These capsules form a kind of cylindrical sac open at the two
extremities, attached by the circumference of its openings, around the
superior and inferior articular surfaces, intermixing at its insertion
with the periosteum. They are so much the more loose, as the motions
of the articulations are the more extended; that of the humerus, for
example, allows a much greater separation of the osseous articular
surfaces, than that of the femur; their length in fact is almost the
same. Now, as on the one hand, the neck of the first bone is much less
than that of the second, and as on the other, both these capsules are
inserted at the base of this neck, it follows that the extent of the
separation is in the inverse ratio of the length of the articular necks.
Much cellular texture surrounds these capsules externally, which the
tendinous fibres and even the tendons coming from the neighbouring
muscles, strengthen remarkably. They are sometimes open to allow
tendons to pass which are inserted in the bone between them and the
synovial membrane, an example of this is seen in the scapulo-humeral
articulation for the sub-scapularis. Anatomists who have observed
the insertion of the tendons in the capsules, have concluded from it,
that the muscles of these tendons were destined to prevent the capsule
from being pinched by the articular surfaces in motion. This appears
to me improbable; but at least the muscles are destined to prevent
the looseness of the capsule during the great motions, which would
have been weakened by this looseness; thus there are many of this
kind of muscles at the humeral capsule, whilst we see none of them at
the femoral, which is, as I have said, much less loose. Within, the
capsules are very closely united to the synovial membrane, especially
in adults; for in infants, this adhesion is less. Near their extremity
however this relation does not exist, because the synovial membrane
being reflected upon the cartilage, a triangular space is left
between it and the capsule which is attached to the bone, and as this
arrangement continues all round the articulation, there results from
it a kind of circular canal, filled with cellular texture, and covered
with vessels, which I have sometimes distended with an injection pushed
into a small opening made for the purpose.
The intimate union of the capsule with the synovial membrane prevents
its folds and also its contusion in the great articular motions.