DIGESTION. 59 



mouth, the pharynx, or throat, and the gullet, a long, narrow 

 tube, passing down through the back part of the thorax and 

 piercing the diaphragm, where it dilates to form the stomach. 



2. The Mouth. The cavity of the mouth has for its 

 boundaries the lips and cheeks in front and at the sides; 

 below, the tongue and lower jaw; and above, the palate. 

 The palate consists of two portions; the front part, resting 

 on the upper jaw, is the hard palate, or roof of the mouth. 

 It separates the mouth from the nasal cavity. The back 

 part, the soft palate, consists of mucous membrane folded 

 upon itself. It is continuous with the floor of the nasal 

 cavity. The soft palate arches downward, and forms a par- 

 tition between the mouth and the pharynx. In the middle of 

 the lower border of the soft palate is a prolongation, like an 

 inverted cone, the uvula, often called the palate. 



3. Mastication. The first steps in the process of diges- 

 tion are taken in the mouth. The food is ground up into a 

 pulpy mass by the teeth. While this process called mastica- 

 tion goes on, the saliva is poured into the mouth and mixes 

 with the food. The tongue also aids, in a mechanical way, 

 by keeping the food between the teeth. 



4. The Teeth. These hard, bone-like structures do not 

 appear until some months after birth. They are arranged in 

 two semi-circular rows, the upper and lower teeth. The former 

 are firmly planted in the borders of the upper jaw, the latter 

 in the lower jaw. In infancy, at about the seventh month, 

 the front teeth begin to appear. The point of the tooth 

 gradually pierces the somewhat dense mucous membrane form- 

 ing the gum, and one after another is cut, until the child, 

 at two years of age, has twenty teeth. But this set of teeth, 

 called the temporary, or milk set, is short-lived. They are 

 all cast off during childhood, and are followed by a new set. 

 These also make their appearance gradually. The same num- 

 ber take the place of the temporary set, and three others are 



