232 MICROSCOPIC ANATOMY OF THE TEETH 



the curve of the tooth sometimes turns directly backwards, 

 penetrating the skull and brain. 



The persistent growth of the great incisor teeth forming 

 the tusks of the elephant sometimes gives rise to remarkable 

 conditions. Several cases have been recorded where an 

 elephant has been struck by a javelin or loaded spear-head 

 which has penetrated the pulp at the free-growing end, 

 and the vitality of the tissues has been so great that the 

 death of the pulp has not followed, but the spear carried 

 forward by the growing tooth has been completely surrounded 



FIG. 141. Skull of Rat (Mus decumamis). Dislocation and overgrowth 

 of incisors. The right upper incisor penetrates the skull. (From a specimen 

 lent to the author by Mr. Montagu Hopson.) 



by the subsequently deposited ivory and has become 

 immovably fixed in the tusk. Such a case is shown in the 

 photograph (fig. 142). The head of the javelin and a portion 

 of the iron shaft are embedded solidly in the ivory. This 

 specimen was obtained from the ivory worker by a merchant 

 in the City, who could not be persuaded to part with it 

 but lent it to the author to be photographed. There are 

 two somewhat similar specimens in the Hunterian Museum 

 at the Royal College of Surgeons. The effects of the con- 

 tinuous growth of teeth with persistent pulps is also shown in 

 fig. 141, the skull of the rat previously referred to on p. 107. 

 Other examples of teeth with persistent pulp are found 

 in the incisor of the Dugong, the large upper canines of the 



