Popular Science Monthly/Volume 35/October 1889/Digestion and Related Functions
|DIGESTION AND RELATED FUNCTIONS.|
PROFESSOR OF PHYSIOLOGY IN MCGILL UNIVERSITY.
IT is a matter well recognized by those of much experience in breeding and keeping animals with restricted freedom and under other conditions differing widely from the natural ones—i. e., those under which the animals exist in a wild state—that the nature of the food must vary from that which the untamed ancestors of our domestic animals used. Food may often with advantage be cooked for the tame and confined animal. The digestive and the assimilative powers have varied with other changes in the organism brought about by the new surroundings. So much is this the case, that it is necessary to resort to common experience and to more exact experiments to ascertain the best methods of feeding animals for fattening, for work, or for breeding. Inferences drawn from the feeding habits of wild animals allied to the tame to be valuable must always, before being applied to the latter, be subjected to correction by the results of experience.
To a still greater degree does this apply to man himself. The greater his advances in civilization, the more he departs from primitive habits in other respects, the more must he depart in his feeding. With the progressive development of man's cerebrum, the keener struggle for place and power, the more his nervous energies are diverted from the lower functions of digestion and assimilation of food; hence the greater need that food shall be more carefully selected and more thoroughly and scientifically prepared. Not only so, but, with our increasing refinement, the progress of digestion to successful issues demands that the senses of man be ministered to in order that there be no interferences in the central nervous system, and every encouragement given to the latter to furnish the necessary nervous impulses to the digestive organs and the tissues in every part of the organism: for it is not enough that food be digested in the ordinary sense; it must also he built up into the tissues, a process depending, as we shall endeavor to show later, on the nervous system.
The "gastronomic art" has, therefore, become of great importance. It is as yet more of an art than a science; the cook has outstripped the physiologist, if not the chemist also, in this direction.
We can not explain fully why food prepared by certain methods and served in courses of a certain established order is so suited to refined man, A part is known, but a great deal remains to be discovered. We may, however, notice a few points of importance in regard to the preparation of food.
It is now well established by experience that animals kept in confinement must have, in order to escape disease and attain the best results on the whole, a diet which not only imitates that of the corresponding wild forms generally, but even in details, it may be, with altered proportions or added constituents, in consequence of the difference in the environment. To illustrate: poultry can not be kept healthy confined in a shed without sand, gravel, old mortar, or some similar preparation; indeed, for the best results they must have green food also, as lettuce, cabbage, chopped green clover, grass, etc. They do not require as much food as if they had the exercise afforded by running hither and thither over a large field. We have chosen this example because it is not commonly recognized that our domesticated birds have been so modified that special study must be made of the environment in all cases if they are not to degenerate. The facts in regard to horned cattle, horses, and dogs are perhaps better known.
But all these instances are simple as compared with man. The lower mammals can live and flourish with comparatively little change of diet; not so man. He demands food not only dissimilar in its actual grosser nature, but differently prepared. In a word, for the efferent nervous impulses, on which the digestive processes depend, to be properly supplied, it has become necessary that a variety of afferent impulses (through eye, ear, nose, palate) reach the nervous centers, attuning them to harmony, so that they shall act, yet not interfere with one another.
Cooking greatly alters the chemical composition, the mechanical condition, and, in consequence, the flavor, the digestibility, and the nutritive value of foods. To illustrate: meat in its raw condition would present mechanical difficulties, the digestive fluids permeating it less completely; an obstacle, however, of far greater magnitude in the case of most vegetable foods. By cooking, certain chemical compounds are replaced by others, while some may be wholly removed. As a rule, boiling is not a good form of preparing meat, because it withdraws not only salts of importance, but proteids and the extractives—nitrogenous and other. Beef-tea is valuable chiefly because of these extractives, though it also contains a little gelatin, albumin, and fats. Salt meat furnishes less nutriment, a large part having been removed by the brine; notwithstanding, all persons at times, and some frequently, find such food highly beneficial, the effect being doubtless not confined to the alimentary tract.
Meat, according to the heat employed, may be so cooked as to retain the greater part of its juices within it or the reverse. With a high temperature (65° to 70° C.) the outside in roasting may be so quickly hardened as to retain the juices.
In feeding dogs it is both physiological and economical to give the animal the broth as well as the meat itself. The poor man may get excellent food cheaply by using not alone the meat of the shank of beef, but the soup (extractives) derived from it. There is much waste not only by the consumption of more food than is necessary, but by the purchase of kinds in which that important class, the proteids, comes at too high a price.
