Part 17

In general, it is those organs whose nutritive substance is gelatine, which have the greatest tendency to be placed in relation with the calcareous substance, and consequently to be encrusted with it. Hence why the cartilages especially are ossified; why those of the sutures disappearing, the bones of the cranium become continuous; why the larynx is finally almost all osseous; why the cartilages of the ribs are often as solid as the ribs themselves; why oftentimes many vertebræ united form a more or less considerable continuous mass. I would observe however that the arteries, which have so great a tendency to ossification, are not so evidently gelatinous as many other substances which ossify much less easily, as the tendons for example.

III. _Peculiar Phenomena of the Development of the Callus._

Nothing is easier after what has been said upon the osseous nutrition, than to understand the formation of the callus. We know that it has three periods, 1st, the development of the fleshy granulations; 2d, their change into cartilage; 3d, the change of this cartilage into bone. This triple phenomenon takes place in a space of time that varies according to the age, the fracture, the kind of bone, &c. but which is in general longer than that of the other cicatrices.

The development of fleshy granulations is a phenomenon common to every species of organ which has experienced a solution of continuity and whose divided edges are not in immediate contact. Here these granulations arise from every part of the divided surface, from the periosteum, the compact texture and that of the cells, the last especially. Those of one side unite to those of the opposite. Thus far the osseous cicatrix does not differ from that of the other parts. This state corresponds with the mucous state of natural ossification. As the fleshy fibres are but the extension of the nutritive parenchyma, they have its vital forces; their organic sensibility follows the same laws as in ordinary nutrition; at first it is in relation with gelatine; this is then exhaled; then commences the cartilaginous state; then the osseous cicatrix takes a peculiar character, which distinguishes it from that of the other organs.

At the end of a longer time, the organic sensibility increases in the parenchyma of cicatrization which the fleshy granulations form; then these become in relation with the calcareous substance which comes to the bone, and which they had until then repelled; they admit it then, as well as the red portion of the blood which always precedes it in every species of ossification.

Hence we see that the callus is cellular and vascular in the first period; that in the second it contains cellular texture and vessels, with gelatine; that in the third, it has cellular texture, vessels, and gelatine, with calcareous substance.

It has not the regular forms of the sound bone, because the parenchyma of cicatrization arising irregularly upon the osseous surfaces, the exhalation and absorption of gelatine cannot be made in a precise and regular manner. The callus is so much the larger in proportion to the separation of the ends of the bone, because the fleshy granulations having had more space to go over in order to meet, are more extensive, and consequently have absorbed more nutritive substance.

If the constant motion of the fractured ends prevents on each side the granulations, or what is the same thing, the two parenchymas of cicatrization from uniting, then, notwithstanding the exhalation of the nutritive substances in each of them, the bone does not unite, and hence the preternatural articulations.

Callus is formed with difficulty when the ends divided and laid bare, suppurate with the neighbouring parts, as happens in compound fractures, because the formation of pus expends the nutritive substances destined to repair the fracture. The further considerations upon this singular production belong to pathology.

I have not exposed in this chapter the ideas of the ancients, who thought that the bones were formed by the hardening of an osseous juice, the existence of which there is nothing to demonstrate; nor those of Haller, who imagined that the heart hollowed out arterial channels in the osseous substance by its own impulse, and hardened this substance by the pulsation of the arteries; nor those of Duhamel, who made every thing depend upon the periosteum; I refer to various works that have a thousand times refuted these opinions.

Without refuting any one in particular, I would remark that they have one fundamental error, viz. that of considering osseous nutrition in an insulated manner, of not presenting it as a division of general nutrition, of admitting for its explanation reasonings only applicable to the bones, and which are not derived as consequences from those which serve to establish the nutrition of all the organs. Let us never lose sight of this essential principle, upon which rest all the phenomena of the economy, viz. that over a multitude of effects, a very small number of causes only presides. Let us mistrust every explanation which is partial and mutilated, which circumscribes the resources of nature according to the limits of our weak understandings.

IV. _Peculiar Phenomena of the Development of the Teeth._

The teeth, differing in part by their texture, from the other bones, have also a peculiar mode of nutrition which we shall now examine. But as the knowledge of this supposes that of the general structure of the teeth, it is proper to explain here that structure, referring their description to the examination of the bones of the face.

_Organization of the Teeth._

The teeth are formed by two substances, one external, of a peculiar nature, called enamel, the other internal, which is the common base of it, and the texture of which is the same as that of the other bones. They have besides a cavity which contains a spongy substance, as yet but little known.

