characters of those of the carnivora and the herbivora. (Fig. 23.) The incisors (a), four in Fig. 23. HUMAN TEETH-UPPER JAW.-a. Incisors. b. Canines. c. Anterior molars. d. Posterior molars. ratively flat surfaces, like those of the herbivora; but instead of presenting curvilinear ridges, are covered with more or less conical eminences, like those of the carnivora. In the human subject, therefore, the teeth are evidently adapted for a mixed diet, consisting of both animal and vegetable food. Mastication is here as perfect as it is in the herbivora, though less prolonged and laborious; for the vegetable substances used by man, as already remarked, are previously separated to a great extent from their impurities, and softened by cooking; so that they do not require, for their mastication, so extensive and powerful a triturating apparatus. Finally, animal substances are more completely masticated in the human subject than they are in the carnivora, and their digestion is accordingly completed with greater rapidity. We can easily estimate, from the facts above stated, the great importance, to the digestive process, of a thorough preliminary mastication. If the food be hastily swallowed in undivided masses, it must remain a long time undissolved in the stomach, where it will become a source of irritation and disturbance; but if reduced beforehand, by mastication, to a state of minute subdivision, it is readily attacked by the digestive fluids, and becomes speedily and completely liquefied. SALIVA. At the same time that the food is masticated, it is mixed in the cavity of the mouth with the first of the digestive fluids, viz., the saliva. Human saliva, as it is obtained directly from the buccal cavity, is a colorless, slightly viscid and alkaline fluid, with a specific gravity of 1005. When first discharged, it is frothy and opaline, holding in suspension minute, whitish flocculi. On being allowed to stand for some hours in a cylindrical glass vessel, an opaque, whitish deposit collects at the bottom, while the supernatant fluid becomes clear. The deposit, when examined by the micro Fig. 24. BUCCAL AND GLANDULAR EPITHELIUM, with Granular Matter and Oil-globules; deposited as sediment from human saliva. scope (Fig. 24), is seen to consist of abundant epithelium scales from the internal surface of the mouth, detached by mechanical irritation, minute, roundish, granular, nucleated cells, apparently epithelium from the mucous follicles, a certain amount of granular matter, and a few oil-globules. The supernatant fluid has a faint bluish tinge, becomes slightly opalescent by boiling, and the addition of nitric acid. Alcohol in excess, causes the precipitation of abundant whitish flocculi. According to Bidder and Schmidt,' the composition of saliva is as follows: The organic substance present in the saliva has been occasionally 1 Verdauungssæfte und Stoffwechsel. Leipzig, 1852. known by the name of ptyaline. It is coagulable by alcohol, but not by a boiling temperature. A very little albumen is also present, mingled with the ptyaline, and produces the opalescence which appears in the saliva when raised to a boiling temperature. The sulpho-cyanogen may be detected by a solution of chloride of iron, which produces the characteristic red color of sulpho-cyanide of iron. The alkaline reaction of the saliva varies in intensity during the day, but is nearly always sufficiently distinct. The saliva is not a simple secretion, but a mixture of four distinct fluids, which differ from each other in the source from which they are derived, and in their physical and chemical properties. These secretions are, in the human subject, first, that of the parotid gland; second, that of the submaxillary; third, that of the sublingual; and fourth, that of the mucous follicles of the mouth. These different fluids have been comparatively studied, in the lower animals, by Bernard, Frerichs, and Bidder and Schmidt. The parotid saliva is obtained in a state of purity from the dog by exposing the duct of Steno where it crosses the masseter muscle, and introducing into it, through an artificial opening, a fine silver canula. The parotid saliva then runs directly from its external orifice, without being mixed with that of the other salivary glands. It is clear, limpid, and watery, without the slightest viscidity, and has a faintly alkaline reaction. The submaxillary saliva is obtained in a similar manner, by inserting a canula into Wharton's duct. It differs from the parotid secretion, so far as its physical properties are concerned, chiefly in possessing a well-marked viscidity. It is alkaline in reaction. The sublingual saliva is also alkaline, colorless, and transparent, and possesses a greater degree of viscidity than that from the submaxillary. The mucous secretion of the follicles of the mouth, which forms properly a part of the saliva, is obtained by placing a ligature simultaneously on Wharton's and Steno's ducts, and on that of the sublingual gland, so as to shut out from the mouth all the glandular salivary secretions, and then collecting the fluid secreted by the buccal mucous membrane. This fluid is very scanty, and much more viscid than either of the other secretions; so much so, that it cannot be poured out in drops when received in a glass vessel, but adheres