DIGESTION. 97 



tion. 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 loiver jaw is capable of making a downward and an upward, 

 a lateral and an anteroposterior movement, dependent upon the c6n- 

 struction of the temporomaxillary articulation. 



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

 mylohyoid, and platysma myoides muscles ; elevated by the temporal, 

 mas set er, and internal pterygoid muscles ; moved laterally by the alter- 

 nate contraction of the external pterygoid muscles ; moved anteriorly 

 by the pterygoid, and posteriorly by the united actions of the genio- 

 hyoid, mylohoid, 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. 



The movements of mastication, though originating in an effort of 

 the will and under its control, are, for the most part, of an automatic 

 or reflex character, taking place through the medulla oblongata and 

 induced by the presence of food within the mouth. The nerves and 

 nerve-centers involved in this mechanism are shown in the follow- 

 ing table : 



Nerve Mechanism of Mastication. 

 A fferent Nerves. Efferent Nerves. 



1. Lingual branch of 5th pair. i. Third branch of 5th pair. 



2. Glossopharyngeal. 2. Hypoglossal 



3. Facial. 



The impressions made upon the terminal filaments of the afferent 

 nerves are transmitted to the medulla ; motor impulses are here gen- 

 erated which are transmitted through motor nerves to the muscles 

 involved in the movements of the lower jaw. The medulla not only 

 generates motor impulses, but coordinates them in such a manner 

 8 