_Hard Portion of the Teeth._

The enamel of the tooth is only seen around the crown; some anatomists have thought that it extended a little upon the root, an opinion founded no doubt upon the extreme whiteness that the root often has in detached teeth, and which makes it impossible to distinguish the line of demarcation. But a very simple experiment proves this demarcation; it consists in macerating the tooth in diluted nitric acid. The acid immediately attacks both the root and the crown which it softens; but the first becomes yellow like almost all animal substances exposed to its action, whilst the other preserves its colour, and even becomes whiter. This experiment also proves that their respective natures are essentially different.

The enamel, thick on the top of the crown, grows thinner towards the root, an arrangement required by its use, which is to defend the tooth, to support principally the efforts of mastication, which are made especially upon the top of the crown.

This substance hard, compact, particularly when it has remained a long time in the air, acted upon with difficulty by the file, is composed of very close fibres, the direction of which cannot be traced. The medullary oil does not appear to penetrate it; it does not burn, but breaks by the action of fire, and is thus separated from the other substance, which, exposed to heat, at first becomes black, then burns like the other bones and gives out the same odour.

Is the enamel organized, or is it only a fluid which, oozing at first from the external surface of the tooth, afterwards becomes there hardened and concrete? This question is not I think easy to be resolved. The enamel has in fact attributes which seem equally favourable to both these opinions. On the one hand it is sensible, like every thing that is organized; it gives us, much more evidently than the hair or the nails, the sensation of bodies which strike it. The diluted acids, the vegetable especially, raise its sensibility so much, that the least touch becomes very painful a long time after their use. The teeth are then, as we call it, on edge. On the other hand the enamel has many characters that seem to denote a want of organization. 1st. It never inflames, or becomes the seat of any tumour, or any alteration which softens its texture; it never experiences any alteration, which by raising its life, renders it more sensible than in a natural state, as happens to the hair, for example, which ordinarily insensible, has a very great vital activity in the plica polonica. In fact we often judge of the vitality of organs more by their morbid alterations, than by their natural state. 2d. It appears that there does not take place in the enamel alternate exhalation and absorption of nutritive matters, or at least if it does, it is not sensible. Rubbing wears away this substance, which is never replaced; this is remarkable in old people, and in those who are in the habit of often striking their teeth together. We know that we file the enamel like an inorganic body, and that it is not reproduced, whilst the hair and the nails evidently grow after being cut. File the extremity of a long bone after amputation; fleshy granulations will soon grow upon the filed surface; the action of the instrument will be a stimulus which will develop the vital phenomena.

The osseous portion of the tooth composes the whole of the root and the interior of the crown; it is formed by the compact texture, very dense, having great resemblance to that of stone. It has none of the texture of the cells. Its fibres, very close to each other, have various directions, very difficult to trace, but which in general follow that of the roots; it is necessary, in order to see this direction perfectly, to soften the teeth in an acid.

Each tooth has a cavity situated in the crown, of the same form as the crown, diminishing in diameter as we advance in age, communicating externally by small canals, the number of which is equal to that of the distinct roots of the tooth, and which open at the end of these roots. This cavity is lined by a very delicate membrane on which the vessels ramify, and which, with its opposite face, covers the marrow.

_Soft Portion of the Tooth._

This is a spongy substance which appears to be formed by the interlacing of the vessels and nerves belonging to each tooth, but the nature of which is not yet well understood; we know only that it has a very great animal sensibility equal to that of the medullary organ. This is proved, 1st, by the pains of carious teeth in which the marrow is bare, and which are, as we know, extremely acute; 2d, by the introduction of a probe into the opening occasioned by caries, this produces no pain until it comes to the marrow, and then it is extreme; 3d, by opening a socket of a very young animal that has not yet cut its teeth. At this age the marrow is very considerable and the tooth being small in proportion, it is easy to raise the tooth without injuring it, because it has as yet no root and the opening at the base of the crown is very large. The tooth being raised and the marrow thus laid bare, if it is irritated in any way the animal gives signs of the most acute pain. I have often made this experiment, always easily done, on account of the want of thickness of the osseous layers which then form the sockets.

The teeth have remarkable sympathies, which extend not only to the solid part, but also to the marrow. As this is much greater in proportion in the early ages, as it is almost the predominant part of the tooth, these sympathies are then more numerous and evident. In these sympathies, sometimes the animal and sometimes the organic properties are brought into action.

