Before passing from this part of the subject, a few remarks of a more general nature seem called for. The nerves of the stomach have a direct relation to undigested but digestible substances. When any body that can not be digested is introduced into the stomach, distinct uneasiness is speedily excited, and an effort is soon made to expel it, either upward by the mouth or downward by the bowels. It is in this way, says Dr. Combe, that bile in the stomach excites nausea, and that tartar emetic produces vomiting. The nerves of the bowels, on the other hand, are constituted in relation to digested food; and, consequently, when any thing escapes into them from the stomach in an undigested state, it becomes a source of irritative excitement. This accounts for the cholic pains and bowel-complaints which so commonly attend the passage through the intestinal canal of such indigestible substances as fat, husks of fruits, berries, and cherry-stones.
The process of digestion, which commences in the stomach, is completed in the intestines. Physiologists have hence sometimes called the former part of the process, or chymification, by the more simple term stomach digestion; and the latter, or chylification, has been termed intestinal digestion. The bowels have distinct coats corresponding with those of the stomach. By the alternate contraction and relaxation of the muscular coat, their contents are propelled in a downward direction, somewhat as motion is propagated from one end of a worm to the other. It has hence been called vermicular, or wormlike motion. Some medicines have the power of inverting the order of the muscular contractions. Emetics operate in this manner to produce vomiting. Other medicines, again, excite the natural action to a higher degree, and induce a cathartic action of the bowels. When medicines become necessary to obviate that kind of costiveness which arises from imperfect intestinal contraction, physicians usually administer rhubarb, aloes, and similar laxatives, combined with tonics. But when the muscular coat of the bowels is kept in a healthy condition by a natural mode of life, and is aided by the action of the abdominal muscles, it rarely becomes necessary to administer laxative medicines.
The inner or mucous coat of the stomach and bowels is generally regarded by physiologists as a continuation of the skin. They greatly resemble each other in structure, and they are well known to sympathize with each other. Eruptions of the skin are very generally the result of disorders of the digestive organs. On the other hand, bowel complaints are frequently produced by a chill on the surface. The mucous coat and the skin are both charged with the double function of excretion and absorption. By the exercise of the former function, much of the waste matter of the system, requiring to be removed, is thrown into the intestines, and, mingling with the indigestible portion of the food, forms the common excrement; while by the exercise of the latter function the nutritive portion of their contents is taken up, and, as we have seen, passes into the general circulation, and contributes either to promote growth or to repair waste.
4. Evacuation.—This is the fourth and last principal step in the process of digestion. After the chyle is separated from the chyme and passes into the circulation, the indigestible and refuse portion of the food, which is incapable of nourishing the system, passes off through the intestinal canal. In its course its bulk is considerably increased by the excretion of waste matter which has served its purposes in the system, and which, mingling with the innutritious and refuse part of the food, is thrown out of the body in the form of excrement. If the contents of the bowels are too long retained, uneasiness is produced. Hurtful matter, also, which should pass off by evacuation, is reabsorbed, passes again into the general circulation, and is ultimately thrown out of the system either by the lungs or through the pores of the skin.
This part of the process of digestion is very important, for it is impossible to enjoy good health while this function is imperfectly performed. To secure full and natural action in the intestinal canal, several principal conditions are necessary. These are, first, well-digested chyme and chyle; second, a due quantity and quality of secretions from the mucous or lining membrane of the bowels; third, a free and full contractile power of the muscular coat, and the unrestrained action of the abdominal and respiratory muscles; and, finally, a due nervous sensibility to receive impressions and communicate the necessary stimulus. The contractile power of the muscular coat, and the free passage of the intestinal contents from the stomach downward, are greatly aided by the constant but gentle agitation which the whole digestive apparatus receives during the act of breathing, and from exercise of every description. By free and deep inhalations of air into the lungs, the diaphragm is depressed and the bowels are pushed down. But when the air is thrown out from the lungs, the diaphragm rises into the chest, and the bowels follow, being pressed upward by the contractile power of the abdominal muscles. During exercise, breathing is deeper and more free, which gives additional pressure to the bowels from above. The abdominal muscular contraction is also, in turn, more vigorous and extensive, and thus the motion is returned from below. Persons that take little or no exercise, or who allow the chest and bowels to be confined by tight clothing, lose this natural stimulus, and frequently become subjects of immense suffering from habits of costiveness. These should be removed if possible, and they generally can be by a proper course of discipline. This should have reference to both diet and exercise. Such articles of food should be used as tend to keep open the bowels. This should be combined with the free exercise of the lungs and the abdominal muscles. In addition to these, there should be a determination to secure a natural evacuation of the bowels at least once a day. This is regarded by physiologists generally as essential to health. Efforts should be continued until the habit is established. Some definite period should be fixed upon for this purpose. Soon after breakfast is, on many accounts, generally preferable.
