less distinctly. But it is an exceedingly difficult thing to follow the single twigs, of which we have spoken, from their origin to their insertion, or, _vice versa_, from their insertion to their origin. And so, adds the great anatomist, being able to observe but a small part of the things I should like to see, I find myself compelled, in my perplexity, to supply by the aid of ratiocination the deficiency of the senses. I therefore maintain that the interior part of a tooth is susceptible of experiencing pain accompanied by a feeling of pulsation (a fact already mentioned by Galen), because a nerve and an artery penetrate into it. In the ox the penetration of bloodvessels into the roots of the teeth can be more readily ascertained than in man. It may be admitted that the same occurs in the human teeth; and this, for the reasons already given, and also because only by admitting the existence of an artery within the cavity of the tooth can be explained the copious flow of florid red blood from a decayed tooth, which has, in some cases, been known to imperil the life of a patient. And I myself, says Eustachius, have observed with my own eyes an accident of this kind. The author then passes on to speak of the eruption of the teeth,[292] but the data with which he furnishes us are neither very precise nor very exact. Eustachius, without declaring himself for or against it, cites, in this chapter, the opinion of those who believe in the possibility of a third dentition in old people. He returns to this subject in the last chapter but one of his book, which treats of dental anomalies: “Ali,” says he, “testifies to old persons having had all their teeth renewed. This has been derided as chimerical by medical men of later date, or at least only admitted under the condition that such teeth be of a nature completely different from the first.” Our teeth, says the author, grow old together with us, and toward the term of life they abandon us, a fact which also distinguishes them from the other bones. When, however, it occurs, through illness, that the teeth are extracted or fall out spontaneously before the period of old age, the alveoli become filled up with a bony substance; and in addition the two osseous scales of the maxillary bones approach one another and unite together in such a manner as to form a sharp margin, every vestige of a cavity being obliterated.