86 HUMAN PHYSIOLOGY. 



DIGESTION. 



Digestion is a physical and chemic process by which the food introduced 

 into the alimentary canal is liquefied and its nutritive principles are trans- 

 formed by the digestive fluids into new substances capable of being absorbed 

 into the blood. 



The digestive apparatus consists of the alimentary canal and its appen- 

 dages viz., teeth, lips and tongue; the salivary, gastric and intestinal glands; 

 the liver and pancreas. 



Digestion may be divided into several stages; prehension, mouth digestion 

 (mastication and insalivation), deglutition, gastric and intestinal digestion, 

 and defecation. 



Prehension, the act of conveying food into the mouth, is accomplished by 

 the hands, lips, and teeth. 



MASTICATION. 



Mastication is the trituration of the food, and is accomplished by the 

 teeth and lower jaw under the influence of muscular contraction. When 

 thoroughly divided, the food presents a larger surface for the solvent action 

 of the digestive fluids, thus aiding the general process of digestion. 



The teeth are thirty-two in number, sixteen in each jaw, and divided into 

 four incisors or cutting teeth, two canines, four bicuspids, and six molars 

 or grinding teeth; each tooth consists of a crown covered by enamel, a neck, 

 and a root surrounded by the crusta petrosa and embedded in the alveolar 

 process; a section through a tooth shows that its substance is made of dentine, 

 in the center of which is the pulp cavity containing blood-vessels and nerves. 



The lower jaw is capable of making a downward and an upward, a lateral 

 and an anteroposterior movement, dependent upon the constructon of the 

 temporomaxillary articulation. 



The jaw is depressed by the contraction of the digastric, geniohyoid, 

 mylohyoid, and platysma my aides muscles; elevated by the temporal, masseter, 

 and internal pterygoid muscles; moved laterally by the alternate contraction of 

 the external pterygoid muscles; moved anteriorly by the pterygoid, and poste- 

 riorly by the united actions of the geniohyoid, mylohyoid, and posterior fibers 

 of the temporal muscles. 



The food is kept between the teeth by the intrinsic and extrinsic muscles 

 of the tongue from within, and the orbicularis oris and buccinator muscles 

 from without. 



