§ II.
OF THE VARIETIES AND THE CAUSES.
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.