404 
MR. TOMES ON THE STRUCTURE OF 
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 be 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 opaque 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 
* Lectures on Dental Physiology and Surgery, p. 102. 
