APPENDIX 487 



able to buy prepared specimens. These may be obtained at 

 slight expense from dealers in microscopic supplies. Such 

 specimens, as well as sections of various organs, are very neces- 

 sary, as it is only by a clear comprehension of the structure of 

 the different tissues and of the organs which they compose that 

 the student can understand the functions of the various parts. 



CHAPTER XIII 



The principal chemical changes taking place during digestion 

 in the higher animals may be illustrated with very simple appa- 

 ratus, and at the cost of but little time. It is not necessary that 

 the student possess any knowledge of chemistry. The object 

 of digestion, viz., the changing of substances which are in- 

 capable of absorption into substances which may be absorbed, 

 can be made plain even to the youngest student. The chemi- 

 cals needed may be obtained of any druggist. 



The" following experiments deal with the three principal di- 

 gestive fluids, viz., saliva, gastric juice, and pancreatic juice ; 

 and with the main kinds of foods, i.e., starchy, albuminous, 

 and fatty substances. 



SALIVARY DIGESTION 



(i) The microscopical appearance of undigested starch and its 

 reaction with iodine 



Into a test tube about one fourth full of water put a pinch of 

 corn starch and shake the tube. Notice that the starch does 

 not dissolve. Examine a drop of the mixture under a micro- 

 scope and note the starch grains floating about in the water. 

 Add a drop or two of dilute iodine solution to the mixture in 

 the tube and note that it turns a deep blue. Examine a drop 

 of this mixture under the microscope and note that each starch 

 grain has turned blue. 



Prepare another test tube with water and starch, and boil the 

 mixture in the flame of an alcohol lamp or of a Bunsen burner, 

 keeping the tube agitated all the time in order to prevent the 