The sympathies of animal sensibility are evident in those pains of which the teeth become the seat from the action of cold or moisture upon the cutaneous system; in those produced in the face and the head by the caries of a tooth. Fauchart relates a case of obstinate hemicrania, which was immediately removed by the extraction of a tooth. The sensibility of the ear and the eyes is changed in some violent cases of tooth ache. The animal contractility is also brought into action in the sympathies of the teeth; nothing is more frequent in dentition, than convulsions of the voluntary muscles. Tissot speaks of a spasm of the muscles of the jaw, which was cured by the extraction of two carious teeth, and of a convulsion in the muscles of the neck that occasioned death, the primitive source of which was in a decayed tooth.

The organic sympathies are not less often produced by affections of the teeth. Spasmodic vomiting, diarrhœa, frequency of the pulse, oftentimes involuntary evacuation of urine, phenomena, over which the sensible organic contractility of the stomach, the intestines, the bladder and the heart presides, are the frequent effects of dentition and violent pains of the teeth, especially of the first. The insensible organic contractility, and the organic sensibility are brought sympathetically into action in the enlargements of the parotid gland, in the general swelling of the face, in the increased secretion of saliva and sometimes in the erysipelatous inflammations which take place from an acute affection of the teeth.

The sympathies of the teeth often take place between the two corresponding teeth of the same row or of the two rows. My first upper molar tooth of the left side is a little carious; from time to time it gives me pain, then invariably the first molar tooth of the right side becomes as painful, though it is sound. There are other cases in which a tooth being painful below, sympathetic pains are felt in that which is above, and vice versa.

The structure of the teeth having been explained, let us see how their different substances are developed. This subject of osseous nutrition does not appear to me to have been clearly illustrated by any author. I shall attempt to explain it better. There are two dentitions, one is provisional and limited to the first age, the other belongs to the whole life; each should be considered before, during, and after the cutting of the teeth.

_First Dentition considered before Cutting._

The phenomena of dentition before the period of cutting are these; the jaws of the fœtus are closed the whole length of their upper edge; they appear to be homogeneous at first view; but examined in their interior, they exhibit a row of small membranous follicles, separated by delicate partitions, disposed like the teeth of which they are to serve as the germ, and having the following arrangement.

The membrane which serves as a covering to the follicle forms a sac without an opening, which lines at first all the parietes of the socket, to which it is attached by elongations. At the place where the vessels and nerves enter, this sac leaves the socket, becomes detached, is folded into the form of a canal which accompanies the vascular and nervous bundle, and afterwards spreads out upon the marrow of the tooth which is the termination of the bundle.

It follows from this that this membrane has the general conformation of the serous membranes, in the shape of some kinds of night-caps. It has two portions, the one attached and lining the socket, the other loose and covering the marrow, as for example, the pleura has a costal and a pulmonary portion. The marrow and the vessels, though contained in its duplicature, are in truth found without the cavity, which is lubricated by a simple exhalation. I have found that this exhalation was like that of the serous membranes, essentially of an albuminous nature; the action of the nitric acid, that of alkohol and of fire incontestably prove it. Subjected to the action of one of these agents, especially the first, the membrane whitens immediately. The layer of albumen which covers it becomes concrete and coagulated, as when we make a similar experiment upon a serous surface.

The marrow, very considerable at this period, is found suspended, like a bunch of grapes, from the extremity of the vessels and the nerves.

It is upon the medullary portion of the membrane of the follicle, and upon the surface of its loose extremity, that the first osseous point is developed; it soon extends, and takes precisely the form of the top of the crown, which it is afterwards to form, that is to say, that it is quadrilateral in the molar teeth, pointed in the canine, and wedge shaped in the incisors. Developed at first nearest the gums, it extends afterwards along the vascular and nervous stem, it is moulded upon it as it approaches the part of the alveolus where it enters; so that it exhibits on this side a concave surface which embraces the pulpy portion of the membrane, and adheres by several vascular elongations; and as this portion is loose, the first rudiment of the tooth floats also in the cavity of the membrane, as we can see very well by cutting the alveolar portion of this membrane, after having destroyed the corresponding part of the alveolus.

