The drainage system consists of very delicate vessels known as the

_lymphatics_, which come together into larger and larger vessels, just

as the veins do, and finally empty into the vena cava just at the point

where it enters the heart. There is no back pressure of blood here, so

that the movement of fluid through the lymphatics is not hindered. There

is no pump for forcing the lymph to move along; the very gradual motion,

which is all that is necessary to keep the fluid from accumulating in

the tissue spaces, is brought about by pressure upon the lymphatics

resulting from the bodily movements. The lymphatic vessels are like the

veins in having valves here and there along them. Whenever by any bodily

movement either a lymphatic vessel or a vein is squeezed, the valves

insure that the liquid shall be passed along in the direction toward the

heart and never in the reverse direction. In the case of the veins this

action is not absolutely necessary, since the heart itself is able to

maintain the circulation, although it does help, particularly in

bringing back the blood from the extremities. In the lymphatics this is

the only way by which movement of fluid is brought about. The result is

that when the body is perfectly quiet, there is very little movement of

fluid through the lymphatics; muscular activity, on the other hand,

leads to rather active movement through these vessels. The fact is well

illustrated in ourselves. One who sits for a long time humped over a

desk finds himself feeling very dull; to obtain relief he stirs about,

stretches, and yawns. The dullness was merely the result of the

stagnation of fluid in his tissues, causing the cells to be more or less

poisoned by their own waste products. By making active movements, these

stagnating fluids were forced along to be replaced by fresh liquid

direct from the blood and the beneficial effect is felt immediately.

_Massage_ properly applied has very much the same effect, although it is

doubtful whether as good results can ever be obtained thus as by actual

vigorous exercise.

At various places along the lymphatics are little spongelike lumps of

tissue known as _lymph nodes_; the particular spongy substance of which

they are composed is called _adenoid tissue_. This adenoid tissue acts

as a filter for the fluid passing through it. Any foreign particles,

living or nonliving, that get into the stream are caught in the lymph

nodes and held there more or less permanently. Most of the nonliving

particles that get into our tissue fluids are from the dust that we

inhale, which works its way through the mucous membrane of the

respiratory passages and so into the body fluids. The lymphatics that

drain the lungs carry along these dust particles and they lodge in lymph

nodes at the base of the neck. Persons who have lived in dusty regions

or have pursued a dusty livelihood, such as coal heaving, will have by

the end of their lives lymph nodes which are literally black with dirt.

The tonsils are lymph nodes at the base of the tongue. Unfortunately

they are so near the surface of the throat that they frequently become

infected from the throat itself, and so become the seats of pus pockets,

as already noted. Closely related to the tonsils are the masses of

adenoid tissue at the back of the throat which frequently grow to an

undue size in children, and are then known as adenoids. The harm done by

adenoids is chiefly mechanical; they may block the Eustachian tubes, and

so cause deafness, as already mentioned in Chapter IX, or they may

interfere with the free movement of the air through the nasal passages.

Children in whom this condition exists are mouth breathers. Adenoids,

like tonsils, are subject to infection, and so may give trouble by

becoming the sites of poison formation. Adenoids represent always an

overgrowth and for that reason may be removed without any possibility of

hampering the proper working of the body. Experience has shown that

children whose development appears to be hindered by the presence of

adenoids are almost invariably benefited by having them removed. The

tonsils are normal parts of the bodily structure and as such undoubtedly

have a regular work to do, but here again experience has shown that harm

from persistent pus pockets is so much greater than harm from loss of

function following their removal as to justify taking them out, whenever

pus pockets develop in them. There are enough lymph nodes in the region

about the throat, so that if tonsils or adenoids are removed any foreign

matters that get into the body fluids will still be filtered out.

[Illustration: Copyright, Keystone View Co.

