348 MICROSCOPIC ANATOMY OF THE TEETH 



It was considered by A. Underwood that eruption may 

 be caused by the movement of the soft parts surrounding 

 the tooth, due to the growth and changes taking place in 

 the periodontal membrane, and comparable to some extent 

 with the movement of the mucous membrane carrying the 

 teeth over the jaw in the Sharks (18). 



A somewhat similar explanation of the process of eruption 

 is given by Thornton Carter. 1 He describes a slight amount 

 of ossification of the margin of the cartilaginous jaw in the 

 Dog-fish, and says: * The crust of bone, which is of a transitory 

 nature, being constantly absorbed and deposited, is instru- 

 mental in causing a progressive movement of the sliding 

 membrane,' and further, * When a functional tooth is shed, 

 absorption of the underlying bone takes place, and also 

 absorption of the fibrous membrane at its outcrop. As 

 a natural consequence of the absorption of the underlying 

 bone and fibrous membrane there is a rapid proliferation 

 of cells at the margin of the cartilage with formation of 

 bony tissue over the same area. The newly formed bony 

 tissue operates on the fibres of the sliding membrane, which 

 is poor in cellular elements and ill-adapted for active growth, 

 and causes the membrane to move upwards and bear with 

 it the next successional tooth.' That the presence of this 

 bony deposit, however, is not essential to the process appears 

 to be indicated by the fact that it is not always found 

 in Elasmobranchs. The section of the edge of the jaw of 

 Lamna at the Natural History Museum does not show the 

 presence of any bone, and no bone is figured in Rldewood's 

 drawing of the eruption of the teeth in Carcharias in the 

 Cambridge Natural History. 



Carter's conclusions regarding eruption of the teeth in 

 man are as follows : ' Thus we may conclude that in man 

 the cause of eruption, or at least an active factor in producing 

 eruption, is to be found in the disproportionate growth 

 occurring in the tissues forming the tooth and the tissues 

 surrounding the tooth.' 



It seems impossible to deny that there is a forward move- 

 ment of the tooth in eruption, which is probably due to 



1 Colyer's Dental Surgery and Pathology, 1919. 



