§ I.
1. The solidity and security of joints are increased in proportion as
the extent of their motions is diminished. This inverse proportion
of these two properties to each other, is in a particular manner
remarkable in the upper extremities, where the connexion of the
humerus with the scapula, of the fore-arm with the humerus, and of the
bones of the wrist among themselves, appear, in regular gradation, to
acquire the one as they lose the other: hence, their predisposition to
luxations is extremely different. We will examine those to which the
fore-arm is subject.
2. An angular ginglymus unites to the humerus the bones of the
fore-arm, which are again connected with each other by a double lateral
ginglymus. Eminences and depressions, reciprocally receiving and
received, constitute the first kind of articulation, where, proceeding
from without inwardly, we find, 1st, the small head of the humerus,
entering or rather joining the upper articular cavity of the radius,
which moves on it: 2dly, the external groove of the humerus, receiving
the rim of the same cavity of the radius: 3dly, a projection which,
rising from the external edge of the coronoide[19] cavity, extends to
the corresponding edge of that of the olecranon, and is received into
the external depression of the sigmoid cavity:[20] 4thly, the large
groove of the humerus, receiving the middle eminence of this same
cavity: 5thly, a considerable projection, obliquely applied to the
internal depression which receives it.
[19] Not generally named in English works of anatomy.
[20] _Echancrure sigmoide._
3. These numerous connexions secure the solidity of the joint, which
is still farther strengthened before by the coronoid apophysis,
together with the fleshy and tendinous extremities of the biceps,
and brachialis muscles, and by the olecranon behind; at the inferior
part, by the anconeus; on the sides by two ligaments which descend
from the two tuberosities, and strong muscular fasciæ running from
the same parts. The whole articulation is also surrounded by a thick
capsule, strengthened by numerous accessory fibres. With such powers of
resistance, how can this joint suffer a luxation? Yet next to that of
the os humeri, it is perhaps most frequently subject to this accident.
§ II.