CHAPTER X 



SPECIAL MUSCULAR MECHANISMS 



Mastication. The teeth of different varieties of mammalia 

 are adapted to the work which they have to perform. In her- 

 bivora they serve to grind the food; in carnivora to tear and cut 

 the food; and in man, omnivorous, they are adapted to serve 

 both grinding and cutting purposes. In man there appear 

 in succession two sets of teeth. The first set are temporary 

 and consist of twenty teeth four incisors, two canines, and 

 four molars on each side. They appear in order central in- 

 cisors at the seventh month after birth, the other incisors at 

 the ninth month, the canines at the eighteenth month and 

 the molars at the twelfth and twenty-fourth months respec- 

 tively. The teeth in the lower jaw appear a little before the 

 corresponding ones of the upper jaw. The second set of teeth 

 or permanent set begin to replace the temporary set between 

 the sixth and seventh years. There appear, in addition, twelve 

 true molars, making the total number of teeth thirty-two. 

 The wisdom teeth appear about the twenty-fifth year. 



In order that the food may readily be swallowed and acted 

 upon by the digestive juices it is finely divided by the action 

 of the jaws, which, by means of the incisors and canines, cut 

 the food, and by means of the bicuspids and molars, crush it. 

 The lower jaw is raised by the masseter, temporal, and internal 

 pterygoid muscles; depression is largely passive, but is aided 

 by the digastrics and slightly by the mylohyoid and geniohyoid 

 muscles. When the infrahyoid group fix the hyoid bone, all 

 the muscles passing between it and the mandible act to depress 

 the latter; it is moved laterally by the external pterygoids act- 

 ing separately, and forward by their joint action, and is retracted 

 by the horizontal fibers of the temporals. The action is volun- 

 tary, the impulses coming through the trigeminal and hypo- 



