ew bone is permanently required and some of the old bone
is absorbed in order to meet the altered physiological strain upon the
bone resulting from the alteration in its architectural form. In
overriding displacement, even the dense cortical bone intervening
between the medullary canal of the two fragments is ultimately
absorbed and the continuity of the medullary canal is reproduced.

The amount of callus produced in the repair of a given fracture is
greater when movement is permitted between the broken ends. It is also
influenced by the character of the bone involved, being less in bones
entirely ossified in membrane, such as the flat bones of the skull,
than in those primarily ossified in cartilage.

If the fragments are widely separated from one another, or if some
tissue, such as muscle, intervenes between them, callus may not be
able to bring about a bony union between the fragments, and
_non-union_ results.

Bones divided in the course of an operation, for example in osteotomy
for knock-knee, or wedge-shaped resection for bow-leg, are repaired by
the same process as fractures.

#Excess of Callus.#--In comminuted fractures, and in fractures in
which there is much displacement, the amount of callus is in excess,
but this is necessary to ensure stability. In fractures in the
vicinity of large joints, such as the hip or elbow, the formation of
callus is sometimes excessive, and the projecting masses of new bone
restrict the movements of the joint. When exuberant callus forms
between the bones in fractures of the forearm, pronation and
supination may be interfered with (Fig. 4). Certain nerve-trunks, such
as the radial (musculo-spiral) in the middle of the arm, or the ulnar
at the elbow-joint, may become included in or pressed upon by callus.

[Illustration: FIG. 4.--Excess of Callus after compound fracture of
Bones of Forearm.]

#Absorption of Callus.#--It sometimes happens that when an acute
infective disease, especially one of the exanthemata, supervenes while
a fracture is undergoing repair, the callus which has formed becomes
softened and is absorbed. This may occur weeks or even months after
the bone has united, with the result that the fragments again become
movable, and it may be a considerable time before union finally takes
place.

#Tumours of Callus.#--Tumours, such as chondroma and sarcoma, and
cysts which are probably of the same nature as those met with in
osteomyelitis fibrosa, are liable to oc