torn at the same time as the bone is broken, blood is poured
out, and clots around and between the fragments. This clot is soon
permeated by newly formed blood vessels, and by leucocytes and
fibroblasts, the latter being derived from proliferation of the cells
of the marrow and periosteum. The granulation tissue thus formed
resembles in every particular that described in the repair of other
tissues, except that the fibroblasts, being the offspring of cells
which normally form bone, assume the functions of _osteoblasts_, and
proceed to the formation of bone. The new bone may be formed either by
a direct conversion of the fibrous tissue into osseous tissue, the
osteoblasts arranging themselves concentrically in the recesses of the
capillary loops, and secreting a homogeneous matrix in which lime
salts are speedily deposited; or there may be an intermediate stage of
cartilage formation, especially in young subjects, and in cases where
the fragments are incompletely immobilised. The newly formed bone is
at first arranged in little masses or in the form of rods which unite
with each other to form a network of spongy bone, the meshes of which
contain marrow.

The reparative material, consisting of granulation tissue in the
process of conversion into bone, is called _callus_, on account of its
hard and unyielding character. In a fracture of a long bone, that
which surrounds the fragments is called the _external_ or _ensheathing
callus_, and may be likened to the mass of solder which surrounds the
junction of pipes in plumber-work; that which occupies the position of
the medullary canal is called the _internal_ or _medullary callus_;
and that which intervenes between the fragments and maintains the
continuity of the cortical compact tissue of the shaft is called the
_intermediate callus_. This intermediate callus is the only permanent
portion of the reparative material, the external and internal callus
being only temporary, and being largely re-absorbed through the agency
of giant cells.

Detached fragments or splinters of bone are usually included in the
callus and ultimately become incorporated in the new bone that bridges
the gap.

In time all surplus bone is removed, the medullary canal is re-formed,
the young spongy bone of the intermediate callus becomes more and more
compact, and thus the original architectural arrangement of the bone
may be faithfully reproduced. If, however, apposition is not perfect,
some of the n