3. The action of external bodies is almost the only known cause of this
fracture, whether these bodies strike the shoulder with violence, or
the shoulder be forcibly driven against them. But this action is not
in every case the same; its application is most frequently mediate or
indirect, but is sometimes immediate or direct.
In the first case there is a true _counter-stroke_, the ordinary
effect, either of a severe blow on the point of the shoulder, which
is the most common occurrence; or, as happens less frequently, of a
fall on the arm when it is extended for the purpose of guarding the
body from the force of the accident. Under these circumstances, being
pressed between the sternum, which makes resistance, and the body which
acts on its extremity, the clavicle is bent in that direction which is
most natural to it; but, not being sufficiently flexible, it gives way
generally in the place where its curvature is the greatest. Thus the
ribs are broken, when the sternum, by being violently driven backward,
forces them to bend in the centre beyond their natural flexibility.
In the second case, the fracture occurs at the spot where the stroke
is given. Here the momentum or quantity of force applied on the bone,
surpassing the solidity which the bone possesses, its continuity is
necessarily destroyed.
4. But in whatever way the fracture is produced, it is either oblique
or transverse, single or double, in the middle or towards the
extremities of the bone, simple or compound.
An oblique fracture is most frequently the effect of a
_counter-stroke_; a transverse fracture is the more common result
of the immediate action of external bodies; a counter-stroke seldom
produces any thing but a simple fracture; while compound fractures are
generally owing to a direct stroke. The one produces a solution of
continuity in the middle of the bone, or thereabout; because in that
part the curvature is most considerable. The other is almost always
the cause of this solution, when it occurs at the extremities. To the
latter alone, is a double division to be attributed. The reason of
these differences is already so plain, that it would be a waste of time
to dwell on an explanation of them.
§ III.