104 OF THE ALVEOLI. 



in their course ; the branches anastomosing together, and com- 

 municating occasionally with very minute calcigerous cells, 

 lodged in the transparent intertubular structure, which may be 

 compared to the corpuscles of ordinary bone. 



The fibres or filled tubes of the enamel are about 5 th of a 

 line in diameter, and are hexagonal. They are striated, 

 arranged parallel to each other, and are applied by their 

 internal extremities to corresponding depressions on the surface 

 of the ivory. 



The ordinary bony tubuli of the cement or cortical substance 

 communicate here and there with the branching tubes of the 

 ivory. 



These minute but interesting details in regard to the structure 

 of the teeth, which are found to vary in the different classes of 

 animals, are important, not only as furnishing one of the best 

 methods of their classification, but in exhibiting the striking 

 analogy that exists, as to their structure, between teeth and 

 bone. The tubes or canals of common bone are occupied by 

 blood-vessels, the calcareous matters being lodged in the bony 

 corpuscles and their reticular tubuli ; while those of the teeth 

 are vascular in the growing state, and become nearly all 

 filled up as well as their corpuscles with earthy matter, to 

 give that great degree of solidity requisite in biting and masti- 

 cation. 



The alveoli or sockets of the teeth, are formed upon the edge 

 of the jaw : the bone, of which they consist, is less firm than 

 any other part of the jaws: they correspond exactly with the 

 roots of the teeth ; and are lined with a vascular membrane, 

 which serves as a periosteum to the roots, and assists in fixing 

 them firmly. 



They are developed pari passu, with the teeth, and solely 

 for the purpose of giving them a lodgment ; hence when the 

 teeth are removed from the jaw, in the living subject, the 

 sockets subsequently disappear by absorption, as being of no 

 further use. There are two sets of alveoli, one for the deci- 

 duous teeth of the child, and one for the permanent teeth of 

 the adult. Their walls are formed of one plate on the external 



