BLOOD CAPILLARIES. 



Wrap the frog in a wet cloth, with one leg projecting, and 

 tie it, thus wrapped, to the shingle. Tie threads around two 

 of the toes, and stretch the web, but not too tightly, over the 

 hole. Place the shingle firmly on the stage of a microscope. 

 Examine first with a low power. The large tubes which grow 

 smaller by subdivision are Arteries. The large tubes which 

 are formed by the union of smaller ones are the Veins. The 

 finer tubes, forming a network in every direction, are the 

 Capillaries. They receive the blood 

 from the arteries, and pass it on to 

 the veins. 



Put on a higher power, a one-fifth 

 or one-sixth objective. It may now 

 be seen that the colored corpuscles 

 float more in the center of the 

 stream, and with a steady motion, 

 while the colorless corpuscles keep 

 close to the walls of the capillary, 

 and seem to adhere to them, ad- 

 vancing w r ith a hesitant motion, 

 seeming to roll along against the 

 wall of the capillary. 



Close your eyes for a moment, 

 and reflect that in all the active 



tissues of your body for example, the muscles, brain, and 

 digestive organs there is a similar network of fine tubes 

 with a current of blood running through them. The current 

 is not so rapid as it seems, for the microscope magnifies the 

 rate of flow as well as the size of the corpuscles. The blood 

 really is moving slowly in the capillaries ; and it is very im- 

 portant that it should be so, for in the capillaries the work 

 of the blood is done. Part of the liquid of the blood soaks 

 through the thin walls of the capillaries, and nourishes the 



Fig. 18. Capillary Blood Tubes 

 of Muscle. 



