Magnified section of a tooth, to illustrate the arrangement of the fibrous crystals composing the enamel. 1. Cavity of the tooth. 2. Bony substance. 3. Enamel, showing the crystals disposed in radii.] 589. In this manner an instrument is constructed possessing the requisite hardness, durability, and insensibility; yet organized, alive, as truly an integrant portion of the living system as the eye or the heart. 590. No less care is indicated in fixing than in constructing the instrument. It is held in its situation not by one expedient, but by many. 1. All along the margin of both jaws is placed a bony arch, pierced with holes, which constitute the sockets, called alveoli, for the teeth (fig. CLXI.). Each socket or alveolus is distinct, there being one alveolus for each tooth (fig. CLXI.). The adaptation of the root to the alveolus is so exact, and the adhesion so close, that each root is fixed in its alveolus just as a nail is fixed when driven into a board. [Illustration: Fig. CLXI. Upper jaw, showing the alveoli.] 2. The roots of the tooth, when there are more than one, deviate from a straight line (fig. CLVI. 6, 7, 8); and this deviation from parallelism, on an obvious mechanical principle, adds to the firmness of the connexion. 3. Adherent by one edge to the bony arch of the jaw, and by the other to the neck of the tooth, is a peculiar substance, dense, firm, membranous, called the gum, less hard than cartilage, but much harder than skin, or common membrane; abounding with blood-vessels, yet but little sensible; constructed for the express purpose of assisting to fix the teeth in their situation. 4. The dense and firm membrane covering the bony arch of the jaw is continued into each alveolus which it lines; from the bottom of the alveolus this membrane is reflected over the root of the tooth, which it completely invests as far as the neck, where it terminates, and where the enamel begins: this membrane, like a tense and strong band, powerfully assists in fixing the tooth. 5. Lastly, the vessels and nerves which enter at the extremity of the root, like so many strings, assist in tying it down; hence, when in the progress of age, all the other fastenings are removed, these strings hold the teeth so firmly to the bottom of the socket, that their removal always requires considerable force. 591. But a dense substance like enamel, acting with force against so hard a substance as bone, would produce a jar which, propagated along the bones of the face and skull to the brain, would severely injure that tender organ, and effectually interfere with the comfort of eating. 592. This evil is guarded against, 1. By the structure of the alveoli (fig. CLXII.), which are composed not of dense and compact, but of loose and spongy bone (fig. CLXII.). This cancellated arrangement of the osseous fibres is admirably adapted for absorbing vibrations and preventing their propagation (90). 2. By the membrane which lines the socket.