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148 DENTAL HISTOLOGY AND OPERATIVE DENTISTRY.
a section through a growing- jjortioii of the })roccs.s around a j)er-
inanent tootli. A higher power (Fig. 128) shows the i)enetrating
iibers and the formation of Ilaversian-systeni hone without fibers, in
the body of the ])ro('ess.
The osteoclasts, or niyehtphujucs, are bone-destroying eells (Fig.
129); they act not only upon bone, but also u])on cenientum and dentin.
They are oval cells, often as nuieh as 30 microns in diameter, and con-
tain many nuclei,—from two or three to fifteen or twenty. They are
often called giant cells. The osteoclasts are not constantly found in the
membrane, but make their appearance whenever calcified tissues are to
be destroyed. In order for them to act U})on the tissues they must lie
in contact with its surface, and therefore the first step in absorption of
the peridental membrane is the cutting off of the fibers imbedded in
the bone or cementum. Where the osteoclasts act upon the surface of

Fig. 131.

liecord in the calcified tissue of an absorption repaired : D, dentin : Cm, cementum filling absorp-
tion cavity. (About 40 X.)

the tissue they produce bay-like excavations, in which they lie, and
which are known as Howship's lacunte. These excavations are shown
in Fig. 132, though the osteoclasts have disappeared. In Fig. 131,
from a ground section, the basin-like excavations are shown filled with
new-formed cementum, thus leaving in the tissue the record of an
absorption repaired. In absorption of the roots of the tem})orary teeth
the osteoclasts are found not only in the membrane and attacking the
surface of the root, but all through the medullary spaces in the bone,
removing the temporary alveolar process.
When absorption is going on at one place on the surface of a root
a compensating formation of cementum is going on at another, so
that not all of the fibers of the membrane are cut off. This is illus-