224 THE RATE OF FLOW. [BOOK i. 



determines the general velocity of the flow at various parts of the 

 system. The slowness of the flow in the capillaries is not due to 

 there being so much more friction in their narrow channels than in 

 the wider canals of the larger arteries. For the peripheral resist- 

 ance caused by the friction in the capillaries and small arteries is 

 an obstacle not only to the flow of blood through these small 

 vessels where the resistance is actually generated, but also to the 

 escape of the blood from the large into the small arteries, and 

 indeed from the heart into the large arteries. It exerts its 

 influence along the whole arterial tract. And it is obvious that if 

 it were this peripheral resistance which checked the flow in the 

 capillaries, there could be no recovery of velocity along the venous 

 tract. 



The blood is flowing through a closed system of tubes, the 

 blood vessels, under the influence of one propelling force, the systole 

 of the ventricle, for this is the force which drives the blood from 

 ventricle to auricle, though as we have seen its action is modified 

 in the several parts of the system. In such a system the same 

 quantity of fluid must pass each section of the system at the same 

 time, otherwise there would be a block at one place, and a 



deficiency at another. If, for instance, 

 a fluid is made to flow by some one 

 force, pressure or gravity, through a 

 tube A (Fig. 34) with an enlargement 

 B, it is obvious that the same quantity 

 of fluid must pass through the section 

 b as passes through the section a in 

 the same time, for instance a second. 

 Otherwise, if less passes through b than a, the fluid would accumu- 

 late in B, or if more, B would be emptied. In the same way just 

 as much must pass in the same time through the section c as 

 passes through a or b. But if just as many particles of water 

 have to get through the narrow section a in the same time as 

 they have to get through the broader section c, they must move 

 quicker through a than through c, or more slowly through c than 

 through a. For the same reason water flowing along a river 

 impelled by one force, viz. that of gravity, rushes rapidly through 

 a 'narrow' and flows sluggishly when the river widens out into 

 a ' broad.' The flow through B will be similarly slackened if B 

 instead of being simply a single enlargement of the tube A consists 

 of a number of small tubes branching out from A, with a united 

 sectional area greater than the sectional area of A. In each of 

 such small tubes, at the line c for instance, the flow will be slower 

 than at a, where the small tubes branch out from A, or at 6, where 

 they join again to form a single tube. Hence it is that the blood 

 rushes swiftly through the arteries, tarries slowly through the 

 capillaries, but quickens its pace again in the veins. 



An apparent contradiction to this principle that the rate of 



