, are also useful. Complete rest and
prolonged immobilisation are to be condemned.

TRAUMATIC DISLOCATIONS

A dislocation or luxation is a persistent displacement of the opposing
ends of the bones forming a joint. We are here concerned only with
such dislocations as immediately follow upon injury. Those that are
congenital or that result from disease will be studied later.

_Causes._--The majority of dislocations are the result of _indirect_
violence, the more movable bone acting as a lever, on a fulcrum
furnished by the natural check to movement in the form of ligament,
bone, or muscle. It is in this way that most dislocations of the
shoulder, hip, and elbow are produced.

At the moment the violence is applied, the muscles are relaxed or
otherwise taken at a disadvantage, so that the joint is for the time
being deprived of their support. The joint is moved beyond its
physiological range, and the end of one of the bones being brought to
bear upon the capsule, tears it, and passes through the rent thus
made. The muscles then contract reflexly, and pull the head of the
bone into an unnatural position outside the capsule. The position
assumed will depend upon such factors as the direction of the force,
the structure of the joint, the position of the limb at the time of
the accident, and the relative strength of the different groups of
muscles acting upon the bone which is displaced.

Violence applied _directly_ to the joint is a much less frequent cause
of dislocation. In this way, however, the knee-joint may be
dislocated, one bone being driven past the other--for example, by a
kick from a horse; or the acromio-clavicular joint by a blow on the
shoulder.

_Muscular contraction_ is not often the sole cause of dislocation,
although, as has been mentioned, it plays an important role in the
production of the majority of these injuries. The shoulder, mandible,
and patella are, however, not infrequently displaced by muscular
action alone. Acrobats sometimes acquire the power of dislocating
certain joints by voluntary contraction of their muscles.

_Age and Sex._--Dislocations occur most frequently in adult males,
doubtless on account of the nature of their occupations and
recreations. In children the epiphyses are separated, and in old
people the bones are broken by such forms of violence as cause
dislocation in the middle-aged.

Muscular debility and undue laxness of ligaments resulting from
disease or previous di