Time for Meals.—Before passing from the subject of digestion, I will submit a few thoughts in relation to the times for eating. It has already been observed that three or four hours are generally necessary for the digestion of a simple meal. Usually, perhaps, a greater length of time is required. It is also an established doctrine, based upon the results of careful examination and experiment, that the stomach requires an interval of rest, after the process of digestion is finished, to enable it to recover its tone before it can again enter upon the vigorous performance of its function. As a general rule, then, five or six hours should elapse between meals. If the mode of life is indolent, a greater time is required; if active, less time will suffice. Where the usages of society will allow the principal meal to be taken near the middle of the day, the following time for meals is approved by physiologists generally: breakfast at 7 o'clock, dinner at half past 12, and tea at 6. Luncheons and late suppers should be avoided; for the former will always be found to interfere with the healthful performance of the function of digestion, and the latter will induce restlessness, unpleasant dreams, and pain in the head. "A late supper," says the author of the Philosophy of Health, "generally occasions deranged and disturbed sleep; there is an effort on the part of the nerves to be quiet, while the burdened stomach makes an effort to call them into action, and between these two contending efforts there is disturbance—a sort of gastric riot—during the whole night. This disturbance has sometimes terminated in a fit of apoplexy and in death."
The Skin.—This membranous covering, which is spread over the surface of the body to shield the parts beneath, serves also as an excreting and secreting organ. By the great supply of blood which it receives, it is admirably fitted for this purpose. The whole animal system, as we have seen, is in a state of transition, decay and renovation constantly succeeding each other. While the stomach and alimentary canal take in new materials, the skin forms one of the principal outlets by which particles that are useless to the system are thrown out of the body. Every one knows that the skin perspires, and that checked perspiration is a powerful cause of disease and death; but few have any just notion of the extent and influence of this exhalation. When the body is overheated by exercise, a copious sweat breaks out, which, by evaporation, carries off the excess of heat, and produces an agreeable feeling of coolness and refreshment. The sagacity of Franklin led him to the first discovery of the use of perspiration in reducing the heat of the body, and to point out the analogy subsisting between this process and that of the evaporation of water from a rough porous surface, so constantly resorted to in the East and West Indies, and in other warm countries, as an efficacious means of reducing the temperature of the air in rooms, and of wine and other drinks, much below that of the surrounding atmosphere. This is the higher and more obvious degree of the function of exhalation. But in the ordinary state of the system, the skin is constantly giving out a large quantity of waste materials by what is called insensible perspiration; a process which is of great importance to the preservation of health, and which is called insensible, because the exhalation, being in the form of vapor, and carried off by the surrounding air, is invisible to the eye. But its presence may often be made manifest, even to the sight, by the near approach of a dry cool mirror, on the surface of which it will soon be condensed so as to become visible. It is this which causes so copious deposits upon the windows of a crowded school-room in cold weather. A portion of these exhalations, however, proceed from the lungs.
There is an experiment that may be easily tried, which affords conclusive evidence that the amount of insensible perspiration is much greater than it is ordinarily supposed to be. Take a dry glass jar, with a neck three or four inches in diameter, and thrust the hand and a part of the fore-arm into it, closing the space in the neck about the arm with a handkerchief. After the lapse of a few minutes, it will be seen, by drawing the fingers across the inside of the jar, that the insensible perspiration even from the hand is very considerable. Many attempts have been made to estimate accurately the amount of exhaled matter carried off through the skin; but many difficulties stand in the way of obtaining precise results. There is a great difference in different constitutions, and even in the same person at different times, in consequence of which we must be satisfied with an approximation to the truth.
Although the precise amount of perspiration can not be ascertained, it is generally agreed that the cutaneous exhalation is greater than the united excretions of both bowels and kidneys. Great attention has been given to this subject. Sanctorius, a celebrated medical writer, weighed himself, his food, and his excretions, daily, for thirty days. He inferred from his experiments that five pounds of every eight, of both food and drink, taken into the system, pass out through the skin. All physiologists agree that from twenty to forty ounces pass off through the skin of an adult in usual health every twenty-four hours. Take the lowest estimate, and we find the skin charged with the removal of twenty ounces of waste matter from the system every day. We can thus see ample reason why checked perspiration proves so detrimental to health; for every twenty-four hours during which such a state continues, we must either have this amount of useless and hurtful matter accumulating in the system, or some of the other organs of excretion must be greatly overtasked, which obviously can not happen without disturbing their regularity and well-being. It is generally known that continued exposure in a cold day produces either a bowel complaint or inflammation of some internal organ. Instead of expressing surprise at this, if people generally understood the structure and uses of their own bodies, they would rather wonder why one or the other of these effects is not always attendant upon so great a violation of the laws of health, which are the laws of God.
The lungs also excrete a large proportion of waste matter from the system. So far, then, their office is similar to that of the kidneys, the liver, and the bowels. In consequence of this alliance with the skin, these parts are more intimately connected with each other, in both healthy and diseased action, than with other organs. Whenever an organ is unusually delicate, it will be more easily affected by any cause of disease than those which are sound. Thus, in one instance, checked perspiration may produce a bowel complaint, and in another, inflammation of the lungs, and so on. Hence the fitness, in prescribing remedies, of adapting them not only to the disease itself, but of taking into the account the cause of the disease. A bowel complaint, for example, may arise either from overeating or from a check to perspiration. The thing to be cured is the same in both cases, but the means of cure ought obviously to be different. In one instance, an emetic or laxative, to carry off the offending cause, would be the most rational and efficacious remedy; in the other, a diaphoretic should be administered, to open the skin and restore it to a healthy action. Facts like these expose the ignorance and impudence of the quack, who undertakes to cure every form of disease by one remedy.