THE MAIN FACTS OF THE CIRCULATION. 143 



this evenness of flow is continued on along the veins as far as we can trace 

 them. Not infrequently variations in velocity and in the distribution of 

 the blood, due to causes which will be hereafter discussed, are witnessed 

 from time to time. 



The character of the flow through the smaller capillaries is very 

 variable. Sometimes the corpuscles are seen passing through the chan- 

 nel in single file with great regularity ; at other times they may be 

 few and far between. Some of the capillaries are wide enough to permit 

 two or more corpuscles abreast. In all cases the blood as it passes through 

 the capillary stretches and expands the walls. Sometimes a corpuscle may 

 remain stationary at the entrance into a capillary, the channel itself being 

 for some little distance entirely free from corpuscles. Sometimes many 

 corpuscles will appear to remain stationary in one or more capillaries for a 

 brief period and then to move on again. Any one of these conditions 

 readily passes into another ; and, especially with a somewhat feeble circu- 

 lation, instances of all of them may be seen in the same field of the micro- 

 scope. It is only when the vessels of the web are unusually full of blood 

 that all the capillaries can be seen equally filled with corpuscles. The 

 long, oval red corpuscle moves with its long axis parallel to the stream, 

 occasionally rotating on its long axis, and sometimes, in the larger chan- 

 nels, on its short axis. The flexibility and elasticity of a corpuscle are 

 well seen when it is being driven into a capillary narrower than itself, or 

 when it becomes temporarily lodged at the angle between two diverging 

 channels. 



These and other phenomena, on which we shall dwell later on, may be 

 readily seen in the web of the frog's foot or in the stretched-out tongue or in 

 the mesentery of the frog ; and essentially similar phenomena may be 

 observed in the mesentery or other transparent tissues of a mammal. All 

 over the body, wherever capillaries are present, the corpuscles and the 

 plasma are being driven in a continuous and though somewhat irregular 

 yet on the whole steady flow through channels so minute that the passage 

 is manifestly attended with considerable difficulties. 



It is obvious that the peculiar characters of the flow through the minute 

 arteries, capillaries, and veins affords an explanation of the great change 

 taking place in the peripheral region between the arterial flow and the 

 venous flow. The united sectional area of the capillaries is, as we have seen, 

 some hundreds of times greater than the sectional area of the aorta ; but 

 this united sectional area is made up of thousands of minute passages, 

 varying in man from 5 to 20 //, some of them, therefore, being in an undis- 

 tended condition, smaller than the diameter of a red corpuscle. Even were 

 the blood a simple liquid free from all corpuscles, these extremely minute 

 passages would occasion an enormous amount of friction, and thus present a 

 considerable obstacle or resistance to the flow of blood through them. Still 

 greater must be the friction and resistance occasioned by the actual blood 

 with its red and white corpuscles. The blood in fact meets with great dif- 

 ficulties in its passage through the peripheral region, and sometimes, as we 

 shall see, the friction and resistance are so great in the peripheral vessels of 

 this or that area that no blood passes through them at all, and an arrest of 

 the flow takes place in the area. 



The resistance to the flow of blood thus caused by the friction generated 

 in so many minute passages is one of the most important physical facts in the 

 circulation. In the large arteries the friction is small ; it increases gradually 

 as they divide, but receives its chief and most important addition in the 

 minute arteries and capillaries; it is relatively greater in the minute arteries 

 than in the capillaries on account of the flow being more rapid in the former, 



