Phenomena subsequent to the Cutting of the Second Teeth.
After being cut, the teeth evidently grow, 1st, in length; 2d, in thickness. It is only the root that is enlarged in the first direction; the crown preserves always the same dimensions; and if in old people it appears longer, it is only because the gums have retracted; a phenomenon which besides we very often observe in persons who have become thin, in those who have made use of mercury, &c.
The growth in the second direction is not made without, but only within; the canal of the root and the cavity of the body are constantly contracting, and are finally obliterated. Then the tooth receiving no longer the blood or the influence of the nerves, dies and falls out. But this death appears also to be hastened by the accumulation of osseous substance, of a very great quantity of the phosphate of lime, which predominates there so much over the gelatine, that the principle of life is entirely destroyed, so that in this respect, the shedding of the teeth exhibits a phenomenon analogous to that of the shedding of the horns of the herbivorous animals, of the calcareous shell of the crustaceous ones, &c.
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.