X-RAYING THE TEETH AND JAW. HEALTH OFTEN DEPENDS ON THEIR GOOD

CONDITION]

[Illustration: Photo, Paul Thompson

A THROAT EXAMINATION--IMPORTANT IN DETERMINING PHYSICAL FITNESS]

One more function of the lymph nodes must be mentioned; this is their

property in preventing the spread of cancer cells. We now know that

so-called secondary cancers are the result of the spread of cancer cells

from the original seat of the cancer to other parts of the body, and

that this spread is much hindered by the ability of the lymph nodes to

catch the cancer cells and hold them. Unfortunately sooner or later some

of the cells will escape beyond the lymph nodes and so spread the

malignant growth throughout the body, but until this happens the cancer

is confined to the region where it started, and it is during this period

that complete cure by surgical means is possible. It is because of the

imminent danger of the escape of cancer cells beyond the restraining

lymph nodes that relief by surgery should be sought at the very earliest

possible moment. Delay, whether due to carelessness or any other cause,

is as certainly fatal in the case of cancer as in any other disease for

which a cure is known.

CHAPTER XV

THE SERVICE OF SUPPLY OF FOOD

In order for the blood that circulates through the body to pass on to

the body cells the materials which they need, it is evident that the

blood itself must have some source from which to obtain the materials.

Our present task is to examine this source. In Chapter IV we talked

about food and its uses; here we are concerned with the way in which the

foodstuffs are taken into the body and prepared for use. We know that

the material which we take in is part solid and part liquid, giving rise

to the familiar distinction between food and drink. For our present

purposes this distinction has no importance and will not be made. What

we do have to note is that of the complex mixture of materials which

makes up any ordinary meal, some of the substances are ready to be taken

up at once by the blood and to be distributed around the body; others

have to go through a preliminary course of preparation. All of the

dietary accessories, so far as we know, pass from the digestive organs

into the blood without change. All or nearly all of the energy-yielding

foods, on the other hand, must have a preliminary preparation to which

we give the name of digestion; the operation of digestion being to break

down the complex foodstuffs that are eaten into simpler materials of

which the body can make use. Digestion is made up of a series of

chemical changes by which the large molecules of the original foodstuffs

are broken into smaller and simpler molecules. This breaking down of

large molecules into smaller is a very common kind of chemical process.

A feature of it is that under ordinary circumstances the breaking down

of the large molecule goes on very slowly, but if the right conditions

are provided the breaking down proceeds rapidly. The particular

condition which is necessary is the presence in the solution where the

large molecules are breaking down of something which will hurry up the

process. There are a good many kinds of substances which have this

ability to hasten molecular decomposition; those that do it in carrying

on digestion are given the name of _enzymes_. We do not know just how

the enzymes act; we merely know that when they are put along with the

substance which is to be decomposed, it goes to pieces very much more

rapidly than if none of them was present. The enzymes themselves are not

used up in the process, so if time enough is allowed, a very small

amount of enzyme can bring about the decomposition of a very large

amount of material. In our bodies the whole digestive process consists

of a succession of decompositions of complex materials into simpler ones

under the speeding-up influence of enzymes. We have a number of

different kinds of foodstuffs to be decomposed and a corresponding

number of enzymes.

The process begins in the mouth; here the food is chewed and moistened

with saliva before being swallowed; both the chewing and the moistening

are important to insure good digestion later on. The enzymes have no

particular ability to penetrate a mass of food material; what they do is

to attack it from the outside and work in as it decomposes. Since

enzyme action is thus a surface action, it is evident that the larger

the surface the more efficient will be the action. Chewing is nothing in

the world but a mechanical breaking up of the food to get the largest

possible surface. We have sufficient proof of its importance in the

digestive disturbances that arise as the direct consequence of improper

chewing. Of recent years it has been realized that undernutrition,

particularly in children, is often a result of the failure to chew the

food properly; it has been found, furthermore, that bad teeth or

improperly shaped mouths are very frequently responsible. For this

reason in most of our large cities dental clinics are being established

for the purpose of inspecting and, if necessary, caring for the mouths

of school children. The result of this work is to improve the general

average of health among children simply by increasing the extent to

which the food is chewed. This fact, together with that described in

Chapter XIII as to pus pockets, should impress upon both parents and

teachers the fundamental importance of proper care of the teeth. This

includes not only the prevention of decay by daily thorough cleaning and

the securing at intervals of not more than six months of dental

inspection with treatment where necessary, but also in the case of

children with deformed jaws the special treatment necessary to bring the

teeth into position for effective chewing. Too much stress can scarcely

be laid on the importance of these precautions.