It is remarkable in the highest degree that man's appetite, or the instinctive choice of food, has proved wiser than our science. It would be impossible even yet to match, by calculations based on any data we can obtain, a diet for each man equal upon the whole to what his instincts prompt. With the lower mammals we can prescribe with greater success. At the same time chemical and physiological science can lay down general principles based on actual experience, which may serve to correct some artificialities acquired by perseverance in habits that were not based on the true instincts of a sound body and a healthy mental and moral nature; for the influence of the latter can not be safely ignored even in such discussions as the present. These remarks, however, are meant to be suggestive rather than exhaustive.
We may with advantage inquire into the nature of hunger and thirst. These, as we know, are safe guides usually in eating and drinking.
After a long walk on a warm day one feels thirsty; the mouth is usually dry; at all events, moistening the mouth, especially the back of it (pharynx), will of itself partially relieve thirst. But if we remain quiet for a little time the thirst grows less, even if no fluid be taken. The dryness has been relieved by the natural secretions. If, however, fluid be introduced into the blood either directly or through the alimentary canal, the thirst is also relieved speedily. The fact that we know when to stop drinking water shows of itself that there must be local sensations that guide us, for it is not possible to believe that the whole of the fluid taken can at once have entered the blood.
Again, in the case of hunger, the introduction of innutritions matters, as earth, or sawdust, will somewhat relieve the urgent sensations in extreme cases, as will also the use of tobacco by smokers, or much mental occupation, though this is rather illustrative of the lessening of the consciousness of the ingoing impulses by diverting the attention from them. But hunger, like thirst, may be mitigated by injections into the intestines or the blood. It is, therefore, clear that, while in the case of hunger and thirst there is a local expression of a need, a peculiar sensation, more pronounced in certain parts (the fauces in the case of thirst, the stomach in that of hunger), yet these may be appeased from within through the medium of the blood, as well as from without by the introduction of food or water, as the case may be.
Up to the present we have assumed that the changes wrought in the food in the alimentary tract were identical with those produced by the digestive ferments as obtained by extracts of the organs naturally furnishing them. But for many reasons it seems probable that artificial digestion can not be regarded as parallel with the natural processes except in a very general way. When we take into account the absence of muscular movements, regulated according to no rigid principles, but varying with innumerable circumstances in all probability, the absence of the influence of the nervous system determining the variations in the quantity and composition of the outflow of the secretions; the changes in the rate of so-called absorption, which doubtless influences also the act of the secretion of the juices—by these and a host of other considerations we are led to hesitate before we commit ourselves too unreservedly to the belief that the processes of natural digestion can be exactly imitated in the laboratory.
What is it which, enables one man to digest habitually what may be almost a poison to another? How is it that each one can dispose readily of a food at one time that at another is quite indigestible? To reply that, in the one case, the digestive fluids are poured out and in the other not, is to go little below the surface, for one asks the reason of this, if it be a fact, as it no doubt is. When we look further into the peculiarities of digestion, etc., we recognize the influence of race as such, and in the race and the individual that obtrusive though ill-understood fact—the force of habit, operative here as elsewhere. And there can be little doubt that the habits of a people, as to food eaten and digestive peculiarities established, become organized, fixed, and transmitted to posterity.
It is probably in this way that, in the course of the evolution of the various groups of animals, they have come to vary so much in their choice of food and in their digestive processes, did we but know them thoroughly as they are; for to assume that even the digestion of mammals can be summed up in the simple way now prevalent seems to us too broad an assumption. The field is very wide, and as yet but little explored.
Human Physiology.—The study of Alexis St. Martin has furnished probably the best example of genuine human physiology to be found, and has yielded a harvest rich in results.
We suggest to the student that self-observation, without interfering with the natural processes, may lead to valuable knowledge; for, though it may lack some of the precision of laboratory experiments, it will prove in many respects more instructive, suggestive, and impressive, and have a bearing on medical practice that will make it telling. Not that we would be understood now or at any time as depreciating laboratory experiments, but we wish to point out from time to time how much may be learned in ways that are simple, inexpensive, and consume but little time.
