MASTICATION. 



171 



Mastication. The chewing of the food or mastication is 

 performed by the teeth. 



The first set of teeth, which are known as temporary, decidu- 

 ous, or milk-teeth (Fig. 93), and which exist during early child- 

 hood are twenty in number, and the second or permanent set 

 (Fig. 95), which begin to take the place of the first set at about 

 the sixth year of life, remain to a greater or lesser extent until 

 old age. The latter set is composed of thirty-two teeth four 

 incisors, two canines, four bicus- 

 pids, and six molars in each jaw 

 (Fig. 95). The incisors, or cut- 

 ting teeth, are adapted to bite the 

 food ; the molar teeth, or grinders, 

 are adapted to grind the food, while 

 the canines and bicuspids in man 

 aid the incisors and molars. In 

 the carnivora, the canines or 

 tushes as they are called are 

 very long and pointed, and are 

 admirably adapted to pierce the 

 body of their prey, even to the 

 vitals, thus killing and subse- 

 quently tearing the animal pre- 

 paratory to feeding upon it. The 

 herbivora need no such aggres- 

 sive weapons, and in them the 

 molars are so constructed as to 

 grind the food, their teeth resem- 

 bling the grindstones of the mil- 

 ler. The teeth of man have char- 

 acters which resemble those of 

 both carnivora and herbivora, and 

 from this fact it may be inferred 

 that it was designed that man 

 should have a mixed diet. 



Movements of Mastication. 

 The movements concerned in 

 mastication are those of the lower jaw upon the upper, produced 

 by the action of the following muscles : Masseter, temporal, 

 internal and external pterygoids, digastric, mi/lohi/oid, and genio- 

 hyoid. The lower jaw is brought against the upper by the con- 

 traction of the masseters, temporals, and internal pterygoid mus- 

 cles. The action of the external pterygoids, when both are 

 acting, is to draw the lower jaw forward, causing it to project 

 beyond the upper. If but one external pterygoid acts, that side 

 of the jaw is drawn forward and the chin deviates to the oppo- 

 site side. 



FIG. 95. The palate and superior 

 dental arch (right side) : 1, median 

 incisors ; 2, lateral incisors ; 3, canine ; 

 4, first bicuspid; 5. second bicuspid; 

 6, first molar; 7, second molar; 8, 

 wisdom-tooth ; 9, mucous membrane 

 of the hard palate continuous behind 

 with that of the soft palate ; 10, the 

 anteroposterior raphe of palate ; 11, 

 pits on each side of the raphe per- 

 forated with the orifices of glands; 

 12, anterior rugosities of the mucous 

 membrane (after Testut). 



