54 



PHYSIOLOGY. 



tubes, digestive tube, etc., they are sometimes called visceral muscles in 

 distinction from the skeletal muscles. 



The Circulation of Blood in the Web of a Frog's Foot. For this 

 get a frog with a pale web. Take a piece of shingle six inches long 

 and three inches wide. Cut a round hole, half an inch in diameter, 

 near one end of it. Wrap the frog in a wet cloth, with one leg project- 

 ing, 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. 

 Keep the web moist. Place the shingle firmly on the stage of a micro- 

 scope. 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 net- 

 work 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, advancing with a hesi- 

 tant motion, seeming to roll along against 

 the wall of the capillary. 



Close your eyes for a moment, and re- 

 flect that in all the active tissues of your 

 body for example, the muscles, brain, and 

 digestive organs there is a similar net- 

 work 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 cor- 

 puscles. The blood really is moving slowly in the capillaries, and it is 

 very important 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 surrounding tissues. All 

 the other parts of the circulatory system exist for the purpose of send- 

 ing a continuous, slow, and steady stream of blood through the 

 capillaries. (See pages 72 and 73.) 



Fig. 24. Capillary Blood 

 Tubes of Muscle. 



