74 THE VITAL PROCESSES 



the best explanation offered as yet for this passage is that the colloidal 

 substances are changed by the cells lining the capillaries into sub- 

 stances that form true solutions and that after the passage they are 

 changed back again to the colloidal condition. 



Summary. Between the cells and the capillaries is a 

 liquid, known as the lymph, which is similar in composi- 

 tion and physical properties to the blood. It consists 

 chiefly of escaped plasma. The vessels that contain it 

 are connected with the system for the circulation of the 

 blood. By adding new material to the lymph and with- 

 drawing waste material from it, the blood keeps this liquid 

 in a suitable condition for supplying the needs of the cells. 

 Supplementing each other in all respects, the blood and 

 the lymph together form the nutrient cell fluid of the 

 body. The interchange of material between the blood 

 and the lymph, and the lymph and the cells, takes place 

 in part according to the principle of osmosis. 



Exercises. I. Explain the necessity for the lymph in the body. 



2. Compare lymph and water with reference to density, color, and 

 complexity of composition. 



3. Compare lymph and blood with reference to color, composition, 

 and movement through the body. 



4. Show how blood pressure in the capillaries causes a flow of the 

 lymph. 



5. Show how contracting muscles cause the lymph to move. Com- 

 pare with the effect of muscular contraction upon the blood in the 

 veins. 



6. Trace ihe lymph in its flow from the right hand to where it enters 

 the blood ; from the feet to where it enters the blood. 



7. What conditions prevail at the cells to cause a movement of food 

 and oxygen in one direction and of waste materials in the opposite 

 direction? 



8. What part does water play in the exchanges at the cells? 



9. Show that the blood and the lymph together fulfill all the re- 

 quirements of a nutrient cell fluid in the body. 



