150 HOW FOOD IS PREPARED FOR BODY USES 



similar action is caused by an enzyme called ptyalin (ti'd-lin), or 

 salivary am'ylase; but this enzyme acts only in. an alkaline medium 

 at about the temperature of the body. 



Mouth Cavity in Man. — In a study of a frog we can find that 

 the mouth cavity has six tubes opening from it (figure on page 149) . 

 These are (a) the gullet or esoph'agus, (b) the windpipe or trachea 

 (tra'ke-d), opening through the glottis, (c) the pair-ed nostril hole^ 

 (posterior nares) , (d) the paired Eustachian (u-sta'ki-dn) tubes, lead- 

 ing to the ear. All of these 

 openings are found also in man. 

 In man the mouth cavity 

 and the internal surface of 

 the food tube are lined with 

 mucous membrane. The mucus 

 secreted from gland cells in this 

 lining makes a slippery surface 

 so that the food can slip down 

 easily. The roof of the mouth 

 is formed by a plate of bone 

 called the hard palate, in front, 

 and a softer continuation to 

 the back, called the soft palate. 

 These separate the nose cav- 

 ity from that of the mouth. 

 The space back of the soft 

 palate is called the pharynx, 

 or throat cavity. From the 

 pharynx lead off the gullet and 

 windpipe, the former back of 

 the latter. The lower part of 

 the mouth cavity is occupied by a muscular tongue. The tongue 

 Ls used in moving food about in the mouth, and in starting it on its 

 way to the gullet ; it also plays an important part in speaking. 



The Teeth. — In man the teeth, unlike those of the frog, are 

 used in the mechanical preparation of the food for digestion. In- 

 stead of holding prey, they crush, grind, or tear food so that more of 

 its surface may be given for the action of the digestive fluids. The 

 first or " milk " teeth of man are only twenty in number, while in 



Mouth cavity of man. e, Eustachian 

 tube ; Hp, hard palate ; sp, soft palate ; 

 Ut, upper teeth ; Lt, lower teeth ; T, 

 tongue ; Ph, pharynx ; Ep, epiglottis ; Lx, 

 larynx or voice box ; E, esophagus or 

 gullet; Tr, trachea. 