The moistening of the food with saliva is likewise important to good

digestion. As a matter of fact, without it even the act of swallowing

would be impossible. One can easily prove that insufficient moistening

prevents swallowing by eating a rather large quantity of a very dry

food, like crackers. A common habit of people is to moisten the food

with water which they drink instead of waiting for it to be properly

moistened by saliva. Contrary to popular belief there is absolutely no

objection to drinking water at mealtime. In fact, the presence of the

additional liquid in the alimentary tract is probably beneficial rather

than the reverse; there is, however, a very serious objection to using

water for washing down half-chewed food. The best possible way to judge

whether the food is sufficiently chewed is by observing whether it has

been sufficiently moistened so that it will be swallowed easily. If so,

the chances are that the chewing has been sufficient. Between mouthfuls

as much water may be taken as one desires, although if ice water is

drunk, it should be held in the mouth until the worst chill is taken off

before being swallowed, so that it will not chill the stomach. Since the

comfort of drinking ice water is in the cooling of the mouth and throat

and not at all in the cooling of the stomach, this increases rather than

diminishes the enjoyment one gets from a glass of it on a hot day. So

much satisfaction can be obtained by proper drinking habits, that it is

a pity to allow the health to be injured by improper habits to such an

extent as to necessitate, as frequently happens, the complete

abandonment of water drinking with meals.

The saliva is primarily for the purpose of moistening the food, but

besides this it is also a definite digestive juice, because it contains

one of the digestive enzymes. This enzyme, to which is given the name

_ptyalin_, acts upon starch, changing it to sugar, but not to the

particular sugar of which the body makes use. Perhaps we should digress

for a moment to say that there are chemically a number of sugars. These

are in many respects alike, although they also vary a good deal among

themselves. For example, some are much sweeter than others; glucose,

which is a sugar that can be made from cornstalks and other plant

products, is not nearly so sweet as cane or beet sugar, although one is

fully equal to the other so far as nutritive value is concerned. Of all

the various kinds of sugar that exist, only those in the glucose class

can actually be used by the body, so that all other sugars have to be

changed by digestion into a glucoselike sugar before passing on into the

blood. The enzyme of saliva converts starch not into glucose but into a

more complex sugar to which is given the name of maltose. It, therefore,

starts digestion, but does not carry it through to completion. Starch

digestion begins during the course of chewing and mixing the food with

saliva; it goes on while the food is being swallowed, and for a short

time after it enters the stomach, being stopped there sooner or later by

means which will be described in a moment. No other real digestion

occurs in the mouth, although the chewing and moistening are of very

great importance in preparing the food for the digestion that is to come

further along.

