DIGESTION. 25 



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

 by the hands, lips and teeth. 



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 greater 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 imbedded 

 in the alveolar process ; a section through a tooth shows that its substance 

 is made of dentine, in the centre 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 antero-posterior movement, dependent upon the construction 

 of the temporo-maxillary articulation. 



The jaw is depressed by the contraction of the digastric, genio-hyoid, 

 mylo-hyoid and platysma myoitles 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 posteriorly by the united actions of the genio-hyoid, mylo' 

 hyoid and posterior fibres of the temporal muscle. 



The food is kept between the teeth by the intrinsic and extrinsic mus- 

 cles 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 centres 

 involved in this mechanism are shown in the following table : 



NERVOUS CIRCLE OF MASTICATION. 



AFFERENT OR EXCITOR NERVES. EFFERENT OR MOTOR NERVES. 



1. Lingual branch of 5th pair. I. 3d branch of 5th pair. 



2. Glosso-pharyngeal. 2. Hypo-glossal. 



3. Facial. 



The impressions made upon the terminal filaments of the sensory nerves 

 are transmitted to the medulla ; motor impulses are here generated which 

 C 



