CIRCULATION OF THE BLOOD. 



55 



Vein 



Artery 



Fig. 25. 



Cross-section of Small Artery 

 and Vein. 



The Blood Flow in the Capillaries. The arteries 



divide and subdivide, and become capillaries, which have 



connecting branches, form- 

 ing a close network of tiny 

 thin-walled tubes. These 

 penetrate nearly every .tis- 

 sue of the body. The blood 

 cannot do its full work till it 

 is in the tissues, and to reach 

 the tissues it must soak 

 through the walls of the 

 capillaries. The work of 

 the heart and arteries is to 

 keep a steady flow of blood 



through the capillaries, that the tissues may be constantly 



supplied. 



How is it that the jerky action of the heart, at each 



contraction sending a jet of 



blood into the arteries, 



shown by a spurt when an 



artery is severed, and also 



indicated by the intermit- 

 tent pulse, how is this 



intermittent flow converted 



into the steady, uniform 



current that we have seen 



in the capillaries ? 



Experiments illustrating the 

 Blood Flow in the Capillaries. 

 A few experiments may make this 



Surface View 



Longitudinal Section 



Capillaries, composed of a single 

 layer of cells. 



Fig. 26. 

 matter more clear. 



Material : I . A common rubber syringe. 



2. A glass tube three feet long and seven sixteenths of an inch 

 outside diameter. 