The food that is swallowed passes down the esophagus and enters the

stomach. Before speaking of this it will be necessary to recall what was

said in Chapter VII about the behavior of the stomach walls. Between

meals these are in a relaxed and flabby state, with the opposite walls

lying more or less in contact. There is usually a little liquid and some

swallowed air in the stomach, so that it is not actually empty, even

when we speak of it as being so. Just before mealtime the walls of the

stomach draw up so that instead of a flabby bag we have a fairly tense

tubular organ. It is at this time that the contractions of the stomach

wall, which we recognize as hunger, begin to come on. There is a

sphincter muscle between the esophagus and stomach which closes the

opening tightly and so prevents the pressure within the stomach from

forcing gas or liquid back up into the esophagus. This sphincter opens

automatically only in connection with the act of swallowing. Every time

we make a swallowing movement a sort of wave passes down the esophagus,

and when this wave arrives at the stomach the sphincter relaxes,

allowing whatever was moving down the esophagus to enter. If one watches

a horse or cow in the act of swallowing, the rather deliberate progress

of this wave down the neck can be followed. What happens as we eat a

meal is that every mouthful, as it is swallowed, is passed through the

sphincter into the stomach and room is made for it by a gradual

relaxation of the stomach wall, so that, as we saw in Chapter VII, the

pressure of the stomach against its contents stays fairly steady, in

spite of the fact that more and more material is being taken into the

stomach from the esophagus. One result of this behavior of the stomach

is that the first food that is swallowed is nearest to the stomach

walls, and that which is swallowed afterward is nearer the middle, being

inclosed on all sides by the previously swallowed food. This is fairly

important because the gastric juice is secreted by glands in the wall

of the stomach and so will get in contact first with the food that is

swallowed first, and only afterward will reach the food that was

swallowed later. Occasionally the sphincter between the stomach and the

esophagus relaxes unexpectedly; this is said to happen more often in

smokers than in nonsmokers. The result is that some of the sharply acid

stomach contents are forced up into the esophagus and vigorous

swallowing is necessary to crowd them back down into the stomach. The

burning sensation which accompanies this is known as heartburn,

although, as we have just seen, it is really entirely a matter of the

esophagus and has nothing whatever to do with the heart.

Although the stomach carries on a certain part of the work of digestion,

its primary purpose, as we shall see presently, is to serve as a storage

place into which a considerable amount of food can be placed in a few

moments and so enable us to do our eating at three or four definite

meals instead of little by little throughout the day. When we rise from

table after any meal, we have in our stomachs roughly one-third of our

total food supply for the day, the exact proportion depending, of

course, on our individual habits as regards our distribution of

food-taking among the three meals. During the two to four hours

following the meal this accumulated material in the stomach will be

passed along little by little to the small intestine, which is the real

digestive organ of the body. In the course of this time there will be

some additional digestion within the stomach, but not enough to prepare

any foodstuffs for actual use by the tissues. The outlet from the

stomach to the small intestine is guarded by

[Illustration: THE ALIMENTARY CANAL

_A_, stomach; _J_, _I_, small intestine; near _CC_, vermiform appendix;

_AC_, _TC_, _DC_, large intestine; _R_, rectum. (Martin’s “Human Body”)]

another sphincter similar to that which closes the opening between the

esophagus and stomach. This sphincter is ordinarily closed tightly,

relaxing occasionally to let a little food through into the small

intestine. It is interesting to note, however, that the sphincter does

not hold against some materials; for example, water that is drunk

between meals passes rapidly from the stomach on into the small

intestine. It is said that raw oysters, raw white of egg, and other

materials of a similar consistency also pass the sphincter immediately.

Most other kinds of food material cannot pass unless the sphincter

actually relaxes.

Shortly after food begins to enter the stomach churning movements are

set up. These consist of regular contractions of the smooth muscle

beginning at about the middle of the stomach in the form of a ring and

so causing a deep depression to form right around the stomach, which

then travels along toward the sphincter at the outlet. This, of course,

crowds the food up toward that end; but if the sphincter does not relax,

the food instead of escaping simply squirts back through the space left

in the middle of the ringlike construction and so is actively churned. A

regular procession of these waves travels over this part of the stomach

during the whole time that gastric digestion is going on. The outer half

of the stomach, the side toward the esophagus, does not join in this

churning motion. The walls here remain quiet, merely pressing upon the

food to crowd it up into the part where the churning is going on as fast

as room is made for it there. At intervals during this process the

sphincter relaxes and a small mass of food is crowded through into the

small intestine; then the sphincter closes again, preventing more from

passing until the former mass has had time to be acted upon by the

digestive juices in the intestine. The way in which this sphincter is

controlled is one of the very interesting facts of physiology, but

before describing it we shall have to say something about actual gastric

digestion.

