THE CIRCULATION. 



313 



700 times greater than that of the first portion of the aorta; there- 

 fore, the velocity of the blood in the capillaries is but 1 / 300 or 1 / 700 

 of that in the aorta. 



Physics show that there is no friction between the walls of the 

 vessels and the contained liquid. The exterior layer of the liquid 

 is adherent to the inner surface of the tube and remains perfectly 

 motionless. Between this still layer and the center of the current 





Fig. 104. Ludwig's Stromuhr. (LANDOIS.) 



there are layers of molecules which by molecular cohesion introduce 

 resistances. The next layers adhere to one another less and less the 

 more central they are. Thus the swiftness of the liquid molecules 

 will not be the same in all parts of the vessel, the maximum being 

 reached at the center of the vessel. 



Rate in the Arteries. From the relation of the arteries to the 

 main central pump, the heart, the velocity of the blood-flow in 

 them is very naturally greater than in the capillary or venous sys- 

 tems. In rough terms, the average velocity in the large arteries is 

 12 inches per second. To measure the velocity we employ Ludwig's 



