 

 conditions are capable of modifying the velocity of the blood-current, 

 it is a very difficult task to ascertain the numerical valuation of that 



