978 THE DIGESTIVE SYSTEM. 
The following statement is simple, and perhaps is sufficient for all ordinary purposes. 
The temporary teeth usually appear in the following order :—Central incisors, lateral 
incisors, first molars, canines and second molars; the er uption commences between the 
sixth and the ninth month, and is usually completed by the twenty-fourth—the lower 
teeth, as a rule, preceding the upper. 
Formation of Enamel and Dentine.— Different opinions are held as to the method in which 
the enamel is produced by the enamel cells. One view maintains that it is secreted and shed 
out by the enamel cells (K6lliker). According to the other view, part of the substance of the 
cells is actually converted or transformed into enamel (Tomes). In connexion with this latter 
view, Which seems to receive more support at present, Tomes has discovered that there pro- 
jects from the base of each enamel cell, towards or into the most recently formed enamel, a 
fibrillar process, which has received the name of Tomes’ process, and he holds that the enamel is 
formed by calcification taking place in or around the process. 
Similarly, two views are held as to the production of dentine by the odontoblasts ; one, that 
the odontoblasts secrete the matrix of the dentine, and the other, that their substance is actually 
converted into the matrix. The odontoblasts, when active, are br anched c ‘olumnar-shaped cells, and 
from their outer ends one or more processes extend towards and into the dentine ; between these 
processes a inatrix appears—produced probably by the odontoblasts—and soon this matrix becomes 
calcified. In this way the dentine is formed, and the process is repeated until its full thickness 
is attained. The branches of the odontoblasts, encased in dentine, just mentioned, are the Tomes’ 
fibrils already described ; the canals in which they lie are the dentinal tubes; and the fibrils 
themselves are concerned in the production of the sheaths of Neumann which line the tubes. 
The tooth-sacs, when fully developed, are large and distinct fibrous bags which 
lie in the alveoli of the maxilla and are continuous above with the tissue of the eum. 
On the lingual side of the sacs of the milk teeth are found the germs of the permanent 
teeth, surrounded by their own sacs. These latter are at first very small, and are partly 
embedded in the posterior wall of the temporary tooth-sacs, but subsequently they come 
to le in distinct but incomplete bony cavities of their own. The bone surrounding the 
tooth-sacs, temporary and permanent, is always wanting over the summit of the sac, and 
the band of connective tissue by which the sac is connected with the overlying gum 
tissue, through the deficiency, is known as the gubernaculum dentis. 
These points are easily demonstrated on the lower jaw of a child at birth, particularly 
when the tissues have been allowed to soften a little. If, in such a specimen, the gum and 
periosteum be reflected upwards from the outer and inner surfaces of the mandible, and 
freed as far as the upper border of the jaw, the gum, with the tooth-sacs depending 
from it like small bags, can be pulled away out of the bony groove of the jaw; and 
if the operation has been successfully performed, the tooth-sacs of the three front 
permanent teeth may be seen, varying in size from a small pin’s-head to a hemp- 
seed, hanging down behind the upper part of the corresponding temporary sacs. As 
already explained, the tooth-sacs are produced simply by a condensation of the connective 
tissue around the developing tooth, the condensation going on to the formation of a dis- 
tinct membranous bag. 
Formation of Alveoli and Eruption.—At first the developing teeth lie in an open 
bony groove or channel between the outer and inner plates of the young jaw. This 
groove is subsequently divided up into separate compartments for the sacs of each of the 
temporary teeth. As development proceeds these compartments or alveoli surround the 
sacs more completely, but never actually close over the summit. When the eruption 
of the temporary teeth is about to take place, the anterior wall and roof of the alveolus 
are absorbed ; the tooth passes through the sac and appears above the gum, and then the 
alveolus, which up to this was much too large to give actual support, is re-formed more 
closely around the tooth. Meanwhile the root, which was only partly formed at the time 
of the eruption, continues to be added to, possibly for a few years more, and, as it 
grows, the alveolus is completed around it. When the permanent tooth, or as much of it 
as is then formed, is about to be erupted, it makes its way from its own bony cell through 
the posterior wall of the alveolus of its temporary predecessor ; the root of the temporary 
tooth undergoes absorption at the same time, but quite independently of pressure from the 
permanent tooth. The alveolus, now occupied by both teeth, is again much enlarged by 
absorption, particularly in front ; what remains of the temporary tooth is shed ; the 
permanent tooth passes onwards through the enlarged alveolus, and, making its way to 
the surface, appears above the gum. After some time, when the tooth has taken its final 
position, the alveolus is again re-formed, first around its neck, and later on, as the root is 
built up, around it also, and thus the tooth is permanently fixed. 
What the force is which causes the eruption, is a question that has not been answered 
satisfactorily. That the growth of the root pushes up the crown was until recently the 
