3. By the membrane which covers the root of the tooth; and, 4. By the gum. [Illustration: Fig. CLXII. View of the upper and lower teeth in the alveoli; the external alveolar plate being cut away to show the cancellated structure of the alveoli, and the articulation of the teeth.] These membranous substances, even more than the cancellated structure of the alveoli, absorb vibrations and counteract the communication of a shock to the bones of the face and head when the teeth act forcibly on hard materials; so many and such nice adjustments go to secure enjoyment, nay to prevent exquisite pain, in the simple operation of bringing the teeth into contact in the act of eating. [Illustration: Fig. CLXIII.—_View of the Muscles of Mastication, which elevate the lower jaw._ 1. The temporal muscle. 2. Its insertion passing beneath. 3. The zygoma. 4. The masseter muscle, its anterior portion reflected to show the insertion of the temporal. The action of these powerful muscles is to pull the lower jaw upwards with great force against the upper jaw, and at the same time to draw it a little forwards or backwards, according to the direction of the fibres of the muscles.] 593. The teeth in mastication are passive instruments put in motion by the jaws. The upper jaw is fixed, the lower only is movable. The lower jaw is capable of four different motions; depression, elevation, a motion forwards and backwards, and partial rotation. These simple motions are capable, by combination, of producing various compound motions. Numerous muscles, some of them endowed with prodigious power, are so disposed and combined as to be able, at the command of volition, to produce any of these motions that may be required, simple or compound. [Illustration: Fig. CLXIV.—_Muscles of the Jaw._ 1. Portion of the zygomatic process of the temporal bone. 2. Ascending plate of the lower jaw removed to expose, 3. External pterygoid, and, 4. Internal pterygoid muscles. The action of these muscles is to raise the lower jaw, and to pull it obliquely towards the opposite side. When both muscles act together, they bring the lower jaw forwards, so as to make the fore-teeth project beyond those of the upper jaw.] 594. By the combination, succession, alternation, and repetition of these motions, the lower is made to produce upon the upper jaw all the variety of pressure necessary for the mastication of the food. In this process the muscles of the tongue perform scarcely a less important part than the muscles of the lower jaw. Some of its muscular fibres shorten the tongue, some give it breadth, others render it concave, and others convex: so ample is the provision for moving this organ to different parts of the mouth and fauces, whether to bruise the softer parts of the aliment against the palate, to mix it with the saliva, or to place it under the pressure of the teeth.