On the structure of the dental tissues of marsupial animals : and more especially of the enamel / by John Tomes.

  • John Tomes
Date:
1849
Catalogue details

Licence: Public Domain Mark

Credit: On the structure of the dental tissues of marsupial animals : and more especially of the enamel / by John Tomes. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    occurs in the majority of teeth ; and this no doubt does happen in the marsupial teeth, and also in some few others. Indeed in all teeth the enamel fibre is in an early stage of formation partially tubular*. I will now proceed to describe the teeth in those species of marsupials that I have examined ; for I find many minor points of dissimilarity, by which, on careful com- parison, the teeth of allied species may be distinguished the one from the other. I may remark, however, before doing so, that the enamel presents other points of peculiarity, though less apparent, than its tubularity. In many marsupial teeth the enamel is studded with small cells, often, but by no means always, arranged in con- tour lines. Then again the fibres are in many teeth so intimately united to each other that their individuality is lost, and this occurs in most teeth in some parts, so that the dimensions of the fibres at such points cannot he taken. Macropus giganteus.—In this animal the differences in the dental tissues composing the incisor and molar teeth, are chiefly confined to the number of curves described by the enamel fibres and tubes. I shall therefore for the present restrict my descrip- tion to a transverse section of a lower incisor through the part most thickly coated with enamel. The dentinal tubes radiate from the pulp-cavity with numerous gentle secondary curves, and when pursuing the latter third of their course before entering the enamel, give off' numerous short fine branches. When near the enamel the small branches suddenly cease to be given off, and the parent tubes, either with or without bifur- cating, enter the enamel. The absence of the minute lateral tubules renders the tooth more transparent at these than in the neighbouring parts. On entering the enamel the tubes dilate into more or less oval or conical cells, from whence they are con- tinued, and follow in delicate undulations the course of the enamel fibres, some few giving off on their way one or two branches. They gradually diminish in size till they are eventually lost near the surface of the enamel, either from their own minute- ness or from their entering small opacjue cells, which are common near the outer sur- face of this texture, Plate XXXV. fig. 1 A, the dentine ; B, the enamel. The enamel fibres in the thickest part of the tooth are subject in their course outwards to four, five, and sometimes to even six flexures, fig. 2 B. As they arise from the periphery of the dentine, they proceed upwards in a tolerably straight line towards the cutting margin of the tooth; they then turn downwards at an angle of ninety degrees with their first course, and after advancing about as far in the second as they did in the first line, they describe several lesser flexures having similar angles to the first. When within two-fifths of their termination on the surface, the enamel fibres take a straight and parallel course at right angles with the surface of the tooth. As the coating of enamel becomes thinner, both towards the cutting edge and at the lower part of the tooth, the lesser curvatures of the fibres are lost, and the direc- tion of the first large flexure is reversed ; so that the fibres proceed first downwards
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