172 MOUTH DIGESTION. 



The digastric, rnylohyoid, and geniohyoid muscles depress the 

 lower jaw, and this is an essential part of mastication, for the 

 jaws must be separated as well as brought together to make the 

 act complete. 



The movements of the jaw are a combination in which these 

 muscles act, sometimes grouped in one way and sometimes in 

 another. The buccinator muscles in the cheeks and the muscles 

 of the tongue assist very materially by keeping the food between 

 the teeth, where it may be comminuted and triturated. 



The function of the teeth in man is to subdivide and com- 

 minute thoroughly the food ; and this function is an essential part 

 of the process of digestion. As will be seen later, during diges- 

 tion certain fluids are poured into the alimentary canal to contrib- 

 ute their part toward the process. These fluids cannot act 

 properly on large, compact masses of food. While their action is 

 not that of solution, still, in order to fulfil perfectly their office, 

 they must come in direct contact with every portion of the food. 

 This contact is the more essential because the given time in which 

 to act is not unlimited, and if the process is not completed within 

 the allotted time, digestion will be performed incompletely. 

 When a chemist desires to dissolve a substance quickly and com- 

 pletely, he first pulverizes it in a mortar. Likewise, in digestion 

 one of the most important steps is this process of comminution 

 or mastication. If mastication is insufficiently performed, the 

 succeeding steps in the process of digestion are seriously interfered 

 with, and indigestion or dyspepsia results. f 



Insufficient mastication is one of the commonest causes of in- 

 digestion, and many dyspeptics are drugged with remedies pre- 

 scribed to overcome some fancied trouble in the stomach, wjien 

 they should be sent to a dentist. Defective mastication may be 

 due to various causes. The teeth may be so decayed as to expose 

 sensitive surfaces, and when food which is at all hard is taken into 

 the mouth the discomfort, or sometimes the pain, caused their 

 possessor in chewing it makes the performance of the act incom- 

 plete, and the food is swallowed half-masticated ; or the eater may 

 be in too great a hurry and not give enough time to this important 

 act. Whatever the cause, the result is the same; therefore too 

 much attention cannot be given to this process, which is so simple 

 as often to be overlooked. 



Insalivation. Coincident with mastication is the act of in- 

 salivation or the incorporation of saliva with the food. Saliva is 

 the mixed secretion of the salivary glands, which comprise the 

 two parotids, the two submaxillaries, and the two sublinyuals, 

 together with the buccal glands of the mucous membrane of the 

 mouth (Fig. 96). 



Physiologic Anatomy of the Salivary Glands. The parotid 

 gland is the largest of all the salivary glands, and is named from 