The law of rhythm is illustrated, both in health and disease, in striking ways in the digestive tract. An individual long accustomed to eat at a certain hour of the day will experience at that time not only hunger, but other sensations, probably referable to secretion of a certain quantity of the digestive juices and to the movements that usually accompany the presence of food in the alimentary tract. Some persons find their digestion disordered by a change in the hours of meals.
It is well known that defecation at periods fixed, even within a few minutes, has become an established habit with hosts of people; and the same is to a degree true of dogs, etc., kept in confinement, taught cleanly habits, and encouraged therein by regular attention to their needs.
Now and then a case of what is very similar to regurgitation of food in ruminants is to be found among human beings. This is traceable to habit, which is bound up with the law of rhythm or periodic increased and diminished activity.
Indeed, every one sufficiently observant may notice in himself instances of the application of this law in the economy of his own digestive organs.
This tendency is important in preserving energy for higher ends, for such is the result of the operation of this law everywhere.
The law of correlation, or mutual dependence, is well illustrated in the series of organs composing the alimentary tract.
The condition of the stomach has its counterpart in the rest of the tract: thus, when St. Martin had a disordered stomach, the epithelium of his tongue showed corresponding changes.
We have already referred to the fact that one part may do extra work to make up for the deficiencies of another.
It is confidently asserted of late that, in the case of persons long unable to take food by the mouth, nutritive substances given by enemata find their way up to the duodenum by antiperistalsis. Here, then, is an example of an acquired adaptive arrangement under the stress of circumstances.
It can not be too much impressed on the mind that in the complicated body of the mammal the work of any one organ is constantly varying with the changes elsewhere. It is this mutual dependence and adaptation—an old doctrine, too much left out of sight in modern physiology—which makes the attempt to completely unravel vital processes well-nigh hopeless, though each accumulating true observation gives a better insight into this kaleidoscopic mechanism.
We have not attempted to make any statements as to the quantity of the various secretions discharged. This is large, doubtless, but much is probably reabsorbed, either altered or unaltered, and used over again. In the case of fistulæ the conditions are so unnatural that any conclusions as to the normal quantity from the data they afford must be highly unsatisfactory. Moreover, the quantity must be very variable, according to the law we are now considering. It is well known that dry food provokes a more abundant discharge of saliva, and this is doubtless but one example of many other relations between the character of the food and the quantity of secretion provided.
Evolution.—We have from time to time either distinctly pointed out or hinted at the evolutionary implications of the facts of this department of physiology. The structure of the digestive organs, plainly indicating a rising scale of complexity with greater and greater differentiation of function, is, beyond question, an evidence of evolution.
The law of natural selection and the law of adaptation, giving rise to new forms, have both operated, we may believe, from what can be observed going on around us and in ourselves. The occurrence of transitional forms, as in the epithelium of the digestive tract of the frog, is also in harmony with the conception of a progressive evolution of structure and function. But the limits of space will not permit of the enumeration of details.
Summary.—A very brief résumé of the subject of digestion will probably sufface.
Food is either organic or inorganic, and comprises proteids, fats, carbohydrates, salts, and water; and each of these must enter into the diet of all known animals. They must also be in a form that is digestible. Digestion is the reduction of food to a form such that it may be further dealt with by the alimentary tract prior to being introduced into the blood (absorption). This is effected in different parts of the tract, the various constituents of food being differently modified, according to the secretions there provided, etc. The digestive juices contain essentially ferments which act only under definite conditions of chemical reaction, temperature, etc.
The changes wrought in the food are the following: Starches are converted into sugars, proteids into peptones, and fats into fatty acids, soaps, and emulsion; which alterations are effected by ptyalin and amylopsin, pepsin and trypsin, and bile and pancreatic steapsin, respectively.
Outside the mucous membrane containing the glands are muscular coats, serving to bring about the movements of the food along the digestive tract and to expel the fæces, the circular fibers being the more important. These movements and the processes of secretion and so-called absorption are under the control of the nervous system.
The preparation of the digestive secretions involves a series of changes in the epithelial cells concerned, which can be distinctly traced, and takes place in response to nervous stimulation.
These we regard as inseparably bound up with the healthy life of the cell. To be natural, it must secrete.
The blood-vessels of the stomach and intestine and the villi of the latter receive the digested food for further elaboration (absorption). The undigested remnant of food and the excretions of the intestine make up the fæces, the latter being expelled by a series of co-ordinated muscular movements essentially reflex in origin.
- From advance sheets of a text-book on "Animal Physiology" in press of D. Appleton & Co.