226 THE RATE OF FLOW. [BOOK i. 



at about '75 millimeters a second but is probably quicker even 

 than this. 



As regards the veins, the flow is very slow in the small veins 

 emerging from the capillaries but increases as these join into larger 

 trunks, until in a large vein, such as the jugular of the dog, the 

 rate is about 200 mm. a second. 



123. It will be seen then that the velocity of the flow is in 

 inverse proportion to the width of the bed, to the united sectional 

 areas of the vessels. It is greatest at the aorta, it diminishes 

 along the arterial system to the capillaries, to the united bases 

 of the cones spoken of in 112, where it is least, and from thence 

 increases again along the venous system. 



And indeed it is this width of the bed and this alone which 

 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. 36) 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 in a 

 second. Otherwise, if less passes through b than a, the fluid would 

 accumulate 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 

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



