64 DIGESTION OF GELATINE. As the alteration takes place from calorifacient to nutritive digestion, the active fluid changes its chemical relations. In the mouth and duodenum, alkaline juices are resorted to; in the stomach and cœcum, acid Whenever there is an accidental inversion of these conditions, the result correspondingly changes; so when bile, which is alkaline, regurgitates into the stomach, the digestion of nutritive food is instantly arrested. ones. In each of these cases the object is the same: it is to obtain the nutrient material under such forms that the absorbent vessels can readily take it up; this, as we have seen, often involves a metamorphosis of the elements of the food where mechanical subdivision would be insufficient. Fibrin has to be brought into a soluble state, and, indeed, albumen itself must be modified. If it has been taken uncoagulated or glairy, it becomes opalescent, and passes into the allied form known as albuminose. In this condition it is neither precipitated by heat nor by nitric acid, though it is by corrosive sublimate. The cause of this transformation probably has reference to the relative facility with which albuminose can transude into the venous capillaries compared with albumen. There is thus reason to suppose that the result of stomach digestion is the reduction of the various nitrogenized constituents of the food to the condition of albuminose. It is plain that fibrin must come into this or some analogous condition, for it can not be absorbed as fibrin, and, accordingly, it is found that the blood of the gastric and mesenteric veins abounds in albuminose. tine. Intermediate between the classes of calorifacient and histogenetic food, Case of gela- belonging, by its composition and conditions of digestion, to the latter, but by the function it discharges to the former, is gelatine, a nitrogenized substance. It appears to be always derived from albumen, and any portion which may have been received in the food is never directly assimilated or used for the fabrication of tissue, but solely ministers to the production of heat. Though thus a calorifacient body, its place of digestion is the stomach. After it has suffered the action of that organ it has lost its power of gelatinizing, can no longer be precipitated by chlorine, nor give the leather precipitate with tannin. The use of it under the form of jellies, soups, etc., is always attended with the appearance of an unusual quantity of urea in the urine, and hence the administration of those domestic preparations, under an idea of their great nutritive value, is to be looked upon as only a popular error. In an indirect way, however, under the conditions of restricted diet, usually met with in the sick-room, gelatine doubtless maintains an interesting relation to the albumenoid bodies in this, that it protects them from destruction. by undergoing oxidation itself, and so satisfying the requirements of the respiratory mechanism; for, were there not such a substance present to RELATIVE DIGESTIBILITY OF FOOD. 65 receive the attack, the respired oxygen would rapidly bring on the waste of the proper nitrogenized tissues. an actual tis In relation to the gelatigenous tissues, it may be remarked that gelatine is not an actual constituent of them, but arises from them Gelatine not by boiling with water. By a like process, sufficiently pro- sue constitulonged, a similar substance may be obtained from cartilage, ent. designated cartilage-gelatine, or chondrine. In these cases the material unites with water in the same manner that starch does in producing glu cose. The food must therefore pass through various stages before it can be fitted for introduction into the circulation, and carried to all parts of the system. It is procured in portions of a suitable size either by the fingers, or, in civilized life, by resorting to artificial implements, the knife and fork. The incisor teeth next cut it up, and the molars crush or grind it, being worked for this purpose by a powerful system of muscles; meantime it is incorporated with saliva and atmospheric air. Passing into the stomach under the condition of a coarse pulpy mass, the gastric juice carries the process still farther, a more intimate disintegration of its structure ensues, and it is eventually brought into a soluble and changed form. The time required to produce this effect varies with Digestibility ticles of food. the nature of the food. Thus it has been noticed that beef of different aris much more quickly acted on than mutton, and mutton sooner than pork. Circumstances interfering with estimates of digestibili Statements respecting the digestibility of different articles of food must, however, be received with many restrictions. If, as the earlier physiologists believed, the stomach was the sole digestive cavity, and the intestine only for the purpose of absorption, they would doubtless be much nearer to the truth. ty. But when we recall that the digestion of fats does not even begin until the intestine is reached, and that the digestion of the nitrogenized substances is only in part accomplished by the gastric juice, but goes on under the influence of the intestinal juice throughout the whole length of the small intestine, we see at once how imperfect and even incorrect are the indications afforded by such experiments as those of Spallanzani, who introduced food articles into the stomach through the oesophagus in perforated silver vessels, or those of Beaumont, who availed himself of a gastric fistula. Neither can we take, in all instances, the time which an article of food will remain in the stomach as a measure of its digestibility, for this is known to vary with many conditions, as, for instance, the quantity introduced at a time, and the condition of the organ itself. As general illustrations of the digestibility of some of the ordinary elements of food, the examples, however, being more or less open to the preceding criticisms, the following facts may be offered. The white of an egg, répE 66 RELATIVE DIGESTIBILITY OF FOOD. resenting soluble albumen, if introduced into the stomach of a fasting dog through a gastric fistula, will disappear in less than an hour; but if the whites of eight eggs be introduced, portions thereof can be recognized after four hours. Lehmann, who made these observations, adds that blood fibrin varies in its time for gastric solution according as it is in a finely comminuted or a massive state; in the former instance disappearing from the stomach of a dog in an hour and a half, but the same weight in the latter condition requiring almost twice the time. Coagulated albumen