The juice which the glands in the stomach wall secrete is called gastric

juice. This contains three important constituents, first of which is

hydrochloric acid. From the standpoint of chemistry it is a very

interesting thing that the gastric glands should he able to manufacture

a mineral acid like hydrochloric acid. That they do so, however, is

proven by the facts of digestion in everybody. In addition to

hydrochloric acid gastric juice contains two enzymes; the first is

pepsin, which begins the digestion of protein foods, although it does

not carry it through to completion. The other constituent is the enzyme

rennin, which clots milk; this property appears to be useful in that

clotted milk will not pass the sphincter into the small intestine as

quickly as it would if it were liquid, and so the digestive processes

can go on in milk after clotting just as it goes on in any other of our

foods. Since milk is the chief, and in many cases the only, food of the

young it is of course very important that its digestion should be very

efficiently carried on. As the food is churned by the stomach muscles,

it becomes mixed with gastric juice. Any milk that is present will be

clotted and any protein will begin to be digested. The food will also be

mixed with hydrochloric acid. It happens that the salivary enzyme,

ptyalin, will not act in the presence of an acid, so that as soon as any

of the food in the stomach comes in contact with hydrochloric acid of

the gastric juice, the digestion of starch by ptyalin stops. Since

gastric juice is mixed only with that part of the food which is being

churned, the other portion, that in the part of the stomach toward the

esophagus, goes right on undergoing salivary digestion. So, contrary to

the old idea that salivary digestion is unimportant because there is not

time enough during the chewing and swallowing of the food for it to go

on effectively, we now know that the latter parts of the meal

particularly may be very thoroughly acted upon by ptyalin before the

action is stopped through the contact of this part of the food with the

acid of the gastric juice.

The sphincter between the stomach and small intestine is operated by

hydrochloric acid, making up a remarkably ingenious arrangement for

securing the emptying of the stomach as fast as the food is ready to be

passed on into the small intestine. In order that gastric digestion may

have full opportunity, it is necessary that the food should be

thoroughly mixed with the gastric juice. As soon as this has happened,

however, it is proper that the food should be passed along to be acted

on by the rest of the digestive juices. Until that part of the food

which is next to the outlet sphincter is thoroughly mixed with gastric

juice there will be no surplus of hydrochloric acid by which to operate

the sphincter, but as soon as the mixing is complete there will be a

surplus. We have here a simple case of stimulation by a chemical

substance; the mere contact of the acid with the part of the sphincter

which fronts on the stomach is sufficient to stimulate the smooth

muscles in it to relax, and, of course, as soon as relaxation occurs,

the pressure of the churning movements upon the food will crowd that

part of it which is nearest to the sphincter through into the small

intestine.

