CIRCULATION OF THE BLOOD. 



55 



Connective 



Tissuf 



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 Fig. 26. 



Surface View 



Longitudinal Section 



matter more clear. 



Capillaries, composed of a single 

 layer of cells. 



Material : i . A common rubber syringe. 



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

 outside diameter. 



