1. Nature, who, according to the wants of different species of animals,
has varied the number of their articulations, knows also how to vary
their structure, according to the uses of the different parts of their
bodies. With great mobility, she has sometimes connected great solidity
and strength, as is the case in the vertebral column; in other
instances, parts very solid and compact, are capable of performing
but feeble motions, as the carpus, the tarsus, &c. And, lastly, other
parts, again, capable of great motion, possess so little solidity and
firmness as to be easily deranged by the action of external bodies.
Such, in man, is the articulation of the humerus with the scapula, of
the sternum with the clavicle, &c.
2. Hence there exist three classes of articulations, very different
from each other. To the last, as enumerated above, belongs, in a
particular manner, the history of luxations, and, in this, as the
solidity varies, the frequency of dislocations is equally various; no
luxation occurs more frequently than that of the humerus; indeed, in a
comparative catalogue of accidents of this kind, it alone has, during
certain years, occurred oftener, in the Hotel-Dieu, than that of all
the other bones, taken collectively.
3. Every thing seems to favour the escape of this bone from its natural
cavity. 1st, On the part of the articulating surfaces, a cavity
somewhat oval and very shallow, aided by a slight cartilaginous ring,
receives a half-spherical head, twice its own diameter from above
downwards, and three times as large from before backwards. 2dly, On
the part of the ligaments, this articulation is strengthened by only
a simple capsule. This capsule is thin and weak on its lower side, a
direction in which there is nothing to prevent a luxation, while it is
thicker on its upper side, where the acromion and coracoid apophyses,
and a strong ligament, present an obstacle almost insurmountable. 3dly,
As far as respects the muscles and the motions of the joint, strong
and numerous bundles of fibres surrounding the articulating surfaces,
communicate to them motions easily performed in every direction, and
which, by pushing the head of the humerus against the different parts
of the capsule, distend it, predispose it to laceration, and indeed
even rupture it, when the quantum of their force is superior to its
resistance. 4thly, As far as relates to external bodies, what bone is
more exposed to their action than this, particularly among that class
of persons, engaged, for a livelihood, in hard labour?
4. Subject to the influence of these different predisposing causes, the
humerus would be constantly liable to luxations, did not the scapula,
moveable like itself, furnish it, by accompanying its motions, with
a point of support, differently disposed, according to the different
position of its superior extremity; so that, to this two-fold mobility
of the articulating surfaces, is to be attributed, in a great measure,
the stability of their connexion.
§ II.