On the structure of the dental tissues of Rodentia

The author in this paper relates the results obtained from an extensive series of investigations on upwards of sixty of the more typical members of the order Rodentia. He finds, that not only are the teeth of animals of this order distinguished by strongly marked structural peculiarities (hitherto not recognised) from other mammalian teeth, but also that the teeth of the several larger groups are distinguished from each other by modifications in what may be called the rodential type of dental tissue. Mr. Tomes pointed out, in a paper published in Part 2 for 1849 of the Phil. Trans., that in the teeth of marsupial animals the dentinal tubes are continued into the enamel. In the present communication he shows that the structural peculiarities which characterise and are confined to the teeth of rodents are also mainly resident in the enamel. The earlier pages of the paper are devoted to a description of those structural conditions which are common to the teeth of the whole order. Amongst these, the author finds that the extremities of the dentinal tubes, which in the lower part of the tooth communicate with the pulp-cavity, become in the extruded portion sealed up by the development of a layer of non-tubular tissue which is formed at and near the apex of the pulp-cavity. This closure of the dentinal tubes is not however confined to the teeth of Rodentia, but occurs in all teeth in those parts exposed to wear. A similar condition is found to obtain in the osseous tissue which forms the antlers of the Deer-tribe. If a portion of an antler previous to its losing its periosteum be examined, ordinary Haversian systems are found: but if an antler which has been shed be examined, each of the larger Haversian systems will be found to be lined by a layer of transparent tissue destitute both of lacunae and canaliculi. The author considers these conditions to indicate the existence of a general law, viz. that dense tubular tissues, when about to be exposed to wear or about to be cast off, are previously deprived of their connection with nutrient fluids by the development of a layer of non-tubular tissue between them and the vascular surface, with which they were originally connected and from which they derived their nutriment. In order to facilitate description, Mr. Tomes proposes to use several terms as indicative of the arrangement and number of the component tissues of teeth, which he defines and describes at length.