THE CIRCULATION 



63 



(lymphatics), and lymphatic glands. Lymph corresponds 

 nearly to the blood without the red corpuscles. It is the 

 familiar liquid seen in a blister, or oozing out where the 

 skin has been grazed without breaking a blood vessel. 



Necessity for Lymph and Lymph Spaces. — The body 

 cannot be nourished with the albumin, sugar, oxygen, and 

 other digested food in the blood, until this food passes out 

 of the blood vessels. The food leaves the blood through 

 the thin walls of the capillaries. Many of the cells do not 

 touch the capillaries, and the lymph penetrates the spaces 

 between the cells to reach them (see colored Fig. 3). If 

 there were no lymph spaces, these cells could not get any 

 food. The lymph bathes the cells, and the cells absorb 

 what they want from the nourishing fluid. The red corpus- 

 cles bearing the oxygen cannot pass through the capillary 

 walls. Oxygen, being a gas, readily passes through the 

 walls and reaches the cells through the lymph in the 

 lymph spaces. The waste materials must go back into the 

 blood ; carbon dioxid passes back through the capillary 

 walls and is taken to the lungs ; how the other waste 

 materials formed in the cells pass back will soon be 

 explained. 



Need of Lymphatics. — The plasma continually passes 



into the tissues, but it cannot return directly into the blood. 



The lymph contains waste material which must be 



removed, and also much unused food which nature, like an 



economical housekeeper, will offer to the tissues again. 



There are vessels called lymphatics that take the lymph back 



into the blood (see Fig. 64). 



The Lymphatic Circulation (Fig. 64). — The blood flow does not 

 begin nor end. but makes a never ending circle. The countless 

 lymphatics begin, with open ends. 1/1 the lymph spaces between the cells 

 (colored Fig. 3). The smaller lymphatics unite into larger ones until 

 finally they all unite into two large ones that empty into the large veins 



