CHAP. XXIII.] MOVEMENTS OF MASTICATION. 181 



a higher secondary law, to which to refer these wonderful pheno- 

 mena of life. 



The periods of eruption of the permanent teeth, though liable, like 

 those of the milk teeth, to some variety, are, according to Mr. Bell, 

 usually as follow : the anterior true molars at 6-J- years of age, the 

 central incisors at 7, the lateral ones at 8, the anterior and posterior 

 bicuspids at 9 and 10, the canines from 11 to 12, the second true 

 molars from 12 to 13, and the wisdom teeth from 17 to 19. 



Of the Jaw-bones at different Ages. The bones undergo some inter- 

 esting changes of form in connexion with the growth and decay of 

 the teeth, which have been well explained by Hunter. The alveolar 

 processes in both jaws appear with the teeth, and disappear when no 

 longer needed to support and enclose them. In the foetus, before 

 the eruption of the teeth, the upper gum is about on a line with the 

 articulation of the jaw, the lower, consequently, is nearly on the same 

 line, and the angle of the jaw very obtuse. But as the teeth pro- 

 trude, and increase in number, the lower jaw is separated from the 

 upper by the depth of the alveoli and crowns of the teeth of both 

 jaws, the body and ascending ramus are both lengthened, and the 

 angle approaches nearly to a right angle. When the teeth are sub- 

 sequently shed, the alveoli disappear, and the lower jaw has to be 

 much more elevated in order to touch the upper. But as its body 

 and ramus cannot return to their former dimensions, its anterior 

 part is thrown a good deal beyond the upper in this action, and it 

 is only the hinder portions in the situation previously occupied by 

 the molar teeth which come into contact. 



Of the Articulation of the lower Jaw >, and the Movements of Mastica- 

 tion. A constant relation subsists in animals between the nature of 

 the food, the shape and structure of the teeth, and the articulations of 

 the jaw; so that, as Cuvier demonstrated, one of these elements being 

 known, the others may be more or less accurately inferred. Thus, 

 the purely carnivorous animals have teeth fitted to seize and lacerate 

 their food, and the jaw is capable only of the simplest hinge motion. 

 In the herbivorous families, 011 the contrary, teeth of a complex kind 

 are provided for pounding and bruising the food, and the joints are 

 so constructed as to allow of extensive sliding motions ; while in all 

 there is an inter-articular fibro- cartilage for protection under the ex- 

 treme pressure exerted. The form of the articulation in man, not 

 less than the dental series, denotes an intermediate condition,and forms 

 a strong physiological argument for the mixed diet, which general cus- 

 tom and taste have decided to be natural to our species. As there are 

 cutting, tearing, and grinding teeth, all in moderate proportions, of 



