is often
_distraction_ or pulling apart of the fragments (Fig. 35). The broken
ends, especially in oblique fractures, may _override_ one another, and
so give rise to shortening of the limb (Fig. 2). Where one fragment is
acted upon by powerful muscles, a _rotatory_ displacement may take
place, as in fracture of the radius above the insertion of the
pronator teres, or of the femur just below the small trochanter. The
fragments may be _depressed_, as in the flat bones of the skull or the
nasal bones. At the cancellated ends of the long bones, particularly
the upper end of the femur and humerus, and the lower end of the
radius, it is not uncommon for one fragment to be _impacted_ or wedged
into the substance of the other (Fig. 28).

_Causes of Displacement._--The factors which influence displacement
are chiefly mechanical in their action. Thus the direction and nature
of the fracture play an important part. Transverse fractures with
roughly serrated ends are less liable to displacement than those which
are oblique with smooth surfaces. The direction of the causative force
also is a dominant factor in determining the direction in which one or
both of the fragments will be displaced. Gravity, acting chiefly upon
the distal fragment, also plays a part in determining the
displacement--for example, in fractures of the thigh or of the leg,
where the lower segment of the limb rolls outwards, and in fractures
of the shaft of the clavicle, where the weight of the arm carries the
shoulder downwards, forwards, and medially. After the break has taken
place and the force has ceased to act, displacement may be produced by
rough handling on the part of those who render first aid, the careless
or improper application of splints or bandages, or by the weight of
the bedclothes.

In certain situations the contraction of unopposed, or of unequally
opposed, groups of muscles plays a part in determining displacement.
For example, in fracture immediately below the lesser trochanter of
the femur, the ilio-psoas tends to tilt the upper fragment forward and
laterally; in supra-condylar fracture of the femur, the muscles of the
calf pull the lower fragment back towards the popliteal space; and in
fracture of the humerus above the deltoid insertion, the muscles
inserted into the inter-tubercular (bicipital) groove adduct the upper
fragment.

REPAIR OF INJURIES OF BONE

In a _simple fracture_ the vessels of the periosteum and the marrow
being