The residue from the pepsin digestion, after washing, is treated for six hours at near 40° with 100 cubic centimeters of pancreas solution, prepared as follows:
Free the pancreas of a healthy steer of fat, pass it through a sausage grinder, rub one kilogram in a mortar with fine sand and allow to stand for a day or longer. Add three liters of lime water, one of glycerol, of 1.23 specific gravity, and a little chloroform and set aside for six days. Separate the liquor by pressure in a bag and filter it through paper. Before using, mix a quarter of a liter of the filtrate with three-quarters of a liter of water and five grams of dry sodium carbonate, or its equivalent crystallized, heat from 38° to 40° for two hours and filter.[561] In order to avoid the trouble of preparing the pancreas solution pure active pancreatin may be used.[562] One and a half grams of pure pancreatin and three grams of sodium carbonate are dissolved in one liter of water and 100 cubic centimeters of this solution are used for each two grams of the sample. In all cases where commercial pepsin and pancreatin are used, their activity should be tested with bodies such as boiled whites of eggs, whose coefficient of digestibility is well known and those samples be rejected which do not prove to have the required activity.[563]
550. Digestion in Pancreas Extract.—In order to save the time required for successive digestions in pepsin and pancreatin Niebling has proposed to make the digestion in the pancreas extract alone.[564] This process and also a slight modification of it have been used with success by Bigelow and McElroy.[565] Two grams of the sample are washed with ether and placed in a digestion flask with 100 cubic centimeters of two-tenths per cent hydrochloric acid. The contents of the flask are boiled for fifteen minutes, cooled, and made slightly alkaline with sodium carbonate. One hundred cubic centimeters of the unfiltered pancreas solution, prepared as directed above, are added and the digestion continued at 40° for six hours. The residue is thrown on a filter, washed, and the nitrogen determined. The method is simplified by the substitution of active commercial pancreatin for pancreas extract. The solution of the ferment is made of the same strength as is specified above.
551. Artificial Digestion of Cheese.—The artificial digestion of cheese is conducted by Stutzer as follows:[566]
The digestive liquor is prepared from the fresh stomachs of pigs by cutting them into fine pieces and mixing with five liters of water and 100 cubic centimeters of hydrochloric acid for each stomach. To prevent decomposition, two and a half grams of thymol, previously dissolved in alcohol, are added to each 600 cubic centimeters of the mixture. The mixture is allowed to stand for a day with occasional shaking, poured into a flannel bag and the liquid portion allowed to drain without pressing. The liquor obtained in this way is filtered, first through coarse and then through fine paper, and when thus prepared will keep several months without change. It is advisable to determine the content of hydrochloric acid in the liquor by titration and this content should be two-tenths of a per cent. The cheese to be digested is mixed with sand as previously described, freed of fat by extraction with ether, and a quantity corresponding to five grams of cheese placed in a beaker, covered with half a liter of the digestive liquor and kept at a temperature of 40° for forty-eight hours. At intervals of two hours the flasks are well shaken and five cubic centimeters of a ten per cent solution of hydrochloric acid added and this treatment continued until the quantity of hydrochloric acid amounts to one per cent. After the digestion is finished, the contents of the beaker are thrown on a filter, washed with water and the nitrogen determined in the usual way in the residue. By allowing the pepsin solution to act for two days as described above, the subsequent digestion with pancreas solution is superfluous.
552. Suggestions Regarding Manipulation.—The filter papers should be as quick working as possible to secure the separation of all undissolved particles. They should be of sufficient size to hold the whole contents of the digestion flask at once, since if allowed to become empty and partially dry, filtration is greatly impeded. The residue should be dried at once if not submitted immediately to moist combustion. After drying, the determination of the nitrogen can be made at any convenient time. Beaker flasks, i. e., lip erlenmeyers with a wide mouth are most convenient for holding the materials during digestion. The flasks are most conveniently held by a crossed rubber band attached at either end to pins in the wooden slats extending across the digestive bath. The bath should be suspended by cords from supports on the ceiling and a gentle rotatory motion imparted to it resembling the peristaltic action attending natural digestion.
553. Natural Digestion.—The digestion of foods by natural processes is determined chiefly by the classes of ferments already noted. The principle underlying digestive experiments with the animal organism may be stated as follows: A given weight of food of known composition is fed to a healthy animal under the conditions of careful control and preparation already mentioned. The solid dejecta of the animal during a given period are collected and weighed daily, being received directly from the animal in an appropriate bag, safely secured, as is shown in the accompanying [figure]. The dejecta are weighed, dried, ground to a fine powder, mixed and a representative part analyzed. The difference between the solid bodies in the dejecta and those given in the food during the period of experiment represents those nutrients which have been digested and absorbed during the passage of the food through the alimentary canal. The urine, containing solid bodies representing the waste of the animal organism, does not require to be analyzed for the simple control of digestive activity outlined above. In a complete determination of this kind the exhalations from the surface of the body and from the lungs are also determined. In the latter case the human animal is selected for the experiment; in the former it is more convenient to employ the lower animals, such as the sheep and cow.
The arrangement of the stalls and of the apparatus for collecting the excreta should be such as is both convenient and effective.[567]
The method of constructing a bag for attachment to a sheep is shown in [Fig. 118]. It is made according to the directions given by Gay, of heavy cloth and in such a way as to fit closely the posterior parts of the animal.[568] When attached, its appearance is shown in [Fig. 119].
