184 THE HEAD AND NECK 



surface. This is cut away, and the two nerves are found lying 

 on the internal pterygoid muscle. 



2. The mucous membrane of the mouth may be divided 

 upwards and backwards from the level of the last molar tooth of 

 the mandible along the inner surface of the ramus. The lingual 

 nerve is found lying in the submucous tissue, and may be traced 

 backwards between the ramus and the internal pterygoid muscle 

 until the lingula of the mandibular foramen can be felt. The 

 inferior alveolar nerve and vessels are found as they enter the 

 mandibular foramen a little posterior to the lingual nerve. 



This operation is carried out when the patient is anxious to 

 avoid a skin incision and the subsequent scar. 



Dentition. The first of the deciduous teeth to appear are the lower 

 central incisors, and they are usually cut between the sixth and eighth months. 

 The lateral incisors soon follow, and by the end of the year the first deciduous 

 molars should be visible. Early in the second year the canine teeth erupt, 

 and the temporary set is completed by the appearance of the second deciduous 

 molar about the commencement of the third year. The lower teeth usually 

 appear somewhat in advance of the upper. 



The first lower molars are the first of the permanent teeth to erupt, and 

 should be present by the end of the seventh year. They are followed during 

 the next two years by the central and lateral incisors. Within the next four 

 years the first and second premolars, the canines, and the second molars 

 appear in that order, but the eruption of the last-named is often delayed till 

 the fifteenth or sixteenth year. The permanent set is completed by the erup- 

 tion of the third molars, which may appear at any time between eighteen 

 and thirty. 



Irregular dentition is common in rickets, and the upper permanent in- 

 cisors are sometimes notched. The notching corresponds to a small segment 

 of a large circle, and has to be distinguished from the notching of the same 

 teeth in congenital syphilis (Hutchinson* s Teeth), which corresponds to a large 

 segment of a small circle. 



The developing tooth is enclosed in a membranous sac, which, under 

 normal conditions, atrophies as the tooth increases in size. Occasionally, 

 however, the sac fails to atrophy, and develops into a dentigerous cyst, which 

 expands the bone. The cyst usually projects more to the lateral than to 

 the medial side, and contains the undeveloped tooth attached to its wall. 



Alveolar Abscess. The roots of the teeth are firmly fixed 

 in their sockets by a highly vascular fibrous membrane, the 

 alveola -dental periosteum. In peri-odontitis this membrane 

 swells up and becomes very sensitive. It tends to extrude the 

 tooth slightly from its socket, so that biting is rendered a painful 

 process. When an alveolar abscess forms, the pus may make 

 its way out around the neck of the tooth or burrow through the 

 alveolus, giving rise to a " gumboil." In the maxilla it may 

 penetrate the bone laterally and appear near the reflection of 

 the mucous membrane from the gum to the cheek, or it may 



