The sesamoid bones exhibit a less marked exception than that of the
teeth, to the general laws of ossification, but one, however, which is
as real.
_General Arrangement of the Sesamoid Bones._
These small bones, commonly of a round form, and of various size, do
not usually exceed that of a pea, except however the patella; they are
in general found only in the extremities; the trunk never has any of
them.
In the superior extremities we hardly see them, except in the hand,
in which the articulation of the thumb with the first bone of the
metacarpus always presents two of them, and in which they are sometimes
found in the analogous articulation of the index finger, rarely in that
of the little one, or in the phalangeal articulation of the thumb.
In the inferior extremities on the contrary, they are numerous and
especially much more evident. Two are seen on each condyle of the
femur, in the tendons of the biceps, behind the knee; in front is the
patella. In the foot, the tendon of the tibialis posticus near its
insertion in the tuberosity of the scaphoid bone, that of the peroneus
longus in its passage under the cuboid, have also sesamoid bones. We
uniformly see two under the metatarso-phalangeal articulations of the
great toe; under most of the analogous articulations of the other toes,
they are also found, though they are more variable. In the phalangeal
articulations I have also seen them many times. In general, the
sesamoid bones exist only in the direction of flexion, which is that
in which the greatest efforts to support are made. In the direction of
extension I know no one but the patella.
These little bones have not, like the others, a separate existence;
they are developed always in a fibrous organ, either in a tendon,
as those of the biceps, the peroneus, the tibialis posticus, as
also the patella; or in a ligament, as all those placed before
the metacarpo-phalangeal articulations, the metatarso-phalangeal
or phalangeal, which have for their basis the great transverse
fibrous fascia, which we have called the anterior ligament of these
articulations.
_Fibro-Cartilaginous State._
The two primitive bases of the sesamoid bones remain for a long
time without exhibiting any rudiments of them, and are at the place
where these bones are to exist as they are everywhere else. Their
organization is generally uniform. Some time after birth, a little
more gelatine than would serve for the nutrition of these two fibrous
bodies begins to be exhaled at the place where the sesamoid bones will
hereafter be found; then arise cartilages, essentially different from
the cartilages of ordinary ossification, which are nearly of the same
nature as those of the extremities of the long bones of adults, whilst
that these belong truly to the class of fibro-cartilaginous substances.
They resemble in their nature the inter-articular fibro-cartilages,
those of the tendinous grooves, &c. These are not cartilages, but
the fibro-cartilages of ossification, of which we distinguish so
much better the fibrous base, as it is nearer the period of their
development that we examine them.
_Osseous State._
Gradually the vessels of these fibro-cartilages, which had only
circulated white fluids, have their sensibility placed in relation with
the blood; this fluid penetrates them; at the same time the phosphate
of lime begins to be deposited in them; then the texture of the cells
is formed in the interior by a mechanism analogous to that of the other
bones; a delicate compact layer is developed on the exterior. But
in the midst of this new bone, the fibrous base always remains; the
fibres of the tendon, above the sesamoid, are continued, if we may so
say, through its substance with those below it; thus the cicatrices of
these bones, when they are fractured, have a peculiar and distinctive
character; it is their fibrous base, which extending itself for their
reunion, produces this difference. We know that the callus of the
patella is not the same as that of the other bones. Often when the
apparatus has not been exactly kept in place, there remains between the
two fragments a fibro-cartilaginous texture as a means of union; now
this texture is the development not only of the cartilaginous portion
of the bone, but also of the portion of the tendon of the extensors,
which makes part of the organization of this bone. The life of the
sesamoid bones partakes almost as much of that of the fibrous as of
that of the osseous system.
As we advance in age, these small bones increase and become more
characterized in the animal economy; oftentimes they are developed very
late, at the age of twenty, thirty, or even forty years. In some old
people they are very large on the foot. I have seen the bodies of two
persons subject to gout, in which they were so developed as probably
to interfere with motion. Was there any connexion between them and this
severe affection? I know only these two facts.
The sesamoid bones elongate their tendons from the centre of motion,
facilitate their sliding upon the bones, defend their articulations and
even contribute to their motions. All those developed in the anterior
ligaments of the metacarpo and metatarso-phalangeal articulations, and
of the phalangeal themselves, contribute also to the motion of these
articulations. A portion of the synovial membrane is spread upon their
face that corresponds with it, and which remains slightly cartilaginous.
The formation of the sesamoid bones is not a mechanical effect of the
pressure of the tendons or the ligaments against the bones, as has been
said, but the result of the laws of ossification. In fact, in the first
supposition, why should all the articulations of the hand and the foot,
other than those pointed out above, being exposed to a motion nearly
equal to the motion of these, be destitute of these bones?