Olsen, _Pure Foods._ Ginn and Company.
Sharpe, _A Laboratory Manual for the Solution of Problems in Biology,_ pp. 226-240. American Book Company.
Stiles, _Nutritional Physiology._ W. B. Saunders Company.
_The Great American Fraud._ American Medical a.s.sociation, Chicago.
_The Propaganda for Reform in Proprietary Medicines._ Am.
Medical a.s.sociation.
Farmers" Bulletin: numbers 23, 34, 42, 85, 93, 121, 128, 132, 142, 182, 249, 295, 298.
Reprint from Yearbook, 1901, At.w.a.ter, _Dietaries in Public Inst.i.tutions._ Reprint from Yearbook, 1902, Milner, _Cost of Food related to its Nutritive Value._ Experiment Station, Circular 46, Langworthy, _Functions and Uses of Food._
XX. DIGESTION AND ABSORPTION
_Problems.--To determine where digestion takes place by examining_:-- _(a) The functions of glands._ -(b) The work done in the mouth._ -(c) The work done in the stomach._ -(d) The work done in the small intestine._ -(e) The function of the liver._ _To discover the absorbing apparatus and how it is used._
LABORATORY SUGGESTIONS
_Demonstration of food tube of man_ (manikin).--Comparison with food tube of frog. Drawing (comparative) of food tube and digestive glands of frog and man.
_Demonstration of simple gland._--(Microscopic preparation.) _Home experiment and laboratory demonstration._--The digestion of starch by saliva. Conditions favorable and unfavorable.
_Demonstration experiment._--The digestion of proteins with artificial gastric juice. Conditions favorable and unfavorable.
_Demonstration._--An emulsion as seen under the compound microscope.
_Demonstration._--Emulsification of fats with artificial pancreatic fluid. Digestion of starch and protein with artificial pancreatic fluid.
_Demonstration_ of "tripe" to show increase of surface of digestive tube.
_Laboratory or home exercise._--Make a table showing the changes produced upon food substances by each digestive fluid, the reaction (acid or alkaline) of the fluid, when the fluid acts, and what results from its action.
Purpose of Digestion.--We have learned that starch and protein food of plants are formed in the leaves. A plant, however, is unable to make use of the food in this condition. Before it can be transported from one part of the plant body to another, it is changed into a soluble form. In this state it can be pa.s.sed from cell to cell by the process of osmosis. Much the same condition exists in animals. In order that food may be of use to man, it must be changed into a state that will allow of its pa.s.sage in a soluble form through the walls of the alimentary ca.n.a.l, or food tube. This is done by the enzymes which cause digestion. It will be the purpose of this chapter to discover where and how digestion takes place in our own body.
Alimentary Ca.n.a.l.--In all vertebrate animals, including man, food is taken in the mouth and pa.s.sed through a _food tube_ in which it is digested. This tube is composed of different portions, named, respectively, as we pa.s.s from the _mouth_ downward, the _gullet_, _stomach_, _small_ and _large intestine_, and _r.e.c.t.u.m_.
[Ill.u.s.tration: The digestive tract of the frog and man. _Gul_, gullet; _S_, stomach; _L_, liver; _G_, gall bladder; _P_, pancreas; _Sp_, spleen; _SI_, small intestine; _LI_, large intestine; _V_, appendix; _A_, a.n.u.s.]
Comparison of Food Tube of a Frog and Man.--If we compare the food tube of a dissected frog with the food tube of man (as shown by a manikin or chart), we find part for part they are much the same. But we notice that the intestines of man, both small and large, are relatively longer than in the frog. We also notice in man the body cavity or s.p.a.ce in which the internal organs rest is divided in two parts by a wall of muscle, the _diaphragm_, which separates the heart and lungs from the other internal organs. In the frog no muscular diaphragm exists. In the frog we can see plainly the silvery transparent _mesentery_ or double fold of the lining of the body cavity in which the organs of digestion are suspended. Numerous blood vessels can be found especially in the walls of the food tube.
Glands.--In addition to the alimentary ca.n.a.l proper, we find a number of _digestive glands_, varying in size and position, connected with the ca.n.a.l.
