ANOMALIES OF THE HUMAN DENTITION 163 



pause for a few moments that we may note the retention of 

 milk teeth. 



Too commonly current is the impression that some defective 

 eruption of the corresponding successional tooth results from this 

 retention and that if the offending temporary member be ex- 

 tracted the permanent tooth will come normally into place. 

 Retention of certain milk teeth is by no means confined to the 

 human race (see Fig. 33). Indeed among the insectivorous 

 Hedgehogs it is said, though not proved, to be the rule. It 

 occurred occasionally in the Neandertal race (Fig. 51) and 

 is present in some cases of general disease such as rickets. 



Any one of the temporary teeth in either jaw may be re- 

 tained for many years beyond the age at which it is normally 

 lost, perhaps even throughout life (Fig. 56-D, J). In many 

 cases a corresponding permanent tooth is present but under- 

 goes mal-eruption. The most significant condition is met with 

 in the case of the second milk molar which is retained perhaps 

 more frequently than any other milk tooth. It may be nor- 

 mally formed even as to the curve of its roots and nor- 

 mally erupted yet the germ of the successional second premolar 

 may be wholly absent without apparent cause. Except after 

 a radiogram has been taken to ascertain the presence and situa- 

 tion of the premolar and the chances of its being induced to 

 come into normal occlusion no sound second milk molar should 

 be extracted. 



Sometimes the milk tooth fails to be in proper occlusion after 

 the permanent teeth erupt on each side of it but this is by no 

 means always the case. The retained deciduous tooth may 

 remain healthy and useful throughout life. Its position with 

 regard to the successional set varies somewhat but very fre- 

 quently its occlusal surface reaches the plane of the occlusal 

 surface of the first permanent molar with which it forms a well 

 marked line of contact. Its longitudinal axis often forms an 

 angle with that of the first molar since it naturally retains the 

 orientation it possessed when occluding with the rest of the 

 milk dentition which is set in an arcade of radius less than 

 that of the permanent dental arch. 



