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.