Part 12

Yet during this entire time his weight may have been nearly uniform. [Footnote: If, however, he were kept on the scale pan of a sensitive balance, he would find that his weight is constantly changing, increasing with each meal, and then gradually decreasing.] Our bodies are but molds, in which a certain quantity of matter, checked for a time on its ceaseless round, receives a definite form. They may be likened, says Huxley, to an eddy in the river, which retains its shape for a while, yet every instant each particle of water is changing.

WHAT FOOD DOES.--We make no force ourselves. We can only use that which nature provides for us. [Footnote: We draw from Nature at once our substance, and the force by which we operate upon her; being, so far, parts of her great system, immersed in it for a short time and to a small extent. Enfolding us, as it were, within her arms, Nature lends us her forces to expend; we receive them, and pass them on, giving them the impress of our will, and bending them to our designs, for a little while; and then--Yes; then it is all one. The great procession pauses not, nor flags a moment, for our fall. The powers which Nature lent to us she resumes to herself, or lends, it may be, to another; the use which we have made of them, or might have made and did not, is written in her book forever.--_Health and its Conditions_.] All our strength comes from the food we eat. Food is force--that is, it contains a latent power which it gives up when it is decomposed. [Footnote: This force is chemical affinity. It binds together the molecules which compose the food we eat. When oxygen tears the molecules to pieces and makes them up into smaller ones, the force is set free. As we shall learn in Physics, it can be turned, into heat, muscular motion, electricity, etc. The principle that the different kinds of force can be changed into one another without loss, is called the Conservation of Energy, and is one of the grandest discoveries of modern science.--_Popular Physics_, pages 35, 39, 278.] Oxygen is the magic key which unlocks for our use this hidden store. [Footnote: We have spoken of the mystery that envelops the process of the conversion of food force into muscular force (note, p. 107). All physiologists agree that muscular power has its source in the chemical decomposition of certain substances whereby their potential energy is released. Probably some of the food undergoes this chemical change before it passes out of the alimentary canal; possibly some is broken up by the oxygen while it is being swept along by the blood; but, probably by far the largest part is converted into the various tissues of the body, and finally becomes a waste product only after there takes place in the tissue itself that chemical disorganization that sets free its stored-up power.-- FOSTER'S _Physiology_.] Putting food into our bodies is like placing a tense spring within a watch; every motion of the body is only a new direction given to this food force, as every movement of the hand on the dial is but the manifestation of the power of the bent spring in the watch. We use the pent-up energies of meat, bread, and vegetables which are placed at our service, and transfer them to a higher theater of action. [Footnote: It is a grand thought that we can thus transform what is common and gross into the refined and spiritual; that out of waving wheat, wasting flesh, running water, and dead minerals, we can realize the glorious possibilities of human life.]

KINDS OF FOOD NEEDED.--From what has been said it is clear that, in order to produce heat and force, we need something that will burn, _i. e._, with which oxygen can combine. Experiment has proved that to build up every organ, and keep the body in the best condition, we require three kinds of food.

1. _Nitrogenous Food_.--As nitrogen is a prominent constituent of the tissues of the body, food which contains it is therefore necessary to their growth and repair. [Footnote: Since this kind of food closely resembles albumen, it is sometimes called _Albuminous_. The term Proteid is also used.] The most common forms are whites of eggs--which are nearly pure albumen; casein--the chief constituent of cheese; lean meat; and gluten--the viscid substance which gives tenacity to dough. Bodies having a great deal of nitrogen readily oxidize. Hence the peculiar character of the quick-changing, force-exciting muscle.

2. _Carbonaceous Food_--_i. e._, food containing much carbon-- consists of two kinds, viz., the _sugars_, and the _fats_.

(1) The _sugars_ contain hydrogen and oxygen in the proportion to form water, and about the same amount of carbon. They may, therefore, be considered as water, with carbon diffused through it. In digestion, starch and gum are changed to sugar, and so are ranked with this class.

(2) The _fats_ are like the sugars in composition, but contain less oxygen, and not in the proportion to form water. They combine with more oxygen in burning, and so give off more heat.

The non-nitrogenous elements of the food have, however, other uses than to develop heat. [Footnote: The heat they produce in burning may be turned into motion of the muscles, according to the principle of the Conservation of Energy (p. 153, note); while all the structures of the body in their oxidation develop heat.] Fat is essential to the assimilation of the food, while sugar and starch aid in digestion and may be converted into fat. [Footnote: In Turkey, the ladies of the harem are fed on honey and thick gruel, to make flesh, which is considered to enhance their beauty. The negroes on the sugar plantations of the South always grow fat during the sugar-making season.] Fat and carbonaceous material both enter into the composition of the various tissues, and when, by the breaking up of the contractile substance of the muscle, their latent energy is set free, they become the source of muscular force, as well as heat. While the tendency of the albuminous food is to excite chemical action, and hence the release of energy, the fats and carbonaceous food may be laid up in the body to serve as a storehouse of energy to supply future needs.

