THE DENTAL TISSUES. 



previously formed. The structure of these masses is very 

 confused. Tubes, of about the same diameter as dentinal 

 tubes, abound ; but they are often arranged in tufts or in 



FIG. 49 



bundles, and without any apparent reference to any com- 

 mon points of radiation. Irregular spaces, partaking of the 

 character of interglobular spaces or of bone lacunae, abound; 

 and vascular canals are also common. 



In the human tooth secondary dentine occurs in the teeth 

 of aged persons, in which the pulp cavity is much contracted 

 in size, and is also very frequently formed as a protection to 

 the pulp when threatened by the approach of dental caries, 

 or by the thinning of the walls of the pulp cavity through 

 excessive wear. The accompanying figure, representing one 

 of the cornua of the pulp chamber from a molar tooth 

 affected by caries, is a good- example of secondary dentine. 



Section of a mass of secondary dentine from the tooth of a sperm 



whale. 



