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358 DENTAL ANATOMY.
loses its formativ^e energy, and may become entirely obliterated. Taken
at the adult stage of the tooth, it is seen to consist of indistinct hnely
fibrous connectiye tissue containing numerous cells. The outermost
layer of the pulp is known as the memhvana eboris, and is made up of
a single layer of highly specialized cells of a dark granular appearance,
somewhat elongated, termed ndonioblasts. These odontoblasts possess
large oyal nuclei, and are proyided with three sets of processes, as fol-
lows : the dentinal processes, which are identical with the dentinal fibrils,
and, as we haye already seen, enter the dental tubes ; the latercd pro-
cesses, by which they are connected with each other ; and, lastly, the
pulp pjrocesses, extending down to a deeper layer of cells. This latter
layer of cells is somewhat intermediate in size between those more deeply
seated and the odontoblasts. Three or more arteries enter at the apical
foramen, and form a rich capillary plexus a short distance beneath the
membrana eboris. The neryes enter by seyeral trunks along with the
arteries, and soon break up into a fine network in the substance of the
pulp. According to Boll, nerye-fibres penetrate the dentinal tubuli in
company with the dentinal fibrils, but this yiew is not fully accepted.
Cementum.—The cementum in human and many other teeth of similar
structure may be said to be confined to the roots, inyesting them exter-
nally, unless the enamel cuticle or membrane of Nasmyth, mentioned
aboye, })ertains to it, which C S. Tomes and others belieye to be the
case. It, like ordinary bone, consists of a gelatinous base combined
with calcareous salts, and is permeated by yascular canals. Its histo-
logical structure j^resents so many characters common to bone that it is
difficult to consider it anything more than a slight modification of that
tissue. Just as in bone, large irregular spaces (laciince), filled with pro-
toplasmic substance and presenting numerous minute radiating canals
(canaliculi), whidi anastomose with those of neighboring lacunae, are
found in ordinarily thick cementum ; certain differences are, howeyer,
seen to exist.
The lacunte of cementum, for example, are more variable in size
and are noted for the great length of their canaliculi. The direction,
too, of the canaliculi is generally parallel with that of the dentinal
tubuli, radiating from two sides only, whereas in bone-tissue they
radiate in all directions. It has been already stated that the dentinal
tubuli sometimes enter the cementum layer. AYhen this is the case
they become continuous with the canaliculi of the most deeply dis-
tributed lacunre. The outermost or granular layer of the dentine goes
so far toward establishing a complete transition in structure between the
cementum and the dentine that it is generally im]X)Ssible to draw a
diyiding-line and say where the one ends and the other begins. As to
limit of distribution of the cementum on the surface of the teeth in man,
monkeys, carniyores, and insectiyores, different yiews haye been expressed,
owing to the yarious constructions that haye been placed upon the nature
and relationship of the enamel cuticle or Xasmyth's membrane, already
mentioned. AValdeyer, Huxley, and K(")lliker hold that it is no way
connected with the cementum, but that it is a product deriyed from the
enamel, and is therefin'e ej)ithelial in origin. C S. Tomes, Magitot,
and AA'edl, on the other hand, maintain that it is a part of the cementum