3. _Mineral Matters_.--Food should contain water, and certain common minerals, such as iron, [Footnote: While the body can build up a solid from liquid materials on the one hand, on the other it can pour iron through its veins and reduce the hardest textures to blood.--HINTON.] sulphur, magnesia, phosphorus, salt, and potash. About three pints of water are needed daily to dissolve the food and carry it through the circulation, to float off waste matter, to lubricate the tissues, and by evaporation to cool the system (see p. 317). It also enters largely into the composition of the body. A man weighing one hundred and fifty-four pounds contains one hundred pounds of water, about twelve gallons--enough, if rightly arranged, to drown him. [Footnote: It is said that Blumenbach had a perfect mummy of an adult Teneriffian, which with the viscera weighed only seven and a half pounds.]

Iron goes to the blood disks; lime combines with phosphoric and carbonic acids to give solidity to the bones and teeth; phosphorus is essential to the activity of the brain. Salt is necessary to the secretion of some of the digestive fluids, and also to aid in working off from the system its waste products. These various minerals, except iron--sometimes given as a medicine, and salt--universally used as a condiment, [Footnote: Animals will travel long distances to obtain salt. Men will barter gold for it; indeed, among the Gallas and on the coast of Sierra Leone, brothers will sell their sisters, husbands their wives, and parents their children for salt. In the district of Accra, on the gold coast of Africa, a handful of salt is the most valuable thing upon earth after gold, and will purchase a slave. Mungo Park tells us that with the Mandingoes and Bambaras the use of salt is such a luxury that to say of a man "he flavors his food with salt," it is to imply that he is rich; and children will suck a piece of rock salt as if it were sugar. No stronger mark of respect or affection can be shown in Muscovy, than the sending of salt from the tables of the rich to their poorer friends. In the book of Leviticus it is expressly commanded as one of the ordinances of Moses, that every oblation of meat upon the altar shall be seasoned with salt, without lacking; and hence it is called the Salt of the Covenant of God. The Greeks and Romans also used salt in their sacrificial cakes; and it is still used in the services of the Latin church--the "_parva mica_" or pinch of salt, being in the ceremony of baptism, put into the child's mouth, while the priest says, "Receive the salt of wisdom, and may it be a propitiation to thee for eternal life." Everywhere and almost always, indeed, it has been regarded as emblematical of wisdom, wit, and immortality. To taste a man's salt, was to be bound by the rites of hospitality; and no oath was more solemn than that which was sworn upon bread and salt. To sprinkle the meat with salt was to drive away the devil, and to this day, nothing is more unlucky than to spill the salt.--LETHEBY, _On Food_.] are contained in small, but sufficient quantities in meat, bread, and vegetables.

ONE KIND OF FOOD IS INSUFFICIENT.--A person fed on starch alone, would die. It would be a clear case of nitrogen starvation. On the other hand, as nitrogenous food contains carbon, the elements of water, and various mineral matters, life could be supported on that alone. But such a prodigious quantity of lean meat, for example, would be required to furnish the other elements, that not only would it be very expensive, but it is likely that after a time the labor of digestion would be too onerous, and the system would give up the task in despair. The need of a diet containing both nitrogenous and carbonaceous elements is shown in the fact that even in the tropical regions oil is relished as a dressing upon salad. Instinct everywhere suggests the blending. Butter is used with bread; rice is boiled with milk; cheese is eaten with macaroni, and beans are baked with pork.

FIG. 45.

THE OBJECT OF DIGESTION.--If our food were cast directly into the blood, it could not be used. For example, although the chemist can not see wherein the albumen of the egg differs from the albumen of the blood, yet if it be injected into the veins it is unavailable for the purposes required, and is thrown out again. In the course of digestion the food is modified in various ways whereby it is fitted for the use of the body, into which it is finally incorporated. We call this change of food into flesh _assimilation_, a name for a work done solely by the vital organs, and so mysterious in its nature that the wisest physiologist gets only glimpses here and there of its operations.