[Ill.u.s.tration: Diagram of a gland. _i_, the common tube which carries off the secretions formed in the cells lining the cavity _c_; _a_, arteries carrying blood to the glands; _v_, veins taking blood away from the glands.]
What a Gland Does. Enzymes.--In man there are the saliva gland of the mouth, the gastric glands of the stomach, the pancreas and liver, the two latter connected with the small intestine, and the intestinal glands in the walls of the intestine. Besides glands which aid in digestion there are several others of which we will speak later. As we have already learned, a gland is a collection of cells which takes up material from within the body and manufactures from it something which is later poured out as a secretion. An example of a gland in plants is found in the nectar-secreting cells of a flower.
Certain substances, called _enzymes_, formed by glands cause the digestion of food. The enzymes secreted by the cells of the glands and poured out into the food tube act upon insoluble foods so as to change them to a soluble form. They are the product of the activity of the cell, although they are not themselves alive. We do not know much about enzymes themselves, but we can observe what they do. Some enzymes render soluble different foods, others work in the blood, still others probably act within any cell of the body as an aid to oxidation, when work is done. Enzymes are very sensitive to changes in temperature and to the degree of _acidity_ or _alkalinity_[42] of the material in which they act. We will find that the enzymes found in glands in the mouth will not act long in the stomach because of the change from an alkaline surrounding in the mouth to that of an acid in the stomach. Enzymes seem to be able to work indefinitely, providing the surroundings are favorable. A small amount of digestive fluid, if it had long enough to work, could therefore digest an indefinite amount of food.
Footnote 42: The teacher should explain the meaning of these terms.
Gland Structure.--The entire inner surface of the food tube is covered with a soft lining of _mucous membrane_. This is always moist because certain cells, called _mucus cells_, empty out their contents into the food tube, thus lubricating its inner surface. When a large number of cells which have the power to secrete fluids are collected together, the surface of the food tube may become indented at this point to form a pitlike _gland_. Often such depressions are branched, thus giving a greater secreting surface, as is seen in the figure on page 298. The cells of the gland are always supplied with blood vessels and nerves, for the secretions of the glands are under the control of the nervous system.
How a Gland Secretes.--We must therefore imagine that as the blood goes to the cells of a gland it there loses some substances which the gland cells take out and make over into the particular enzyme that they are called upon to manufacture. Under certain conditions, such as the sight or smell of food, or even the desire for it, the activity of the gland is stimulated.
It then pours out its secretion containing the digestive enzyme. Thus a gland does its work.
Salivary Glands.--We are all familiar with the substance called _saliva_ which acts as a lubricant in the mouth. Saliva is manufactured in the cells of three pairs of glands which empty into the mouth, and which are called, according to their position, the _parotid_ (beside the ear), the _submaxillary_ (under the jawbone), and the _sublingual_ (under the tongue).
[Ill.u.s.tration: Experiment showing non-osmosis of starch in tube _A_, and osmosis of sugar in tube _B_.]
Digestion of Starch.--If we collect some saliva in a test tube, add to it a little starch paste, place the tube containing the mixture for a few minutes in tepid water, and then test with Fehling"s solution, we shall find grape sugar present. Careful tests of the starch paste and of the saliva made separately will usually show no grape sugar in either.
If another test be made for grape sugar, in a test tube containing starch paste, saliva, and a few drops of any weak acid, the starch will be found not to have changed. The digestion or change of starch to grape sugar is caused by the presence in the saliva of an _enzyme_, or _digestive ferment_. You will remember that starch in the growing corn grain was changed to grape sugar by an enzyme called _diastase_. Here a similar action is caused by an enzyme called _ptyalin_. This ferment acts _only_ in an alkaline medium at about the temperature of the body.
[Ill.u.s.tration: The mouth cavity of man. _e_, Eustachian tube; _hp_, hard palate; _sp_, soft palate; _ut_, upper teeth; _bc_, buccal cavity; _lt_, lower teeth; _t_, tongue; _ph_, pharynx; _ep_, epiglottis; _lx_, voice box; _oe_, gullet; _tr_, trachea.]
