It would be surprising, then, if we were to find no attention paid to dentistry during the Middle Ages. As a matter of fact, a number of the old surgeons include in their textbooks of surgery the discussion of oral surgery. Aëtius evidently knew much about the hygiene of the teeth, and discusses extraction and the cure of fistulæ of the gums as well as the surgical treatment of many other lesions of the mouth. Paul of Ægina in the century after Aëtius has even more details; and while they both quote mainly from older authors, there seems no doubt that they themselves must have had considerable practical experience in the treatment of the teeth and had made not a few observations. The Arabians took up the subject, and discussed dental diseases and their treatment rationally and in considerable detail. Abulcassis particularly has much that is of significance and interest. We have pictures of two score of dental instruments that were used by him. The Arabs not only treated and filled carious teeth, and even replaced those that were lost, but they also corrected deformities of the mouth and the dental arches. Orthodontia is usually thought of as of much later origin, yet no one who knows Abulcassis's work can speak of efforts at straightening the teeth as _invented_ after his time. [Illustration: SURGICAL INSTRUMENTS OF THE ARABS, ACCORDING TO ABULCASIM _After plates in Gurlt's "Geschichte der Chirurgie"_ 1. A pincher for extracting foreign bodies from the ear 2. An ear syringe for injections 3. A tongue depressor 4. Concave scissors for the removal of tonsils 5. Curved pinchers for foreign bodies in the throat 6 to 29. Instruments for the treatment of the teeth 19 and 20. Forceps 21 to 25. Levers and hooks for the removal of roots 26. Strong pinchers for the same 27. A tooth saw 28 and 29. Files for the teeth] The great surgeons of the later Middle Ages in their textbooks of surgery usually include remarks on oral surgery, and suggest treatment for the various diseases of the teeth. Guy de Chauliac in "La Grande Chirurgie" lays down certain rules for the preservation of the teeth, and shows that the ordinary causes of dental decay were well recognized in his time. Emphasis was laid by him on not taking foods too hot or too cold, and above all on the advisability of not having either hot or cold food followed by something very different from it in temperature. The breaking of hard things with the teeth was warned against as responsible for such fissures in the enamel as gave opportunity for the development of decay. The eating of sweets, and especially the sticky sweets, preserves, and the like, were recognized as an important source of caries. The teeth were supposed to be cleaned frequently, and not to be cleaned too roughly, for this would do more harm than good.