THE TEETH OF PROBOSCIDEA. 353 



I ain told by Mr. Erxleben that he is acquainted with 

 another instance in which a spear head had become com- 

 pletely enveloped in ivory. 



FIG. 150 ('). 



There is also a specimen of a javelin head solidly im- 

 bedded in ivory in the Museum of the Royal College of 

 Surgeons. 



Ivory is one of the most perfectly elastic substances 

 known, and it is on this account that it is used for billiard 

 balls ; it owes its elasticity to the very small size of the 

 dentinal tubes and the frequent bends (secondary curva- 

 tures) which they make ; to the arrangement of the tubes 

 the peculiar engine-turning pattern of ivory is due. Tt 

 differs from other dentine in its containing from 40 to 43 

 per cent, of organic matter (human dentine contains only 

 about 25), and in the abundant concentric rows of inter- 

 globular spaces. Along these ivory when it decomposes 

 breaks up, so that a disintegrated segment of a tusk con- 

 sists of detached concentric rings ; in this condition many 

 mammoth teeth are found, although sometimes where they 

 have remained frozen and protected from the air until the 

 time of their discovery they are hardly affected by the lapse 



( l ) Iron spear-head, irremovably fixed in the interior of a tusk, believed 

 to be from an African Elephant. From a specimen in the possession of 

 Mr. Bennett. 



