imperfect one. We may make a tolerably accurate calculation, however, by ascertaining how much is really secreted during a meal, over and above that which is produced at other times. We have found, for example, by experiments performed for this purpose, that wheaten bread gains during complete mastication 55 per cent. of its weight of saliva; and that fresh cooked meat gains, under the same circumstances, 48 per cent. of its weight. We have already seen that the daily allowance of these two substances, for a man in full health, is 19 ounces of bread, and 16 ounces of meat. The quantity of saliva, then, required for the mastication of these two substances, is, for the bread 4,572 grains, and for the meat 3,360 grains. If we now calculate the quantity secreted between meals as continuing for 22 hours at 556 grains per hour, we have:— Saliva required for mastication of bread = 4572 grains. 66 66 66 66 secreted in intervals of meals = 12232 66 Total quantity in twenty-four hours = 20164 grains; or rather less than 3 pounds avoirdupois. The most important question, connected with this subject, relates to the function of the saliva in the digestive process. A very remarkable property of this fluid is that which was discovered by Leuchs in Germany, viz., that it possesses the power of converting boiled starch into sugar, if mixed with it in equal proportions, and kept for a short time at the temperature of 100° F. This phenomenon. is one of catalysis, in which the starch is transformed into sugar by simple contact with the organic substance contained in the saliva. This organic substance, according to the experiments of Mialhe,' may even be precipitated by alcohol, and kept in a dry state for an indefinite length of time without losing the power of converting starch into sugar, when again brought in contact with it in a state of solution. This action of ordinary human saliva on boiled starch takes place sometimes with great rapidity. Traces of glucose may occasionally be detected in the mixture in one minute after the two substances have been brought in contact; and we have even found that starch paste, introduced into the cavity of the mouth, if already at the temperature of 100° F., will yield traces of sugar at the end of half a minute. The rapidity, however, with which this action is mani 'Chimie appliquée à la Physiologie et à la Thérapeutique, Paris, 1856, p. 43. fested, varies very much, as was formerly noticed by Lehmann, at different times; owing, in all probability, to the varying constitution of the saliva itself. It is often impossible, for example, to find any evidences of sugar, in the mixture of starch and saliva, under five, ten, or fifteen minutes; and it is frequently a longer time than this before the whole of the starch is completely transformed. Even when the conversion of the starch commences very promptly, it is often a long time before it is finished. If a thin starch paste, for example, which contains no traces of sugar, be taken into the mouth and thoroughly mixed with the buccal secretions, it will often, as already mentioned, begin to show the reaction of sugar in the course of half a minute; but some of the starchy matter still remains, and will continue to manifest its characteristic reaction with iodine, for fifteen or twenty minutes, or even half an hour. The above action of the saliva on starch, according to the experiments of Magendie, Bernard, Bidder and Schmidt, &c., does not reside in either the parotid, submaxillary or mucous secretions taken separately; but only in the mixed saliva, as it comes from the cavity of the mouth. The submaxillary and mucous secretions, however, taken together, produce the change; though neither of them has any effect alone, nor even when mixed artificially with the saliva of the parotid. It was supposed, when this property of converting starch into sugar was first discovered to exist in the saliva, that it constituted the true physiological action of the secretion, and that the function of the saliva was, in reality, the digestion and liquefaction of starchy substances. It was very soon noticed, however, by the French observers, that this property of the saliva was rather an accidental than an essential one; and that, although starchy substances are really converted into sugar, if mixed with saliva in a test-tube, yet they are not affected by it to the same degree in the natural process of digestion. We have already mentioned above the extremely variable activity of the saliva, in this respect, at different times; and it must be recollected, also, that in digestion the food is not retained in the cavity of the mouth, but passes at once, after mastication, into the stomach. Several German observers, as Frerichs, Jacubowitsch, Bidder, and Schmidt, maintained at first that the saccharine conversion of starch, after being commenced in the mouth, might be, and actually was, completed in the stomach. We have convinced ourselves, however, by frequent experiments, that this is not the case. If a dog, with a gastric fistula, be fed with a mixture of meat and boiled starch, and portions of the fluid contents of the stomach withdrawn afterward through the fistula, the starch is easily recognizable by its reaction with iodine for ten, fifteen, and twenty minutes afterward. In forty-five minutes, it is diminished in quantity, and in one hour has usually altogether disappeared; but no sugar is to be detected at any time. Sometimes the starch disappears more rapidly than this; but at no time, according to our observations, is there any indication of the presence of sugar in the gastric fluids. Bidder and Smith have also concluded, from subsequent investigations,' that the first experiments performed under their direction by Jacubowitsch were erroneous; and it is now acknowledged by them, as well as by the French observers, that sugar cannot be detected in the stomach, after the introduction of starch, in any form or by any method. In the ordinary process of digestion, in fact, starchy matters do not remain long enough in the mouth to be altered by the saliva, but pass at once into the stomach. Here they meet with the gastric fluids, which become