THE COURSE AND PHYSICS OF THE CIRCULATION 231 



draining the head, arms, and chest, and the inferior the 

 rest of the body. 



Why does the velocity fall from a maximum in the 

 aorta to a minimum in the capillaries and then augment 

 again along the course of the veins? The underlying 

 principle is simple if the student will not allow himself to 

 be led away from it. It is merely that 

 in any stream the velocity is greatest 

 where the cross-section of the channel 

 is least and lowest where it is greatest. 

 When the application is made we find 

 that we are required to regard the 

 aorta as the narrowest and the capil- 

 laries as the widest part of the sys- 

 tem. This is not readily admitted 

 until the enormous number of the 

 capillaries is emphasized. It is not a 

 few capillaries but millions combined 

 which we have to compare with the 

 aorta. It seems to be a general truth 

 that when a vessel forks the sum of 

 the cross-sections of the branches is 

 greater than that of the parent stock. 

 So if an artery divides the velocity will 

 be reduced while there will be a quick- 

 ening at the point where two veins 

 unite to make one. If the velocity in 

 the veins never equals that in the aorta 

 it is simply because the two venae cavse have unitedly 

 a somewhat larger cross-section than the great artery. 



The Facts of Blood-pressure. A vein is easily 

 flattened under the finger; an artery offers a strong re- 

 sistance. We have here the sign of a great difference 

 between the pressure exerted by the blood in the two 

 vessels. The difference is shown still more strikingly 

 when an artery and a vein are cut ; the blood springs from 

 the artery in a pulsating jet while the flow from the vein 

 is copious but easily checked. We say that arterial pres- 



FIG. 47. If a ves- 

 sel (a) divides into 

 two branches, (6) and 

 (c), these will be in- 

 dividually of less 

 cross-section than the 

 main trunk but 

 unitedly they will ex- 

 ceed it. Linear ve- 

 locity will be lower 

 in the branches than 

 in the parent stock. 



