186 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



resembling that of incipient cartilage ; whilst its exterior is composed 

 of a dense, structureless, pellucid membrane. The cellular mass is not 

 at first permeated by vessels ; but a small arterial branch is distributed 

 to each papilla, and spreads out into a tuft of capillaries at its base 

 (Fig. 52). The papilla gradually enlarges, by the formation of new cells 



Fig. 52. 



Vessels of Dental Papilla. 



at the part immediately adjacent to the blood-vessels, which supply the 

 material requisite for .their development; and ^vhen it has acquired 

 its full size, the process of calcification takes place by which it is con- 

 verted into Dentine, the substance most characteristic of teeth. 



311. This Dentine, which in the Elephant's tusk is known as Ivory, 

 is a firm substance, in which mineral matter predominates to a greater 

 extent than in bone ; but which still has a definite animal basis, that 

 retains its form when the calcareous matter has been removed by mace- 

 ration in acid. In every 100 parts, the animal matter forms about 28 ; 

 and of the mineral portion, phosphate of lime constitutes about 64J 

 parts, carbonate of lime 5 parts, and phosphate of magnesia and soda, 

 with chloride of sodium, about 2J parts. When it is fractured, it seems 

 to possess a fibrous appearance ; the fibres radiating from the centre of 

 the tooth towards its circumference. But when a thin section of it is 

 submitted to the microscope, it is seen that this fibrous appearance is 

 due to a peculiar structure in the dentine, which the unaided eye cannot 

 discover. The dentinal substance is itself very transparent ; but it is 

 traversed by minute tubuli, which appear as dark lines, generally in very 

 close approximation, running from the internal portion of the tooth 

 towards the surface, and exhibiting numerous minute undulations, and 

 sometimes more decided curvatures, in their course (Fig. 53). They 



