316 STEUCTUEE AND ADAPTATION OF TEETH 



succeed each other from behind forwards, moving 

 not in a right line, but in the arc of a circle. 

 The position of the growing tooth in the closed 

 alveolus is almost at right angles with that of the 

 molar in use ; the grinding surface being at first 

 directed backwards in the upper jaw, and forwards 

 in the lower jaw, and brought by the revolving 

 course into a horizontal line in both jaws, so that 

 they oppose each other when developed for use. 

 The bony plate forming the sockets is more than 

 usually distinct from the body of the maxillary, 

 and participates in this revolving course, advan- 

 cing forwards with the teeth. The partition 

 between the tooth in use and its successor is 

 perforated near the middle, and in its progress 

 forward the part next the grinding surface is first 

 absorbed, the rest disappearing with the absorp- 

 tion of the roots of the preceding grinder. From 

 this mode of advance, the upper molars are 

 convex, and the lower concave, from behind for- 

 wards." The first molar, cut a fortnight after 

 birth, is shed at two years old. Previously to this 

 time the second molar has been for some time in 

 use, and is in turn shed at the end of the fifth 

 year. The third molar attains its full growth 

 between the third and sixth, and is shed at the 

 ninth year. 



Professor Owen regards these three as the re- 

 presentatives of the deciduous molars of the other 

 animals. The fourth, fifth, and sixth molars re- 



