DIGESTION. 153 



As a result of the action of these fluids the nutritive principles are 

 prepared for absorption into the blood; the non-nutritive principles, 

 along with certain waste products, pass into the large intestine to be 

 finally extruded from the body. 



MOUTH DIGESTION. 



The digestion of the food as it takes place in the mouth comprises 

 a series of physical and chemic changes, the result of the action of 

 the teeth, the tongue, and the saliva. The mechanic division of the 

 food and the incorporation of the saliva with it are termed respec- 

 tively mastication and insalivation. 



MASTICATION. 



Mastication is the mechanic division of the food, and is accom- 

 plished by the teeth and the movements of the lower jaw under the 

 influence of muscle contractions. Complete mechanic disintegration 

 of the food is essential to its subsequent solution and chemic trans- 

 formation; for when finely divided it presents a larger surface to the 

 action of the digestive fluids and thus enables them to exert their 

 respective actions more effectively and in a shorter period of time. 



The Teeth. In man passing from childhood to adult life two 

 sets of teeth make their appearance. The first set constitute the 

 temporary, deciduous, or milk teeth; the second set constitute the 

 permanent teeth, which should last with proper care through life or 

 to an advanced age. 



The temporary teeth, twenty in number, ten in each jaw, though 

 smaller than the permanent teeth, have the same general conforma- 

 tion. They are divided into four incisors, two cuspids or canines, 

 and four molars for each jaw. 



The permanent teeth, thirty-two in number, sixteen in each jaw, 

 are divided into f our incisors, two cuspids or canines, four bicuspids 

 or premolars, and six molars for each jaw. 



Each tooth may be said to consist of three portions: (i) the 

 crown, or that portion which projects above the gums; (2) the root 

 or fang, that portion embedded in the alveolar socket; (3) the con- 

 stricted portion or neck, which is surrounded by the free margin of 

 the gum. The teeth are firmly secured in their sockets by a fibrous 

 membrane, the peridental membrane, which is attached, on the one 

 hand, to the alveolar process, and, on the other, to the cementum. 



A vertical section of a tooth shows that it consists of three distinct 

 solid structures, the enamel, the dentine, and the cementum, which 

 have the anatomic relationship as represented in Fig. OcT" In the 

 center of the dentine there is a cavity the general shape of which varies 

 in different teeth, and which is occupied during the living condition by 

 the tooth pulp. 



Microscopic examination of the tooth reveals the presence of 



