Chapters XIX and XX are, comparatively speaking, of little importance. In the former the author undertakes especially to examine the opinions of Galen on dental bloodvessels and nerves, and discusses whether it were known to him that these vessels and nerves penetrate into the internal part of the teeth. In the latter, Eustachius speaks of the great difficulties that are encountered in dissecting dental bloodvessels and nerves, and reproves those who, by inaccurate illustrative figures, convey the erroneous idea that these parts are very clearly and easily observable. In Chapter XXI the author goes on to speak of the best mode of proceeding in order to make successful observation of the small nerves and vessels going to the roots of the teeth. These researches are much more easily made in large animals than in man; and therefore such things as cannot be observed well in the latter must be studied in the former. In the first place, it is necessary to dissect the lower jaw; and after having done so several times, with all the accuracy required in making researches of this kind, one may proceed to study the dental nerves and vessels of the upper jaw, which is much more difficult. Having opened up the inside of the lower jaw, one observes a cavity full of marrow, and within this a nerve enclosed entirely in its own sheath. Having removed the marrow, and opened the sheath lengthwise, one perceives that the nerve therein enclosed is constituted (analogous to what may be observed in the large nerves of the limbs) by several nervous strings, and that among these runs a comparatively large artery, besides small vascular branches of minor importance. If one then removes the sheath from the bone, together with the nerve and the vessels contained in it, raising it very gently, one sees, issuing therefrom, some very slender fibers, on the nature of which it is, however, difficult to pronounce; and, considering their thinness, one can hardly conceive that they are composed of three different elements, that is, of small nervous, arterial, and venous twigs. At any rate, the author admits that this may be so. On arriving at the lesser teeth, the nerve and the artery that accompanies it divide into two branches, one of which traverses the opening presented by the bone at that point (_mental foramen_), and is destined to the lower lip; the other directs its course toward the roots of the incisors. The small twigs which penetrate into the roots of the incisor and canine teeth are less slender than those which enter the roots of the molars, and are easily to be seen not only in large animals, but also in man. If the tooth of an ox or that of a ram be split through the middle, the mucous substance contained in the interior is seen to be traversed by small bloodvessels; and one perceives, besides, certain fibers, which are probably nerves. All these things, says Eustachius, I have observed many times in different animals, in some cases more, in others