PROBLEM 3. Hov) Materials Are Moved to and from Cells 



Tissue cells Capillaries 



221 



Fig. 223 Body cells are surrounded by lymph. Find the tubes (lymphatics) through 

 which ly^nph is returned to the blood. 



puscles too. Thev are constantly pushing 

 their way through the walls of the capil- 

 laries by means of their pseudopods. Like 

 plasma, lymph is an everchanging mix- 

 ture of water, digested foods, wastes, and 

 other substances. Various gases are dis- 

 solved in it. Its composition in different 

 parts of the body varies according to the 

 activities of the nearby cells. 



Substances could not move into and 

 out of cells were it not for lymph, since 

 diffusion occurs only when there is a 

 liquid on both sides of a membrane. That 

 is, lymph makes possible a constant in- 

 terchange of substances between tissue 

 cells and capillaries. 



How lymph is removed from tissues. 

 After washing over the cells some of 

 the lymph diffuses back into the capil- 

 laries. But much of it is drained off by 

 special thin-walled vessels which carry 

 nothing but lymph. They are called lym- 

 phatics. Tiny lymphatics are found 

 among the cells in all parts of the body. 

 They meet, join together, and form ever 

 wider and wider tubes. Finally, one 

 good-sized lymphatic (the thoracic duct) 

 connects with one of the large veins on 

 the left side of the body under the collar 



bone. A second one connects with a vein 

 on the right side. The lymph enters the 

 blood at these points. 



If you now look back at the diagram 

 of a villus (see Fig. 206) you will see 

 tubes called lacteals (lac'tea'ls). Lacteals 

 are microscopic lymphatics which con- 

 nect with other lymphatics. Digested 

 fats pass into the lacteal instead of into 

 the capillaries. In this way absorbed fats 

 reach the blood stream in a roundabout 

 way, by means of the lymph system. 



Lymph in the lymphatics is pushed 

 along by the pressure of the liquid be- 

 hind. Since there is no pumping, the 

 lymph sometimes tends to stand still. 

 Exercise of the whole body helps to 

 prevent this just as in the case of blood 

 in the veins. And, as in the veins, there 

 are pocketlike valves along the length of 

 the lymphatics which keep the lymph 

 from flowing backward. The movement 

 of lymph is slower than the movement 

 of blood. 



Lymph glands and the spleen. Along 

 the course of the medium-sized lym- 

 phatics there are roundish bodies called 

 lymph glands or lyjnph nodes as Figure 

 224 shows. These act as sieves separating 



