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?