595. By the combined action of the muscles of the lower jaw and tongue, and that of the teeth, the food is bruised, cut, torn, and divided into minute fragments. This operation is of so much importance that the whole process of digestion is imperfect without it. It is proved by direct experiment that the stomach acts upon the aliment with a facility in some degree proportionate to the perfection with which it is masticated. If an animal swallow morsels of food of different bulks, and the stomach be examined after a given time, digestion is found to be the most advanced in the smallest pieces, which are often completely softened, while the larger are scarcely acted upon at all. 596. At the same time that, by the operation of mastication, the aliment undergoes mechanical division, it imbibes a quantity of fluid derived from various sources. 1. From the membrane which lines the internal surface of the mouth, and which affords a covering to all the parts contained in it. 2. From numerous minute glands placed in clusters about the cheeks, gums, lips, palate, and tongue. Each of these glands is furnished with its own little duct, which, piercing the mucous membrane, opens into the cavity of the mouth. From these glands is derived the fluid with which the interior of the mouth is lubricated. It consists of a glutinous, adhesive, transparent fluid, of a light grey tint, salt taste, and slightly alkaline nature, termed mucus. [Illustration: Fig. CLXV.—_View of the Parotid Gland with the Muscles of the Face._ 1. Parotid gland. 2. Parotid duct. 3. Masseter muscle. 4. Buccinator. 5. Triangularis, or depressor of the angle of the mouth. 6. Depressor of the lower lip. 7. Orbicularis, or circular muscle of the mouth. 8. Great zygomatic, or the distorter of the mouth, as in laughing. 9. Elevator of the angle of the mouth. 10. Elevator of the upper lip, and wing of the nose. 11. Compressor of the cartilage of the nose. 12. Orbicularis, or circular muscle of the eyelids. 13. Occipito frontalis; elevator of the eyelids; motor of the scalp, &c., an important muscle of expression. 14. Tendinous portion of the occipito frontalis. 15. Elevator of the ear.] 3. From six large glands placed symmetrically, three on each side, termed the salivary glands, namely, the parotid (fig. CLXV. 1), situated before the ear; the submaxillary (fig. CLIII. 4), situated beneath the lower jaw; and the sublingual (fig. CLIII. 5), situated immediately under the tongue. Each of these glands is provided with a duct (figs. CLXV. 2, and cliii. 4, 5), by which it pours the fluid it elaborates, called saliva, into the mouth.