THE GENERAL PLAN OF DIGESTION.--Nature has provided for this purpose an entire laboratory, furnished with a chemist's outfit of knives, mortars, baths, chemicals, filters, etc. The food is (1) chewed, mixed with the saliva in the mouth, and swallowed; (2) it is acted upon by the gastric juice in the stomach; (3) it is passed into the intestines, where it receives the bile, pancreatic juice, and other liquids which completely dissolve it; [Footnote: Digestion, says Berzelius, is a process of rinsing. The digestive apparatus secretes, and again absorbs with the food which it has dissolved, not less than three gallons of liquid per day.-- BARNARD, BIDDER, SCHMIDT, and others.] (4) the nourishing part is absorbed in the stomach and intestines, and thence thrown into the blood vessels, whence it is whirled through the body by the torrent of the circulation. These processes take place within the _alimentary canal_, a narrow tortuous tube which commences at the mouth, and is about thirty feet long. [Footnote: The digestive apparatus is lined with mucous membrane that possesses functions similar to those of the outer skin. It absorbs certain substances and rejects waste matter. On account of this close connection between the inner and the outer skin, it is not surprising to find that in the lowest animals digestion is performed by means of the external skin. The amœba, which is merely a gelatinous mass, when it takes its food, extemporizes a stomach for the occasion. It simply wraps itself around the morsel, and, like an animated apple dumpling with the apple for food and the crust for animal, goes on with the process until the operation is completed, when it unrolls itself again and lets the indigestible residue escape. The common hydra of our brooks can live when turned inside out, like a glove; either side serving for skin or stomach, as necessity requires.]

FIG. 46.

I. MASTICATION AND INSALIVATION.--l. _The Saliva_.--The food while being cut and ground by the teeth is mixed with the saliva. This is a thin, colorless, frothy, slightly alkaline liquid, secreted [Footnote: By secretion is meant merely a separation or picking out from the blood.] by the mucous membrane lining the mouth, and by three pairs of salivary glands (parotid, submaxillary, and sublingual) opening into the mouth through ducts, or tubes. The amount varies, but on the average is about three pounds per day, and in health is always sufficient to keep the mouth moist. [Footnote: The presence and often the thought of food will "make one's mouth water." Fear checks the flow of saliva, and hence the East Indians sometimes attempt to detect theft by making those who are suspected chew rice. The person from whom it comes out driest is adjudged the thief.] It softens and dissolves the food, and thus enables us to get the flavor or taste of what we eat. It contains a peculiar organic principle called _ptyalin_, [Footnote: One part of ptyalin will convert eight thousand parts of starch into sugar.--MIALEE.

The saliva has no chemical action on the fats or the albuminous bodies. Its frothiness enables it to carry oxygen into the stomach, and this is thought to be of service. The action of the ptyalin commences with great promptness, and sugar has been detected, it is said, within half a minute after the starch was placed in the mouth. The process, however, is not finished there, but continues after reaching the stomach.--VALENTIN. The saliva thus prepares a small portion of food for absorption at once, and so insures at the very beginning of the operation of digestion a supply of force-producing material for the immediate use of the system.] which, acting upon the starch of the food, changes it into glucose or grape sugar.

2. _The Process of Swallowing._--The food thus finely pulverized, softened, and so lubricated by the viscid saliva as to prevent friction as it passes over the delicate membranes, is conveyed by the tongue and cheek to the back of the mouth. The soft palate lifts to close the nasal opening; the epiglottis shuts down, and along this bridge the food is borne, without danger of falling into the windpipe or escaping into the nose. The muscular bands of the throat now seize it and take it beyond our control. The fibers of the œsophagus contract above, while they are lax below, and convey the food by a worm-like motion into the stomach. [Footnote: We can observe the peculiar motion of the œsophagus by watching a horse's neck when he is drinking.]

II. GASTRIC DIGESTION.--1. _The Stomach_ is an irregular expansion of the digestive tube. Its shape has been compared to that of a bagpipe. It holds about three pints, though it is susceptible of some distension. It is composed of an inner, mucous membrane, which secretes the digestive fluids; an outer, smooth, well-lubricated serous one, which prevents friction, and between them a stout, muscular coat. The last consists of two principal layers of longitudinal and circular fibers. When these contract, they produce a peculiar churning motion, called the _peristaltic_ (_peri_, round; _stallein_, to arrange) movement, which thoroughly mixes the contents of the stomach. At the farther end, the muscular fibers contract and form a gateway, the _pylorus_ (a gate), as it is called, which carefully guards the exit, and allows no food to pass from the stomach until properly prepared. [Footnote: With a wise discretion, however, it opens for buttons, coins, etc., swallowed by accident; and when we overload the stomach, it seems to become weary of constantly denying egress, and, finally, giving up in despair, lets everything through.]

FIG. 47.