strongly to the surface of the glass. According to Bernard,' the principal distinction between these Leçons de Physiologie Expérimentale, Paris, 1856, p. 93. different salivary fluids resides in the character of the organic matter peculiar to each one. The organic ingredient of the parotid saliva is small in quantity, perfectly fluid, and analogous in some respects to albumen, since it coagulates by a boiling temperature. That of the submaxillary is moderately viscid, and has a tendency to solidify or gelatinize on cooling; while that of the sublingual and mucous secretions is excessively viscid, but does not gelatinize at a low temperature. The saliva proper consists, therefore, of a nearly homogeneous mixture of all these different secretions; of which that from the parotid is the most abundant, that of the sublingual and of the mucous follicles of the mouth the least so. Bidder and Schmidt obtained, from one of the parotid glands of the dog, one hundred and thirty-six grains of fluid in an hour; from the submaxillary, eighty-seven grains; and from the mucous follicles of the mouth, after ligature of both Wharton's and Steno's ducts, thirty-one grains. The saliva, as a whole, is not secreted with uniform rapidity at all times. While fasting, and while the tongue and jaws are at rest, it is supplied in but small quantity, just sufficient to keep the mucous membrane of the mouth moist and pliable. Any movement of the jaws, however, increases the rapidity of its flow. It is still more powerfully stimulated by the introduction of food, particularly that which has a decided taste, or which requires an active movement of the jaws for its mastication. The saliva is then poured out in abundance, and continues to be rapidly secreted until the food is masticated and swallowed. A very curious fact has been observed by M. Colin, Professor of Anatomy and Physiology at the Veterinary School of Alfort,' viz., that in the horse and ass, as well as in the cow and other ruminating animals, the parotid glands of the two opposite sides, during mastication, are never in active secretion at the same time; but that they alternate with each other, one remaining quiescent while the other is active, and vice versa. In these animals, mastication is said to be unilateral, that is, when the animal commences feeding or ruminating the food is triturated, for fifteen minutes or more, by the molars of one side only. It is then changed to the opposite side; and for the next fifteen minutes mastication is performed by the molars of that side only. It is then changed back again, and so on alternately, so that the direction of the lateral movements of 1 Traité de Physiologie Comparée, Paris, 1854, p. 468. the jaw may be reversed many times during the course of a meal. By establishing a salivary fistula simultaneously on each side, it is found that the flow of saliva corresponds with the direction of the masticatory movement. When the animal masticates on the right side, it is the right parotid which secretes actively, while but little saliva is supplied by the left; when mastication is on the left side, the left parotid pours out an abundance of fluid, while the right is nearly inactive. It is probable, however, that this alternation of function does not exist, to the same extent at least, in man and the carnivora, in whom mastication is performed very nearly on both sides at once. Owing to the variations in the rapidity of its secretion, and also to the fact that it is not so readily excited by artificial means as by the presence of food, it becomes somewhat difficult to estimate the total quantity of saliva secreted daily. The first attempt to do so was made by Mitscherlich,' who collected from two to three ounces in twenty-four hours from an accidental salivary fistula of Steno's duct in the human subject; from which it was supposed that the total amount secreted by all the glands was from ten to twelve ounces daily. As this man was a hospital patient, however, and suffering from constitutional debility, the above calculation cannot be regarded as an accurate one, and accordingly Bidder and Schmidt' make a higher estimate. One of these observers, in experimenting upon himself, collected from the mouth in one hour, without using any artificial stimulus to the secretion, 1500 grains of saliva; and calculates, therefore, the amount secreted daily, making an allow ance of seven hours for sleep, as not far from 25,000 grains, or about three and a half pounds avoirdupois. On repeating this experiment, however, we have not been able to collect from the mouth, without artificial stimulus, more than 556 grains of saliva per hour. This quantity, however, may be greatly increased by the introduction into the mouth of any smooth unirritating substance, as glass beads or the like; and during the mastication of food, the saliva is poured out in very much greater abundance. The very sight and odor of nutritious food, when the appetite is excited, will stimulate to a remarkable degree the flow of saliva; and, as it is often expressed, "bring the water into the mouth." Any estimate, therefore, of the total quantity of saliva, based on the amount secreted in the intervals of mastication, would be a very Simon's Chemistry of Man. Phila. ed., 1846, p. 295. 2 Op. cit., p. 14. |