IO4 



ELEMENTS OF PHYSIOLOGY 



any force to aid the heart? No. When the arteries 

 contract as the pulse passes, it is the force of the heart 

 that is doing it. When you open a door or gate that 

 is closed by an elastic spring, it is the force furnished 

 by you to the spring that shuts the door after you 

 release your hold. 



THE LYMPHATIC SYSTEM 



174. How the Nourishment gets from the Blood into the 

 Tissues. We left the food and oxygen in the capillaries. 

 How does it get out of them into the tissues ? We found 

 that the blood flows very slowly in the capillaries (g 1 ^ of an 



inch per second), and that the 

 capillary walls are very thin, 

 being made of only the inner 

 of the three coats that make 

 the veins and arteries. Here, 

 then, are two favorable con- 

 ditions for giving up the nu- 

 trition (Fig. 96). We learned 

 that the albumin, sugar, and 

 fats were dissolved in the 

 plasma, or liquid portion of 

 the blood. The plasma 

 passes through the thin cap- 



Capillaries connecting a small artery, a, iH ary walls, Carrying the 

 with a small vein, b. J J 



food with it. When it gets 



outside the capillaries, it is next to the walls of the 

 cells that make up the tissues. These spaces are called 

 lymph spaces. Thus the lymph bathes the cells in the 

 nutritious fluid, and the hungry cells absorb what they 

 need. 



175. The red corpuscles bearing the oxygen cannot get 

 through the capillary walls. Oxygen is a gas, however, 



FIG. 96. 



