300 



DIGESTION AND ABSORPTION 



starch paste, saliva, and a few 

 drops of any weak acid, the starch 

 will be found not to 'have changed. 

 The digestion or change of starch 

 to grape sugar is caused by the 

 presence in the saliva of an enzyme, 

 or digestive ferment. You will re- 

 member that starch in the grow- 

 ing corn grain was changed to 

 grape sugar by an enzyme called 

 diastase. Here a similar action is 



caused by an enzyme called ptyalin. 



f This ferment acts only in an alka- 



Expenment showing non-osmosis of ^ 



starch in tube A, and osmosis of line medium at about the tempera- 



sugar in tube B. 



ture of the body. 



Mouth Cavity in Man. 

 In our study of a frog we 

 find that the mouth cavity 

 has two unpaired and four 

 paired tubes leading from 

 it. These are (a) the gullet 

 or food tube, (6) the wind- 

 pipe (in the frog opening 

 through the glottis), (c) the 

 paired nostril holes (pos- 

 terior nares), (d) the paired 

 Eustachian tubes, leading to 

 the ear. All of these open- 

 ings are found in man. 



In man the mouth cavity, 

 and all internal surfaces of 

 the food tube, are lined 

 with a mucous membrane. 

 The mucus secreted from 

 gland cells in this lining 

 makes a slippery surface so 



It 



The mouth cavity of man. e, Eustachian 

 tube; hp, hard palate; sp, soft palate; 

 ut, upper teeth ; be, buccal cavity ; It, lower 

 teeth ; t, tongue ; ph, pharynx ; ep, epiglot- 

 tis; lx, voice box; oe, gullet; tr, trachea. 



