THE COURSE AND PHYSICS OF THE CIRCULATION 233 



superior draining the head, arms, and chest, and the in- 

 ferior 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 appears that if the united 



FIG. 54. If a ves- 

 (o) divides into 



section of all the capillaries were rep- two branches, (&) and 



(c), these will be in- 

 dividually of less 

 cross-section than the 

 main trunk but 



resented by a circular opening in a 

 wall this would be nearly, if not quite, 

 large enough to crawl through! It 



unitedly they will ex- 



seems to be a general truth that when cee( j ; t Linear ve- 



locity will be lower 

 in the branches than 

 in the parent stock. 



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 quickening 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 vense 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 resistance. 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 th'e- artery in a pulsating jet 



