Popular Education. Ira Mayhew
coat of the stomach, which is excited to action by the presence of food, a kind of churning motion is communicated to its contents that greatly promotes digestion; for by this means every portion of food in turn is brought in contact with the gastric juice as it is discharged from the internal surface of the stomach. This motion continues until the contents of the stomach are converted into chyme, and conveyed into the first intestine, where they undergo another important change.
3. Chylification.—As fast as chyme is formed, it is expelled by the contractile power of the stomach into the duodenum, or first intestine. It there meets with the bile from the liver, and with the pancreatic juice. By the action of these agents, the chyme is converted into two distinct portions: a milky white fluid, called chyle, and a thick yellow residue. This process is called chylification, or chyle-making. The chyle is then taken up by the absorbent vessels, which are extensively ramified over the inner membrane or lining of the bowels. From the white color of the contents of these vessels, they have been named lacteals or milk-bearers, from lac, which signifies milk. These lacteals ultimately converge into one trunk, called the thoracic duct, which terminates in the great vein under the clavicle or collar bone, hence called the subclavian vein, just before that vein reaches the right side of the heart. Here the chyle is poured into the general current of the venous blood, and, mingling with it, is exposed to the action of the air in the lungs during respiration. By this process, both the chyle and the venous blood are converted into red, arterial, or nutritive blood, which is afterward distributed by the heart through the arteries, to supply nourishment and support to every part of the body. The change which takes place in the lungs is called sanguification, or blood-making. The chyle is not prepared to impart nourishment to the system until this change takes place. Respiration, then, is, in reality, the completion of digestion. This interesting and vital part of the process of digestion will be considered more fully in the following chapter.
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