ENCLOSED IN THE TUSKS OF THE ELEPHANT. 01 



traces of dvietl pulp in their interior. From these canals, 

 which correspond to the Haversian canals of true Lone, se- 

 condary medullary canals, similar to those in the teeth of 

 certain fishes, radiate. The sides and extremities of these 

 secondary medullary canals send off numerous minute tubes, 

 which are true Retzian tubes, and similar to those in the re- 

 gular ivory, l.)ut not so closely set. These lietzian tubes have 

 a general radiating direction, and proceed in irregular wavy 

 bundles, which sweep past one another without mingling, but 

 branching particularly at their extremities. 



The great central medullary canals are very numerous, and 

 each of them has its own system of secondary canals and 

 Ketzian tubes. 



These individual systems, when seen in a mass of irregular 

 ivory, appear globular or spindle-shaped; when \'iewed in 

 section, they resemble circular or oval opaque spots with a 

 hole in the centre. These individual systems, however, are 

 not isolated ; for they communicate, first, by means of the 

 central canals, which constitute an inosculating system ; and 

 secondly, by the ramifying extremities of the Eetzian tubes, 

 which communicate through the medium of cells more or less 

 minute, and which are more numerous in some places than in 

 others. 



The formation of the irregular ivory does not go on inde- 

 finitely ; a limit is set to its increase, and the changes which 

 ensue at this stage of the process are highly interesting. I 

 have already mentioned the existence of the orifices of Haver- 

 sian or medullaiy canals on the surface of the mass of irregular 

 ivory. When the further formation of this is to terminate, 

 tliese orifices are gradually closed, and appear like imperforated 

 projections on the surface. It is evident, therefore, that the 

 enclosed vascular contents of the canals — that is to say, the 

 ramified processes of the tusk-pulp in the iiTcgular ivor}'— are 

 cut off from the svstf-in Tlu'V (h-v u]\ and the formation of 