Mouth Cavity in Man.--In our study of a frog we find that the mouth cavity has two unpaired and four paired tubes leading from it. These are (_a_) the _gullet_ or food tube, (_b_) the _windpipe_ (in the frog opening through the _glottis_), (_c_) the paired nostril holes (_posterior nares_), (_d_) the paired _Eustachian tubes_, leading to the ear. All of these openings are found in man.
In man the mouth cavity, and all internal surfaces of the food tube, are lined with a _mucous membrane_. The _mucus_ secreted from gland cells in this lining makes a slippery surface so that the food may slip down easily.
The roof of the mouth is formed in front by a plate of bone called the _hard palate_, and a softer continuation to the back of the mouth, the _soft palate_. These separate the nose cavity from that of the mouth proper. The part of the s.p.a.ce back of the soft palate is called the _pharynx_, or throat cavity. From the pharynx lead off the _gullet_ and _windpipe_, the former back of the latter. The lower part of the mouth cavity is occupied by a muscular tongue. Examination of its surface with a looking-gla.s.s shows it to be almost covered in places by tiny projections called _papillae_. These papillae contain organs known as _taste buds_, the sensory endings of which determine the taste of substances. The tongue is used in moving food about in the mouth, and in starting it on its way to the gullet; it also plays an important part in speaking.
[Ill.u.s.tration: I. Teeth of the upper jaw, from below. _1, 2_, incisors; _3_, canine; _4, 5_, premolars; _6, 7, 8_, molars. II. longitudinal section of a tooth. _E_, enamel; _D_, dentine; _C_, cement; _P_, pulp cavity.]
The Teeth.--In man the teeth, unlike those of the frog, are used in the mechanical preparation of the food for digestion. Instead of holding prey, they crush, grind, or tear food so that more surface may be given for the action of the digestive fluids. The teeth of man are divided, according to their functions, into four groups. In the center of both the upper and lower jaw in front are found eight teeth with chisel-like edges, four in each jaw; these are the _incisors_, or cutting teeth. Next is found a single tooth on each side (four in all); these have rather sharp points and are called the _canines_. Then come two teeth on each side, eight in all, called _premolars_. Lastly, the _flat-top molars_, or grinding teeth, of which there are six in each jaw. Food is caught between irregular projections on the surface of the molars and crushed to a pulpy ma.s.s.
Hygiene of the Mouth.--Food should simply be chewed and relished, with no thought of swallowing. There should be no more effort to prevent than to force swallowing. It will be found that if you attend only to the agreeable task of extracting the flavors of your food, Nature will take care of the swallowing, and this will become, like breathing, involuntary. The instinct by which most people eat is perverted through the "hurry habit" and the use of abnormal foods. Thorough mastication takes time, and therefore one must not feel hurried at meals if the best results are to be secured. The stopping point for eating should be at the _earliest_ moment after one is really satisfied.
Care of the Teeth.--It has been recently found that fruit acids are very beneficial to the teeth. Vinegar diluted to about half strength with water makes an excellent dental wash. Clean your teeth carefully each morning and before going to bed. Use dental silk after meals. We must remember that the bacteria which cause decay of the teeth are washed down into the stomach and may do even more harm there than in the mouth.
How Food is Swallowed.--After food has been chewed and mixed with saliva, it is rolled into little b.a.l.l.s and pushed by the tongue into such position that the muscles of the throat cavity may seize it and force it downward.
Food, in order to reach the gullet from the mouth cavity, must pa.s.s over the opening into the windpipe. When food is in the course of being swallowed, the upper part of this tube forms a trapdoor over the opening.
When this trapdoor is not closed, and food "goes down the wrong way," we choke, and the food is expelled by coughing.
[Ill.u.s.tration: Peristaltic waves on the gullet of man. (A bolus means little ball.)]
