ANATOMY AND PHYSIOLOGY 



9 



digestion. This is to be done in the form of a table such as the one 

 shown in Fig. 12. 



Fig. 12. — Form of table for record of Exercise 9. 



Exercise 10. — The Absorption of Food. 



(i) Food in the cavity of the digestive tract is surrounded by the 

 mucous membrane which lines the digestive tract. The thin-walled 

 capillaries and lymphatics which carry blood and lymph, respectively, 

 run in the submucosa (Fig. 13). The food must pass through the 

 mucous membrane and into the capillaries and lymphatics before it 

 can be distributed by the circulatory fluids to all parts of the body. 

 The mucous membrane and the walls of the blood and lymph vessels 

 are semipermeable membranes, that is, some substances can pass 

 through them but others cannot. Some foods are in a condition to be 

 absorbed when they enter the body; others must be changed by di- 

 gestion into substances which can pass through the mucous membrane 

 and the walls of the capillaries and lymphatics. Parchment and simi- 

 lar animal membranes which are no longer alive act as semipermeable 

 membranes and can be used in the construction of a simple apparatus 

 to demonstrate the passage of diffusible food substances through such 

 membranes. A piece of the parchment is tied over the end of a good- 

 sized tube. Into this tube is put a solution of the food to be tested, 

 and the tube is then lowered into a larger vessel containing water. 

 After some time, tests are made of the water in the outer vessel to 

 determine whether any of the substance in the inner tube has passed 

 through the parchment membrane. 



(j) Such an apparatus will be constructed and demonstrated be- 

 fore the class, and the tests to determine the presence of the substances 



