ty
also may be absent--as, for instance, when there is no displacement of
the fragments, or when only one of two parallel bones is broken, as in
the leg or forearm. Similarly, crepitus may be absent when impaction
exists, when the fragments completely override one another, or are
separated by an interval, or when soft tissues, such as torn
periosteum or muscle, are interposed between them. A sensation
simulating crepitus may be felt on palpating a part into which blood
has been extravasated, or which is the seat of subcutaneous emphysema.
The creaking which accompanies movements in certain forms of
teno-synovitis and chronic joint disease, and the rubbing of the
dislocated end of a bone against the tissues amongst which it lies,
may also be mistaken for the crepitus of fracture.

It is not advisable to be too diligent in eliciting these signs,
because of the pain caused by the manipulations, and also because
vigorous handling may do harm by undoing impaction, causing damage to
soft parts or producing displacement which does not already exist, or
by converting a simple into a compound fracture.

It is often necessary for purposes of diagnosis to administer a
general anaesthetic, particularly in injuries of deeply placed bones
and in the vicinity of joints. Before doing so, the appliances
necessary for the treatment of the injury should be made ready, in
order that the fracture may be reduced and set before the patient
regains consciousness.

_Radiography in the Diagnosis of Fractures._--While radiography is of
inestimable value in the diagnosis of many fractures and other
injuries, particularly in the vicinity of joints, the student is
warned against relying too implicitly on the evidence it seems to
afford.

A radiogram is not a photograph of the object exposed to the X-rays
but merely a picture of its shadow, or rather of a series of shadows
of the different structures, which vary in opacity. As the rays
emanate from a single point in the vacuum tube, and as they are not,
like the sun's rays, approximately parallel, the shadows they cast are
necessarily distorted. Hence, in interpreting a radiogram, it is
necessary to know the relative positions of the point from which the
rays proceed, the object exposed, and the plate on which the shadow is
registered. The least distortion takes place when the object is in
contact with the plate, and the shadow of that part of the object
which lies perpendicularly under the lig