1M DIGESTION. 



but in the lower jaw it generally has two distinct branches. 

 The first molars are the first of the permanent teeth ; making 

 their appearance between the sixth and the seventh year. 

 The second molars appear from the twelfth to the thirteenth 

 year; and the third molars from the seventeenth to the 

 twenty-first year, and sometimes even much later. In 

 some instances the third molars are never developed. 



The upper jaw has ordinarily a somewhat longer and 

 broader arch than the lower; so that when the mouth is 

 closed the teeth are not brought into exact apposition, but 

 the upper teeth overlap the lower teeth both in front and lat- 

 erally. The lower teeth are all somewhat smaller than the 

 corresponding teeth in the upper jaw, and generally make 

 their appearance a little earlier. 



The physiological anatomy of the maxillary bones and of 

 the temporo-maxillary articulation necessarily precedes the 

 study of the muscles of mastication and the mechanism of 

 their action. 1 



The superior maxillary bones are immovably articulated 

 with the other bones of the head, and do not usually take 

 any active part in mastication ; but their inferior borders, 

 with the upper teeth embedded in the alveolar cavities, 

 present fixed surfaces against which the food is pressed by 

 the action of the muscles which move the lower jaw. 



The inferior maxilla is a single bone. Its body is hor- 

 izontal, of a horse-shoe shape, and in the alveolar cavities in 

 its superior border are embedded the lower teeth. Below 

 the teeth, both externally and internally, are surfaces for the 

 attachments of the muscles concerned in the various move- 

 ments of the jaw, and one of the muscles of the tongue. 



Behind this horizontal body, on either side, is a vertical 



1 The mechanism of mastication in the human subject is more complex than 

 in any of the inferior animals ; and it is absolutely necessary to enter into tolera- 

 bly minute details of the anatomy of the parts concerned in this function, in or- 

 der to comprehend their physiology. 



