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DEVELOPMENT OF THE TEETH. 975 
junction of the two, see Fig, 649), and some of the dentinal tubes are continuous with 
the lacunze of the cementum. Like true bone, it is laminated, it possesses lacunvze, 
canaliculi, and, when in large masses, it may even contain a few Havens an canals. 
The tooth pulp occupies the pulp cavity and the root canals of the teeth. It is 
composed of a number of branched connective tissue cells, the anastomosing processes 
of which form a fine connective tissue network, containing in its meshes a jelly-like 
material, in addition to numerous vessels and nerves, but no lymphatics. The most 
superficial of these cells form in the young tooth a continuous layer of columnar, 
epithelium-like cells, lying on i surface of the pulp against the dentine ; they are 
known as odontoblasts, we they are the active agents in the formation of dentine. From 
the outer ends of the ee processes are continued into the dentinal tubes, where 
they have been already referred to as Tomes’ fibrils. The vessels of the pulp are 
numerous, and form a capillary plexus immediately within the odontoblasts. The nerves 
form rich plexuses throughout the pulp, but their exact mode of ending is unknown. 
The alveolar periosteum (alveolo-dental periosteum or root-membrane) is a layer 
of connective tissue free froin elastic fibres, but well supplied both with blood-vessels and 
nerves, which fixes the root of the tooth in the alveolus, being firmly united by perforat- 
ing fibres of Sharpey, to the crusta petrosa on the one hand, and to the bone of the 
alveolus on the other. It establishes a communication between the bone of the jaw 
and the cementum, and above it is continuous with the tissue of the gum. Its blood 
comes chiefly from the arteries, which subsequently enter the apical canals for the 
supply of the pulp, but in part also from the vessels of the bone and those of the gum 
(hence the relief obtained in dental periostitis by lancing the gum). 
DEVELOPMENT OF THE TEETH. 
At the beginning of this chapter a tooth was described as a calcitied papilla of the 
mucous membrane, composed of two chief parts—namely, the enamel formed by the 
epithelial layer, and the dentine by the connective tissue layer of the mucous membrane. 
The details of the process by which such a tooth is 
developed from the two layers of the mucous mem- 
brane are both numerous and intricate, and can be 
but briefly described here. 
In lower vertebrates (sharks, rays, etc.), teeth 
which correspond essentially, both in structure and 
development, to those of mammals, are found on the 
surface of the body, and are known as dermal teeth. 
The following outline of the development of the 
dermal tooth of a shark may assist in rendering the 
development of the human teeth more intelligible :— 
First, a papilla is formed from the corium or 
connective tissue layer of the skin (Fig. 658, B), which 
papilla is covered over by the epithelial layer. 
Next the superficial (connective tissue) cells of the 
papilla begin to forma layer of dentine on the surface 
of the papilla (Fig. 658, Cy which it soon encases, the 
remains of. the papilla persisting in the interior as the 
future pulp. At the same time the deepest cells of 
the epithelium deposit a layer of enamel, outside the 
dentine, over the summit of the papilla (Fig. 658, C), 
A, Section of skin 
showing epithelium 
é¢, basement mem- 
brane b, and connec- 
tive tissue layer c. 
B shows the papilla of 
the connective tissue 
layer growing up 
covered by the epi- 
thelial layer. 
In C thesuperficial cells 
of the papilla 2 begin 
to deposit dentine d 
over the papilla, and 
at the same time the 
deepest cells of the 
epithelium deposit 
enamel a. 
D shows the tooth 
breaking through 
the epithelium and 
reaching the surface. 
and subsequently the two—enamel and dentine— Fic. 658.—D1acRaM TO ILLUSTRATE THE 
become inseparably united, thus giving rise to the DEVELOPMENT OF A DERMAL TOOTH 
IN THE SHARK. 
substance of the tooth. 
In all figures—a, enamel; 6, basement mem- 
At a later period the epithelium covering the brane ; ¢, connective tissue layer of skin ; 
summit disappears and the tooth comes to the surface ; ieee nia eae En Ree 
this constitutes its eruption (Fig. 658, D). 
In the case of the mammalian tooth a similar process takes place, not, however, on the 
surface, but deep down in the substance of the gum, into which a downgrowth of epithel- 
ium has previously taken place. This epithelial downgrowth spreads out in the substance 
of the jaw, and into it the papilla grows up, and goes through the other changes described 
above, as if the whole process took place on the surface. 
Development of Human Teeth.—The following is a brief summary of the chief 
events in the development of a human tooth. For convenience in expression and terms, 
