Chap. XV] DIGESTIVE PROCESSES 289 



group different. For instance, n for the starch molecule is large, 

 while for the dextrin molecule it is smaller, so that a single starch 

 molecule in digestion may split into several molecules of dextrin 

 of the same relative composition. 



Fats. — Fats are composed of carbon, hydrogen, and oxygen, but 

 the two latter elements, hydrogen and oxygen, are not in the pro- 

 portion to form water. They are not simple substances, but are 

 mixtures of palmitin, stearin, and olein, which are derived from the 

 fatty acids named respectively palmitic, stearic, and oleic. Each 

 molecule of a simple fat is made from one molecule of glycerine 

 and three molecules of a fatty acid. 



Glycerine Stearic Acid Stearin Water 



C3H5(OH)3 + 3 H . C18H35O0 -> C3H5(CisH3502)3 + 3 H2O. 



In general, fats and oils are practically the same, and the mixture 

 of fats found in the body is liquid at the body temperature. They 

 are soluble in ether, chloroform, and hot alcohol, but are insolu- 

 ble in water. 



Decomposition of fats. — Under the influence of steam, mineral 

 acids, and certain ferments found in the body, fats split up into the 

 substances out of which they are built, i.e. glycerine and fatty 

 acid. 



Stearin Water Glycerine Stearic Acid 



C3H5(Ci8H3502)3 + 3 H2O -> C3H5(OH)3 + 3 H . C18H35O2. 



The process of saponification is similar to the above, only that 

 instead of water a base is used and the final products are glyce- 

 rine and soap. 



Stearin Potassium Hydroxide Glycerine Soap 



C3H5(Ci8H3502)3 + 3 KOH -^ C3H5(OH)3 + 3 K(Ci8H3502). 



Necessity for digestion. — Digestion is necessary because 

 organic foods, with the exception of simple sugars, are not soluble 

 in water, hence they cannot be absorbed. Inorganic foods are 

 absorbed directly, because salts dissolve in water. 



DIGESTIVE PROCESSES 



Digestion consists of two processes, i.e. mechanical and chemical. 



Mechanical processes. — Mechanical processes consist of vari- 

 ous movements that result from the action of the muscles in the 

 alimentary canal. They serve two important purposes: (1) in 



