Page:The American Cyclopædia (1879) Volume VI.djvu/113

DIGESTION 105 licles of the raucous membrane of the mouth. Saliva is but little heavier than water, contains minute corpuscles and epithelial scales, and in health has an alkaline reaction greatest during and after meals. It consists of about 995 parts of water in 1,000, and 5 parts of solid matters. Of the latter the most remarkable is ptyaline, to which the peculiar properties of the fluid are due ; it closely resembles, but is not identi- cal with, albumen and caseine ; it acts the part of a ferment, and, according to Mialhe, 1 part is sufficient to convert 2,000 parts of starch into sugar; it also contains a compound of sulpho-cyanogen, not known to occur in any other animal product, and interesting in a medico-legal point of view. Its salts are near- ly those of the blood, and its alkaline reaction seems to be due to the basic phosphate of soda. The "tartar" of the teeth and sali- vary concretions consist principally of earthy phosphates and animal matter. The limpid secretion of the parotid and sublingual glands saturates the food during proper mastication, while the viscid submaxillary fluid facilitates swallowing. The amount of saliva secreted daily by man will average, according to Bid- der and Schmidt, 3 Ibs., though it varies with the character and frequency of the meals. Besides its mechanical action, the saliva, by its peculiar ferment, has the power of acting chemically upon the farinaceous elements of the food, leading to the conversion of the starch into sugar ; but there is reason to believe that this action does not go on in presence of the acid of the stomach. There is no satisfactory evidence that saliva exerts any other than a physical action upon nitrogenized substances. When the food reaches the stomach the diges- tion is continued by the gastric juice, secreted by the numerous follicles of the mucous mem- brane, lined with glandular epithelium. The nature of the digestive process has been the subject of much speculation. It was at first supposed that the aliments underwent a coc- tion similar to that which they would experi- ence in a vessel with hot water; to this suc- ceeded the theory of acid fermentation, then of putrefaction, of trituration, and of macera- tion, till the present belief in the solvent action of the gastric juice was established. The gas- tric juice is transparent, nearly colorless, and without viscidity. Its most characteristic fea- ture is acidity, which is even perceptible to the taste. Many eminent chemists maintain that the real agent in the solvent process is free lactic acid, while others are in favor of free hydrochloric acid; the former opinion seems to be more fully borne out by the re- sults of experiment. The peculiar organic fer- ment of the gastric juice is pepsin, which dis- poses albuminous matters to undergo solution by the contained acid, which they would oth- erwise only partially do unless exposed to a high temperature. The secretion of the empty stomach is neutral or alkaline, but it becomes acid on the introduction and during the diges- tion of food, resuming its neutral character when this process is finished. From the ex- periments of Dr. Dalton, it appears that an ounce of gastric juice of the dog will dissolve a little over 30 grains of fresh lean meat ; at this rate the full digestion of a pound of raw meat would require two gallons of gastric juice ; and this apparently enormous quantity will not be considered incredible, if it be recol- lected that this fluid after it has done its work of solution is at once reabsorbed into the circulation, so that even this quantity might be secreted during the three or four hours of the digestive process, at an expense to the blood of not more than 2 or 3 oz. of fluid at any one time ; the fluid does not accumulate in the stomach, but its watery portions are in continual process of secretion and reabsorption as long as any food remains undigested, with- in reasonable limits as to quantity ingested. Many of the most important phenomena of gastric digestion were first demonstrated about 1830 by the experiments of Dr. Beaumont on Alexis St. Martin, through an opening in whose stomach the effect of food, stimulants, and sedatives could be. seen. The color of the membrane was pale pink, its appearance velvet-like, and its surface lined with a transpa- rent viscid mucus ; the stimulus of food caused the gastric follicles to enter into activity, and to pour out the acid gastric juice ; small quan- tities of very cold water, or ice, after the pri- mary sedative effect, caused turgidity of the membrane and copious secretion, while ice in large amount and long continued retarded the process. The amount of gastric juice secreted depends on the requirements of the system, and not on the quantity of food taken into the stomach ; this is most important to be remem- bered, since, after the fluid secreted has dis- solved all it can, any excess of food must re- main undigested, pass into the intestines in a crude state, and become a source of pain and irritation until it is expelled. When the sys- tem is diseased, there is no craving for food, which if taken would not cause the secretion of the gastric juice, but would remain undi- gested for an indefinite time, adding its irrita- tion to the general diseased state. The secre- tion of gastric juice is influenced by, though not dependent on, nervous agency ; it is well known that strong mental disturbance will put a stop to the digestive process, and section of the pneumogastric nerves arrests for a time the elaboration of the gastric fluid. There can be no doubt that gastric digestion is essentially a process of chemical solution, the solvent fluid being prepared by the follicles of the stomach, and its action assisted by the peristaltic mus- cular movements of the organ; the experi- ments on St. Martin, and those subsequently performed on the lower animals, fully prove these facts, both in natural and artificial diges- tion. Rapidity of digestion depends so much on the quantity and quality of the food, the state of health, the condition of the mind, and