thought formerly, that they became elongated to form the dental tubes, and that the intertubular substance was merely intercellular substance between these prolongations. But I was compelled to relinquish that idea in consequence of there being no such appearance in the human tooth, and because the explanation led to difficulties with respect to the teeth of the pike, in which, according to Retzius, an immediate transition of the dental into the osseous substance takes place. If one of the largest teeth in the lower jaw of the pike be sawn off, deprived of its earthy matter by means of hydrochloric acid, and then divided into thin longitudinal sections, the dental substance will be seen to form a hollow cone, which is filled with osseous substance. The dental substance is transparent,and consists of fibres which have a direction from the point towards the base of the cone. Canals traverse the osseous substance, resembling the Haversian medullary canals of ordinary bone, only they are not so regular. The dental tubes then are connected with these canals of the proper osseous substance, and may be distinctly seen issuing from them in a funnel-shaped form. The canaliculi soon ramify in the dental substance, and, as they run across the thickness of the dental cone, interlace with the dental fibres. According to this view, the dental tubes would correspond to the medullary or Haversian canaliculi of bone, and not to the calcigerous canaliculi proceeding from the osseous corpuscles. It appears impossible, however, to be assured of the right explanation of all the structural relations of the dental substance, until its development is examined in teeth differing widely from each other in construction.
c. Osseous substance of the teeth.
This requires no particular explanation, as it entirely accords with the ordinary osseous substance.
Having examined in detail the tissues comprehended in this class, and compared them one with another, we have yet to consider the entire class in relation to those which have been previously discussed, and to observe how much our knowledge of the transformations which the cells are capable of undergoing, has been advanced by the study of it.
