Part 27

Why has nature given to the life of the teeth a shorter term than to that of the other bones, which do not cease to exist but with the other organs, whilst the teeth die a long time before? Is it because the stomach becoming weak with age, the animals are thence compelled to nourish themselves in their old age, with soft substances, adapted to the languid state of the gastric forces? Undoubtedly in man, a thousand causes, arising especially from the nature of the aliments, their degree of heat and cold, the manner in which they are cooked, their infinitely various qualities, hasten the natural period of the death and the fall of the teeth, because by incessantly exciting and stimulating these organs, they keep them in a state of constant activity, which exhausts their life sooner than it otherwise would have been. Thus a thousand causes arising from society, make the term of the general life much shorter than that fixed by nature. But in general in all animals, the death of the teeth precedes that of the other organs, though they are not under the influence of society, and they masticate only aliments destined by nature to be in contact with their teeth.

The jaws destitute of teeth in old age, contract; the sockets are effaced; the texture of the gums becomes firmer, and mastication is continued, though with more difficulty. In this change of conformation, the alveolar edge is thrown back; hence the prominence of the chin before. It diminishes in height; hence the approximation of this part to the nose, a phenomenon that arises especially from the absence of the teeth.

V. Particular Phenomena of the Development of the Sesamoid Bones.

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?

MEDULLARY SYSTEM.

Though the medullary system is only met with in the bones, and though its principal uses appear to relate entirely to them, yet its properties and life differ so much from the properties and life of these organs, that we are compelled to examine them in a separate manner.

We distinguish two kinds of medullary systems; one occupies the texture of the cells of the extremities of the long bones, and the whole of the interior of the short and flat bones; the other is found only in the middle part of the first; let us examine each separately.

ARTICLE FIRST.

MEDULLARY SYSTEM OF THE FLAT AND SHORT BONES, AND THE EXTREMITIES OF THE LONG ONES.

I. Origin and Conformation.

This system appears to be the expansion of the vessels which penetrate the bones through the foramina of the second order, that is to say, through those that go to the common texture of the cells. These vessels having arrived on the internal surface of the cells, divide ad infinitum and anastomose in a thousand ways. Their interlacing gives to the interior of the texture of the cells that red appearance that characterises it, and which is so much the more evident, as it is examined at an age nearer infancy, because in fact the vascular system which is very evident at this period, becomes contracted and effaced as we recede from it.

These are the vessels which, in the section of the bones of the cranium by the trephine, give to the saw-dust the redness that is observed. It is these that produce the same phenomenon in the amputation of the extremity of the limbs. Though in general they remain loaded with blood at the moment of death, yet we can, as I have often done, accumulate in them still more by fine injections, which drive before them that which is found in the vessels, and concentrate it at their extremities; then the spongy texture of the adult is almost as red as that of the child which has not been prepared.

II. Organization.

Authors speak of a delicate membrane that lines the interior of all the osseous cells, and which they consider to be the exhalant organ of the medullary fluid. I have never been able, though my researches have been numerous, to discover a similar membrane. We see only the vascular elongations of which I have spoken, which, greatly multiplied, appear in fact to form a membrane, but when examined attentively are found to be very distinct from each other, not continuous, except at the place of the anastomoses, and leaving between them many small spaces in which the bone is not covered, but is in contact with the medullary fluid.

The exhalation then of this fluid appears to arise only from this vascular net-work, and in this respect it is analogous to that of the compact substance, which evidently contains no membrane, and the pores of which are however found filled with this medullary fluid, as is proved by the combustion of the compact texture and its exposure to the sun or artificial heat.

III. Properties.