BLOOD-FLOW IN CAPILLARIES. 



79 



Vein 



Artery 



Fig. 27. Cross Section of Small Artery 

 and Vein. 



The blood cannot do its work till it gets into 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 tis- 

 sues may be constantly sup- 

 plied. 



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 intermittent 

 pulse, how is this inter- 

 mittent flow converted into the steady, uniform current that 

 we have seen in the capil- 

 laries ? 



A few experiments may 

 make this matter more clear. 

 Material : 



1. A common rubber syr- 

 inge. 



2. A glass tube three 

 feet long and seven-six- 

 teenths of an inch outside 

 diameter. 



3. Four inches of the 

 same size glass tubing, for 

 making connections. 



4. Several nozzles, made 



of the same size glass tubing, all fine, but of varying degrees 

 of fineness. 



Surface View 



Nuclei 



Cross Sections 



Longitudinal Section 



Fig. 28. Capillaries, composed of a single 

 layer of cells. 