2. _The Gastric Juice_.--The lining of the stomach is soft, velvety, and of a pinkish hue; but, as soon as food is admitted, the blood vessels fill, the surface becomes of a bright red, and soon there exudes from the gastric glands a thin, colorless fluid--the gastric juice. (See p. 319.) This is secreted to the amount of twelve pounds per day. [Footnote: The amount secreted by a healthy adult is variously estimated from five to thirty-seven pounds. As it is reabsorbed by the blood, there is no loss.] Its acidity is probably due to muriatic or lactic acid--the acid of sour milk. It contains a peculiar organic principle called _pepsin_ [Footnote: Pepsin is prepared and sold as an article of commerce. The best is said to be made from the stomachs of young, healthy pigs, which, just before being killed, are excited with savory food that they are not allowed to eat. One grain is sufficient to dissolve eight hundred grains of coagulated white of egg. A temperature of 130° renders pepsin inert.] (_peptein_, to digest), which acts as a ferment to produce changes in the food, without being itself modified.

The flow of gastric juice is influenced by various circumstances. Cold water checks it for a time, and ice for a longer period. Anger, fatigue, and anxiety delay and even suspend the secretion. The gastric juice has no effect on the fats or the sugars of the food; its influence being mainly confined to the albuminous bodies, which it so changes that they become soluble in water. [Footnote: The question is often asked why the stomach itself is not digested by the gastric juice, since it belongs to the albuminous substances. Some have assigned as the probable reason that life protects that organ, and assert that living tissues can not be digested; but the fallacy of this has been clearly shown by experiments that have been made with living tissues in the course of scientific research. The latest opinion is that the blood which circulates so freely through the vessels of the lining of the stomach, being alkaline, protects the tissue against the acidity of the gastric juice.]

The food, reduced by the action of the gastric juice to a grayish, soupy mass, called _chyme_ (kime), escapes through that jealously guarded door, the pylorus.

Fig. 48.

III. INTESTINAL DIGESTION--The structure of the intestines is like that of the stomach. There is the same outer, smooth, serous membrane (peritoneum) to prevent friction, the lining of mucous membrane to secrete the digestive fluids, and the muscular coating to push the food forward. The intestines are divided into the _small_ and the _large_. The first part of the former opens out of the stomach, and is called the _du-o-de'-num_, as its length is equal to the breadth of twelve fingers. Here the chyme is acted upon by the _bile_, and the _pancreatic juice_.

FIG. 49.

1. _The Bile_ is secreted by the liver. This gland weighs about four pounds, and is the largest in the body. It is located on the right side, below the diaphragm. The bile is of a dark, golden color, and bitter taste. About three pounds are secreted per day. When not needed for digestion, it is stored in the gall cyst. [Footnote: A gall bladder can be obtained from a butcher, and the contents kept in a bottle for examination.] Its action on the food, though not fully understood, is necessary to life. [Footnote: The bile is produced, unlike all the other animal secretions, from venous blood; that is, the already contaminated blood of the portal vein. Its complete suppression produces symptoms of poisoning analogous to those which follow the stoppage of respiration, and the patient dies, usually in a comatose condition, at the end of ten or twelve days.--DALTON. The alkaline bile neutralizes the acid contents of the stomach as they flow into the duodenum, and thus prepares the way for the pancreatic juice. It has also a slight emulsifying power (note, p. 167).]

2. _The Pancreatic Juice_ is a secretion of the pancreas, or "sweetbread"--a gland nearly as large as the hand, lying behind the stomach. It is alkaline, and contains a ferment called _trypsin_. This juice has the power of changing starch to sugar. Its main work, however, is in breaking up the globules of fat into myriads of minute particles, that mix freely with water, and remain suspended in it like butter in new milk. The whole mass now assumes a milky look, whence it is termed _chyle_ (kile) and passes on to the small intestine. [Footnote: It is curious to observe that while the gastric juice is decidedly acid, the fluids with which the food next comes into contact are alkaline. It is thus submitted to the operation alternately of alkaline, acid, and again of alkaline secretions. In the herbivora there is also a second acid juice. The reason of these alternations is not known, but it can hardly be doubted that they serve to make the digestion of the food more perfect. And although the solvent power of the gastric juice is placed in abeyance when its acidity is neutralized by the alkaline fluids, yet it appears to be the case here, as in respect to the saliva, that effects are produced by the mixture of the various secretions which are poured together into the digestive tube, that would not result from either alone.--HINTON.]

3. _The Small Intestine_ is an intricately folded tube, about twenty feet long, and from an inch to an inch and one half in diameter. As the chyle passes through this tortuous channel, it receives along the entire route secretions which seem to combine the action of all the previous ones--starch, fat, and albumen being equally affected.