ject to occasional violence involving powerful muscular contractions,
or are less often exposed to casualties similar to those which result
in luxations in the human skeleton, but because it requires the
cooperation of conditions--anatomical, physiological, and perhaps
mechanical--present in the human race and lacking in the others, which,
however, can not in every case be clearly defined. Perhaps the greater
relative length of the bony levers in the human formation may constitute
a cause of the difference.

Among the predisposing causes in animals may be enumerated caries of
articular surfaces, articular abscesses, excessive dropsical conditions,
degenerative softening of the ligaments, and any excessive laxity of the
soft structures.

_Symptoms and diagnosis._--Three signs of dislocation must usually be
taken into consideration. They are: (1) An alteration in the shape of
the joint and in the normal relationship of the articulating surfaces;
(2) an alteration in the length of the limb, either shortening or
lengthening; (3) an alteration in the movableness of the joint, usually
an unnatural immobility. Only the first, however, can be relied upon as
essential. Luxations are not always complete; they may be partial; that
is, the articulating surfaces may be displaced but not separated. In
such cases several symptoms may not be present. And not only may the
third sign be absent, but the mobility of the first be greatly increased
when the character of the injury has been such as to produce extensive
lacerations of the articular ligaments.

In addition to the above signs, a luxation is usually characterized by
pain, swelling, hemorrhage beneath the skin from damaged or ruptured
blood vessels, and even paralysis, when important nerves are pressed on
by the displaced bones.

Sometimes a bone is fractured in the immediate vicinity of a joint. The
knowledge of this fact requires us to be able to diagnose between a
dislocation and such a fracture. In this we generally have three points
to assist us: (1) The immobility of a dislocated joint as against the
apparently remarkable freedom of movement in fracture; (2) in a
dislocation there is no true crepitus--that peculiar grating sensation
heard as well as felt on rubbing together the rough ends of fractured
bones; however, it must be remembered that in a dislocation two or three
days old the inflammatory changes around the joint may give rise to a
crackling sensation similar t