220 THE DIGESTIVE SYSTEM 



masticate its food satisfactorily while the deciduous molars 

 are being shed. The medial and lateral incisors appear 

 during the seventh and eighth years respectively, and are 

 followed by the first and second premolars, which erupt 

 during the ninth and tenth years. The premolars cannot 

 appear above the gum until the deciduous molars have been 

 removed or have dropped out. The interval between the first 

 premolar and the lateral incisor is filled by the canine, which 

 has usually erupted by the end of the twelfth year. The 

 second permanent molar varies somewhat in its time of ap- 

 pearance, and it is not unusual for its eruption to be delayed 

 till the fifteenth or sixteenth year. A similar variation is found 

 in the date of eruption of the third molar tooth, which com- 

 pletes the permanent set. It may appear at any time between 

 the seventeenth and the thirtieth years. 



The roots of the molar and of the premolar teeth lie in 

 relation to the floor of the maxillary sinus (antrum of High- 

 more), and, when it is necessary to drain the sinus, access can 

 be obtained by removing one of these teeth, preferably a 

 premolar. 



Failure of a tooth to erupt is never caused by failure to 

 develop. It may remain embedded in the bone or, if a 

 maxillary tooth, it may be found in the hard palate. This 

 condition may affect any of the teeth, but it is found most 

 frequently in connexion with the third permanent molar. 

 Such a misplaced tooth may give rise to very pronounced 

 reflex symptoms, of which acute neuralgia in the area of dis- 

 tribution of the fifth nerve is the most common. 



In cases where congenital syphilis is suspected, the condition 

 of the teeth may offer valuable evidence. In congenital syphilis 

 the incisors are short and peg-shaped, and their cutting edges 

 are definitely notched, Hutchinson's Teeth. The notching 

 is not natural, for, on eruption, the teeth are normal in 

 appearance. The adamant (enamel) is very thin and soon 

 becomes broken off, leaving the dentine exposed and causing 

 the characteristic notches. It is only when the upper central 