The following consequences result from this kind of development; 1st. The crown is first formed, and the root is not produced but as the ossification in length advances upon the portion of membrane lining the vascular and nervous bundle. 2d. As all the vessels that come to the tooth enter at its internal surface, and as the external is entirely free in the cavity of the membrane, the ossification in thickness is made especially at the expense of the internal cavity which is constantly contracting, as well as the marrow, an arrangement, the reverse of that of the other bones, the ossification of which commences at a point placed in the centre of the cartilage, and which at first solid in the middle, afterwards become hollow for the medullary cavities and those of the cells, which are always enlarging. 3d. After the ossification of the tooth, the portion of the membrane of the follicle which lined the alveolus, remains the same, whilst that its portion corresponding with the marrow, originally free at the other side, becomes adherent on this side to the whole dental cavity which it lines, of which it forms the membrane noticed above in the article on the structure of the teeth, and which is thus found between the marrow and the osseous substance. 4th. The marrow of the tooth is the part first formed, and the most considerable in the first periods of life. It appears that the osseous substance is next formed, and that the enamel afterwards arises on the exterior of this. I have not yet been able to make evident the manner of its origin.

It is difficult to ascertain at what period the membraneous follicle is formed; that of the first ossification appears to be from the fourth to the fifth month. At the time of birth, we find the twenty teeth of the first dentition already advanced; the whole crown is formed; the beginning of the root appears also in the form of a broad tube, with extremely delicate parietes, and which is constantly becoming longer and thicker; when it reaches the bottom of the socket, the tooth immediately appears externally as this is too narrow to contain it.

The number of teeth, less in the first than in the second dentition, gives a peculiar form to the jaws of the fœtus and the infant, especially to the lower one, which is less elongated in front, and consequently wider in proportion than in the adult, in whom in order to receive all the teeth, the alveolar border must necessarily be more extended. This arrangement of the lower jaw has a great influence in the expression of the physiognomy.

_First Dentition considered at the period of Cutting._

The following phenomena take place about the sixth or seventh month after birth, very rarely sooner, still more rarely before birth, though there are examples of this, as is proved by the history of Louis XIV. At first the two small incisor teeth of the lower jaw appear, sometimes together, sometimes separately; soon after the corresponding incisors of the superior jaw. A month or two later, the four other incisors are cut. At the end of the first year, the four canine teeth usually appear. At the end of the second, or often later, two molar are cut in each jaw and two others soon follow. Each cutting almost always begins in the lower jaw. At the age of four years, four and a half, sometimes five or six, always at a very uncertain period, there appear below two other molars and then two above, which complete the number of twenty-four teeth forming the first dentition; all these except the last four fall out and are replaced by new ones.

The following is the mechanism of this first dentition; the ossification extending constantly towards the root, the tooth can no longer be contained in the socket; it pierces the alveolar portion of the membrane and the mucous membrane of the mouth and an intermediate medullary texture that separates them, with much ease, as this triple layer gradually becomes thinner as the cutting approaches. Is this phenomenon owing only to the mechanical pressure of the tooth? I think that there is another cause; for observe that here the membranes are very little raised before rupturing; whilst that in polypi and other tumours that sometimes arise under the membrane of the gums, it is infinitely more stretched, still it does not break, but is only lifted up. The mechanism of the opening of the gums is not more known than the principle of the severe accidents which are sometimes connected with it. The sac which formed the original membrane of the follicle being thus open, its portion which lines the socket unites to the membrane of the mouth, becomes continuous with it and at the same time adheres intimately to the neck of the tooth; and as during the development of the root, the internal face of this membranous portion, at first loose as we have seen, has gradually contracted adhesions with it, it follows that this root is found fastened between the alveolar portion which lines its exterior, and the medullary portion which covers the interior; it is this which gives it solidity. As the adhesions of the membrane increase, we can less easily distinguish it. It is rare that in the first dentition the formation of the root is finished as completely as in the second; its internal cavity remains also very broad, and the marrow is more developed.

_Second Dentition considered before Cutting._

It is necessary, as in the preceding case, to distinguish the nutritive phenomena into those which take place before, during and after the cutting. Before the cutting, we observe by opening the jaw, a row of dental follicles, corresponding to the row of teeth already formed, situated below or at the side, and separated from them by little partitions, the thickness of which is found greater in proportion as it is examined nearer infancy.

These follicles have precisely the same arrangement as those of the first dentition; like them they form sacs without an opening, the alveolar portion of which is attached, and the loose medullary portion is covered on its surface with the first osseous layers for the crown. The manner of growth is the same; that is to say, it takes place from the exterior to the interior, the reverse of the other bones; an arrangement, which gives the part of the tooth immediately in contact with foreign bodies, being the first formed, time to acquire the solidity necessary for its functions.