3. The operation of external bodies, active, when they are thrown
against the shoulder, passive, when the shoulder, or the arm, is
forcibly driven against them, is always the cause of a fracture of the
neck of the humerus. From the mechanism of the part, the division is
sometimes direct, and sometimes the effect of a counter-stroke.
The first of these arises very generally from a fall on the point of
the shoulder, and as in such a case, the commotion or shock must be
very great, to extend with sufficient force through the thick mass
which forms the deltoid muscle, that muscle sometimes suffers both
contusion and an echymosis. Blood may even escape from a rupture of
some of the arteries or veins of the joint, and form, as Desault has
observed, a collection or tumour which it would be imprudent to open.
The other is the effect of a fall on the elbow, separated, at the time,
some distance from the trunk, or on the hand, which, by a natural
instinct, is thrown out, together with the arm and fore-arm, in order
to break the violence of the fall.
4. The varieties of this kind of fracture originate, 1st, from the
spot which it occupies, being either the middle or the lower part,
rarely the upper part, of the neck of the humerus: 2dly, from the state
of the surrounding soft parts, which sometimes remain quite natural,
and at other times become distended and tumefied. This circumstance
always involves the diagnosis in more or less uncertainty; 3dly, from
the direction of the fracture, which is sometimes transverse, but
usually oblique, particularly when produced in the second mode, that
is, by a counter-stroke (3); 4thly, from the relative situation of
the fragments, which may remain in contact, an occurrence however but
very rare, or may separate from one another in a direction inwardly or
upward; and, 5thly, from different complications, with which it may be
attended.
§ III.