mingled with them, and prevent the change which would otherwise be effected by the saliva. We have found that the gastric juice will interfere, in this manner, with the action of the saliva in the testtube, as well as in the stomach. If two mixtures be made, one of starch and saliva, the other of starch, saliva, and gastric juice, and both kept for fifteen minutes at the temperature of 100° F., in the first mixture the starch will be promptly converted into sugar, while in the second no such change will take place. The above action, therefore, of saliva on starch, though a curious and interesting property, has no significance as to its physiological function, since it does not take place in the natural digestive process. We shall see hereafter that there are other means provided for the digestion of starchy matters, altogether independent of the action of the saliva. The true function of the saliva is altogether a physical one. Its action is simply to moisten the food and facilitate its mastication, as well as to lubricate the triturated mass, and assist its passage down the oesophagus. Food which is hard and dry, like crusts, crackers, &c., cannot be masticated and swallowed with readiness unless moistened by some fluid. If the saliva, therefore, be prevented from entering the cavity of the mouth, its loss does not interfere directly with the chemical changes of the food in digestion, but only with its mechanical preparation. This is the result of direct experi Op. cit., p. 26. ments performed by various observers. Bidder and Schmidt,' after tying Steno's duct, together with the common duct of the submaxillary and sublingual glands on both sides in the dog, found that the immediate effect of such an operation was "a remarkable diminution of the fluids which exude upon the surfaces of the mouth; so that these surfaces retained their natural moisture only so long as the mouth was closed, and readily became dry on exposure to contact with the air. Accordingly, deglutition became evidently difficult and laborious, not only for dry food, like bread, but even for that of a tolerably moist consistency, like fresh meat. The animals also became very thirsty, and were constantly ready to drink." Bernard' also found that the only marked effect of cutting off the flow of saliva from the mouth was a difficulty in the mechanical processes of mastication and deglutition. He first administered to a horse one pound of oats, in order to ascertain the rapidity with which mastication would naturally be accomplished. The above quantity of grain was thoroughly masticated and swallowed at the end of nine minutes. An opening had been previously made in the oesophagus at the lower part of the neck, so that none of the food reached the stomach; but each mouthful, as it passed down the œsophagus, was received at the oesophageal opening and examined by the experimenter. The parotid duct on each side of the face was then divided, and another pound of oats given to the animal. Mastication and deglutition were both found to be immediately retarded. The alimentary masses passed down the oesophagus at longer intervals, and their interior was no longer moist and pasty, as before, but dry and brittle. Finally, at the end of twenty-five minutes, the animal had succeeded in masticating and swallowing only about three-quarters of the quantity which he had previously disposed of in nine minutes. It appears, also, from the experiments of Magendie, Bernard, and Lassaigne, on horses and cows, that the quantity of saliva absorbed by the food during mastication is in direct proportion to its hardness and dryness, but has no particular relation to its chemical qualities. These experiments were performed as follows: The œsophagus was opened at the lower part of the neck, and a ligature placed upon it, between the wound and the stomach. The animal was then supplied with a previously weighed quantity of food, and this, as it passed out by the œsophageal opening, was received into 1 Op. cit., p. 3. 2 Leçons de Physiologie Expérimentale, Paris, 1856, p. 146. appropriate vessels and again weighed. The difference in weight, before and after swallowing, indicated the quantity of saliva absorbed by the food. The following table gives the results of some of Lassaigne's experiments,' performed upon a horse:— It is evident, from the above facts, that the quantity of saliva produced has not so much to do with the chemical character of the food as with its physical condition. When the food is dry and hard, and requires much mastication, the saliva is secreted in abundance; when it is soft and moist, a smaller quantity of the secretion is poured out; and finally, when the food is taken in a fluid form, as soup or milk, or reduced to powder and moistened artificially with a very large quantity of water, it is not mixed at all with the saliva, but passes at once into the cavity of the stomach. The abundant and watery fluid of the parotid gland is most useful in assisting mastication; while the glairy and mucous secretion of the submaxillary gland and the muciparous follicles serve to lubricate the exterior of the triturated mass, and facilitate its passage through the oesophagus. By the combined operation of the two processes which the food undergoes in the cavity of the mouth, its preliminary preparation is accomplished. It is triturated and disintegrated by the teeth, and, at the same time, by the movements of the jaws, tongue, and cheeks, it is intimately mixed with the salivary fluids, until the whole is reduced to a soft, pasty mass, of the same consistency throughout. It is then carried backward by the semi-involuntary movements of the tongue into the pharynx, and conducted by the muscular contractions of the oesophagus into the stomach. THE GASTRIC JUICE, AND STOMACH DIGESTION. The mucous membrane of the stomach is distinguished by its great vascularity and the abundant glandular apparatus with which it is provided. Its entire thickness is occupied by certain glandular organs, the gastric tubules or follicles, which are so closely set as to leave almost no space between them except what is required for the Comptes Rendus, vol. xxi. p. 362. |