The Gullet, or Esophagus.--Like the rest of the food tube the gullet is lined by soft and moist mucous membrane. The wall is made up of two sets of muscles,--the inside ones running around the tube; the outer layer of muscle taking a longitudinal course. After food leaves the mouth cavity, it gets beyond our direct control, and the muscles of the gullet, stimulated to activity by the presence of food in the tube, push the food down to the stomach by a series of contractions until it reaches the stomach. These wavelike movements (called _peristaltic_ movements) are characteristic of other parts of the food tube, food being pushed along in the stomach and the small intestine by a series of slow-moving muscular waves. Peristaltic movement is caused by muscles which are not under voluntary nervous control, although anger, fear, or other unpleasant emotions have the effect of slowing them up or even stopping them entirely.
Stomach of Man.--The stomach is a pear-shaped organ capable of holding about three pints. The end opposite to the gullet, which empties into the small intestine, is provided with a ring of muscle forming a valve called the _pylorus_. There is also another ring of muscle guarding the entrance to the stomach.
Gastric Glands.--If we open the stomach of the frog, and remove its contents by carefully washing, its wall is seen to be thrown into folds internally. Between the folds in the stomach of man, as well as in the frog, are located a number of tiny pits. These form the mouths of the _gastric glands_, which pour into the stomach a secretion known as the _gastric juice_. The gastric glands are little tubes, the lining of which secretes the fluid. When we think of or see appetizing food, this secretion is given out in considerable quant.i.ty. The stomach, like the mouth, "waters" at the sight of food. Gastric juice is slightly acid in its chemical reaction, containing about .2 per cent free _hydrochloric acid_.
It also contains two very important enzymes, one called _pepsin_, and another less important one called _rennin_.
Action of Gastric Juice.--If protein is treated with artificial gastric juice at the temperature of the body, it will be found to become swollen and then gradually to change to a substance which is soluble in water. This is like the action of the gastric juice upon proteins in the stomach.
The other enzyme of gastric juice, called _rennin_, curdles or coagulates a protein found in milk; after the milk is curdled, the pepsin is able to act upon it. "Junket" tablets, which contain rennin, are used in the kitchen to cause this change.
[Ill.u.s.tration: A peptic gland, from the stomach, very much magnified. _A_, central or chief cell, which makes pepsin; _B_, border cells, which make acid. (From Miller"s _Histology_.)]
The hydrochloric acid found in the gastric juice acts upon lime and some other salts taken into the stomach with food, changing them so that they may pa.s.s into the blood and eventually form the mineral part of bone or other tissue. The acid also has a decided antiseptic influence in preventing growth of bacteria which cause decay, and some of which might cause disease.
Movement of Walls of Stomach.--The stomach walls, provided with three layers of muscle which run in an oblique, circular, and longitudinal direction (taken from the inside outward), are well fitted for the constant churning of the food in that organ. Here, as elsewhere in the digestive tract, the muscles are involuntary, muscular action being under the control of the so-called _sympathetic nervous system_. Food material in the stomach makes several complete circuits during the process of digestion in that organ. Contrary to common belief, the greatest amount of food is digested _after_ it leaves the stomach. But this organ keeps the food in it in almost constant motion for a considerable time, a meal of meat and vegetables remaining in the stomach for three or four hours. While movement is taking place, the gastric juice acts upon proteins, softening them, while the constant churning movement tends to separate the bits of food into finer particles. Ultimately the semifluid food, much of it still undigested, is allowed to pa.s.s in small amounts through the pyloric valve, into the small intestine. This is allowed by the relaxation of the ringlike muscles of the pylorus.
Experiments on Digestion in the Stomach.--Some very interesting experiments have recently been made by Professor Cannon of Harvard with reference to movements of the stomach contents. Cats were fed with material having in it bis.m.u.th, a harmless chemical that would be visible under the X-ray. It was found that shortly after food reached the stomach a series of waves began which sent the food toward the pyloric end of the stomach. If the cat was feeling happy and well, these contractions continued regularly, but if the cat was cross or bad tempered, the movements would stop. This shows the importance of _cheerfulness_ at meals. Other experiments showed that food which was churned into a soft ma.s.s was only permitted to leave the stomach when it became thoroughly permeated by the gastric juice. It is the _acid_ in the partly digested food that causes the stomach valve to open and allow its contents to escape little by little into the small intestine.
The partly digested food in the small intestine almost immediately comes in contact with fluids from two glands, the liver and pancreas. We shall first consider the function of the pancreas.