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.