THE CIRCULATION 



105 



and the walls of the capillaries are so thin that they offer 

 no hindrance to the passage of the oxygen into the cells in 

 the tissues. Carbon dioxid, which is one of the products 

 of the combination of oxygen with the food material in 

 the tissues, is also a gas, and it passes back through 

 the capillary walls into the blood, which takes it back 

 to the heart, and thence to the lungs to be sent out of 

 the body. 



176. Why cannot the capillaries themselves carry the 

 food into the tissues ? Because they are tubes, and as long 

 as the food is in the blood it 



cannot reach the cells ; the 

 capillary walls prevent. The 

 lymph spaces and the lym- 

 phatics act as middlemen be- 

 tween the blood and the cells 

 (Fig. 97). 



177. Necessity for the Lym- FIG. 97. Diagram to show Function 



phatic System. If the plasma 

 kept coming into the tissues 

 without any way of getting back into the blood vessels, 

 the blood would soon be lacking in plasma and the tissues 

 would be oppressed with it. We see, then, the absolute 

 necessity for some provision to get this liquid bark into 

 the blood vessels, from which it is constantly overflowing, 

 This is done by a system of tubes called the lymphatics 

 (Fig. 98). 



178. What is lymph ? The blood plasma is called 

 lymph after it gets out of the capillaries. But it soon be- 

 comes changed by the addition of substances thrown out 

 by the cells, and by giving up to the cells the digested 

 food brought by the blood. We should have said also that 

 the white blood corpuscles may pass out into the lymph, 

 especially if there is some condition in the tissues that they 

 can correct (Fig. 82). 



of Lymph. 

 A, tissue cells ; C, capillary; L, lymphatic. 