indicates the commencement of digestion, and even its local completion, in from five minutes to a quarter of an hour; but here again much depends on the condition of the stomach and the general state of the system, whether the animal has been fasting, and whether the gastric juice is exuding in a dilute or concentrated state. gestion, as of fat, does not begin in the stomach. So far as such examinations go, they do not exhibit any marked difRespiratory di- ference between albumen, fibrin, and casein. Gelatine, however, is acted on with remarkable rapidity. Beaumont observed that in an hour 150 grammes of jelly had disappeared. The experiments which have been made on the digestibility of vegetable food introduced through gastric fistula are obviously of no use, since the chief constituents thereof, such as starch and fat, are not even influenced in those circumstances until they have reached the intestine. Their passage from the stomach in this unchanged state, or changed only so far as their nitrogenized ingredients are concerned, may teach us the important fact, which should in these inquiries be always borne in mind, that disappearance from the stomach is one thing and digestion another, and that even though a substance may have passed the pyloric valve, its digestion, far from having been completed, may not as yet have commenced. The digestion of nutritive or nitrogenized material-histogenetic digestion-is therefore carried on in the stomach mainly; and though first mechanical, and then chemical agencies are resorted to, the object is throughout the same—to obtain the food in such a divided and changed state that it can pass, dissolved in water, into the capillary vessels. INTESTINAL DIGESTION. 67 CHAPTER IV. OF CALORIFACIENT OR INTESTINAL DIGESTION. Nature of Intestinal Digestion.—Structure of the Intestine.-Digestive Fluids of the Intestine.The Pancreatic Juice. The Enteric Juice.-Juice of Lieberkuhn. - Secretion of Peyer's Glands.-Bile.-Digestion of the Carbohydrates and Hydrocarbons.-Properties and Varieties of Lactic Acid.—Doctrine of the Effects of Acidity and Alkalinity of the Digestive Juices. -Illustration of Intestinal Digestion from the making of Wine.—Making of Bread.-Influence of Heat over Ferments.—Comparison of Gastric and Intestinal Digestion.—Changes of the Intestinal Contents.-The Facal Residues. AFTER the chyme formed in the stomach has passed through the pyloric valve into the small intestine, the influence of the gastric juice continues for a certain time, even after the bile and pancreatic juices. have been reached. Since their action must be necessarily, in the first instance, superficial, the interior of the mass is still undergoing stomach digestion. But, setting aside this incidental result, which at the most can not be of long duration, the digestive operation taking place in the Nature of inpart of the intestinal tract now under consideration is di- testinal digesrected to the heat-making food. tion. The organ in which calorifacient digestion takes place may be described as a tube bounded by two valves, the pyloric above Structure of and the ileo-cocal below. Its length may be estimated at the intestine. about twenty feet. The digestive surface, making a due allowance for its increase by reason of its valvular structure presently to be described, can not be much under 3500 square inches. The dimensions of the calorifacient digesting surface are therefore far greater than those of the nutritive. Double appa The interior and acting portion of this tube presents two different systems of apparatus, and is occupied in the discharge of two totally distinct functions, digestion and absorption. It is, ratus of intesperhaps, this double duty which demands so extensive a surface, and not the necessities of heat-making digestion alone. tine. ventes. Like the stomach, this tube consists of three coats-a serous, a muscular, and a mucous. The latter is gathered up in its inte- Action of the rior into numberless projecting folds—the valvulæ conniven- valvulæ connites. These serve to increase the surface to which the food is exposed, and perhaps afford a mechanical obstacle to its passing too quickly forward. They tend also to break the continuous motion, and bring the interior parts of the chyme to the surface. The onward move 68 INTESTINAL DIGESTIVE FLUIDS. ment is of course due to the pressure exerted conjointly by the straight and circular fibres of the muscular coat. Anatomists divide the tube into three portions-the duodenum, jejunum, and ileum. Digestive fluids of the in testine. Fig. 19. Posterior view of the duodenum. In Fig. 19 we have a posterior view of the duodenum, a being its superior or pyloric extremity, the middle portion, o the jejunum, d the gallbladder, the cystic duct, p hepatic duct, c the ductus communis, m pancreatic duct. Soon after the chyme has escaped through the pyloric valve into the duodenum, it comes under the influence of the bile and pancreatic juices, which are sometimes discharged upon it at a common point, and sometimes at a little distance apart. Almost simultaneously it is submitted to the mechanical action of the valvulæ conniventes, which make their appearance in the vertical portion of the duodenum, and continue in large numbers until within the last two or three feet of the end of the tube. As the intestine is distended, these project with a certain degree of turgidity, and accomplish their mechanical object. Pancreatic tion and prop But, besides the pancreatic and biliary fluids, there are other juices thrown upon the passing chyme-the enteric juice, which comes from Brunner's glands, and a liquid oozing from the follicles of Lieberkuhn. Moreover, the organisms known as Peyer's glands are affecting the contents of the tube. Of each of these it is necessary therefore to speak. 1st. The pancreatic juice, secreted by the pancreas, an organ bearing a resemblance in its anatomical construction to the salivary juice, constitu- glands, and hence usually regarded as one of that group. erties of. The juice itself is analogous to saliva, being viscid, and in its reaction alkaline: its specific gravity is about 1.008. Alcohol coagulates it. It is said to contain no sulphocyanide nor any suspended particles. It acts upon starch even more energetically than saliva, transmuting it into sugar and lactic acid, and upon fats by forming them into an emulsion, so that they are readily absorbed. This has been found to take place in artificial experiments by submitting fat substances to the juice at a temperature of 100°. Constitution of Pancreatic Juice of Dog. (From Schmidt.) 900.76 90.38 8.86 1000.00 |