Now we encounter the second part of the action which is just as

interesting as the first part. The small intestine is a rather narrow

tube; its cavity is not much more than three-quarters of an inch or so

in diameter. If the outlet sphincter from the stomach were to stay

relaxed after it once let go, the rapid pressing of the food from the

stomach would fill the small intestine quickly to a distance of several

feet. It happens, however, that the next of the digestive juices to act

upon the food are the pancreatic juice and bile, which are secreted

respectively by the pancreas and liver, and are poured into the small

intestine by ducts from these organs just a few inches beyond the outlet

sphincter from the stomach. In order for these juices to mix well with

the food it is important that only a little food come into the small

intestine at a time. Otherwise a large part of it would pass the opening

of these ducts so quickly that there would be no chance for it to become

mixed with the juice from them. What actually happens is that the

sphincter relaxes under the stimulus of surplus hydrochloric acid and

then as soon as a little food has passed through closes again. The

closing as well as the opening is operated by hydrochloric acid, but

there is this difference: the opening is the result of the stimulation

of acid upon the stomach side of the sphincter; when the same acid comes

in contact with the intestinal side of the sphincter, its presence there

causes the sphincter to contract. Of course, this means that whenever

any food passes out from the stomach into the small intestine the

sphincter is stimulated by acid simultaneously from both sides, but

under that condition the stimulus which causes the sphincter to contract

is more potent than that which causes it to relax, so that whenever

there is acid on both sides the sphincter will be shut. The only way in

which it can be opened to allow more food to pass out is to get rid of

the acid on the intestinal side. This is fairly quickly accomplished,

because both the pancreatic juice and bile are strongly alkaline

liquids, so that as fast as they mix with the food they tend to

neutralize the acid; the result is, of course, that by the time these

digestive juices have impregnated the food mass thoroughly and so have

accomplished their purpose, the stimulus which keeps the sphincter

closed has disappeared and there is left only the stimulus to open it,

due to the presence of acid in the stomach. As soon as this condition is

reached, the sphincter will open again, another mass of food will pass

through bringing its acid with it, which, of course, closes the

sphincter promptly, and so the whole story goes on again. This action

has been described in detail because it is one of the very best examples

we have of the remarkably ingenious arrangements by which the complex

bodily functions are carried on automatically.

From what we have just said it is evident that the proper emptying of

the stomach and, therefore, the proper carrying on of the whole process

of digestion, depends upon the formation and outpouring of ample

supplies of acid-containing gastric juice. A failure of gastric

secretion is bound to be followed by a failure of the stomach to empty

itself. Instead of going on to the small intestine where digestion is

completed and the digestion products passed on to the blood stream to be

used by the body, the food mass lies inert in the stomach. After a few

hours, if not immediately, the acute symptoms of indigestion are certain

to develop. We are describing not a theoretical possibility but an

actual happening in the life of thousands of people, sometimes almost

daily. Indigestion is one of the great causes of misery and of impaired

efficiency throughout the civilized world. It is said that Napoleon lost

the Battle of Waterloo on account of an acute indigestion which befell

him on the morning of the crucial day.

There are several causes of indigestion. Among them should be mentioned

food poisoning; that is, the inadvertent taking of some material which

instead of being a food proves to be actively poisonous. A second type

about which we shall have something to say presently is the result of

poisoning by substances which are products of the putrefaction of the

intestinal contents. A third type, and the one in which we are

particularly interested at this moment, is the one which results from

the failure of gastric juice to be properly secreted. A fourth

occasional cause of indigestion, which should be mentioned in passing,

is that which results from eyestrain, and is avoided by properly fitted

glasses. There is comparatively little food poisoning; most persons who

are susceptible to poisoning from particular kinds of foods find it out

promptly and learn to avoid the article which poisons them. A good deal

of indigestion is the direct result of intestinal sluggishness; this

usually comes on gradually enough to give warning of its approach. Sharp

attacks for which there seems to be no justification are nearly always

the result of the failure of gastric secretion. Because of the practical

importance of this topic a very large amount of study has been devoted

to it, and its story makes one of the very fascinating chapters of

physiology.

Our space permits us to touch on it only very lightly. It begins with

the time back in the middle of the eighteenth century when a couple of

Italian investigators fed sponges inside perforated metal capsules to

birds and animals and after recovering and squeezing the sponges

demonstrated that the juice which had been soaked up would dissolve

meat. The scene then shifts from Italy in the middle of the eighteenth

century to our own country at the close of the first quarter of the

nineteenth, when a French-Canadian lad, by the name of Alexis St.

Martin, was brought in to the frontier army post on Mackinac Island with

a gaping gunshot wound in his side. The victim of this accident

recovered, but meanwhile a permanent opening into his stomach was left.

The army doctor on duty at that post, William Beaumont by name,

perceiving the unique opportunity for the study of digestion that was in

his hands, employed the young man in his family and for many years

studied the digestive process as it went on in his stomach. Next we turn

to Russia, where an eminent physiologist, who was still living at last

reports, demonstrated by making artificially an opening into the stomach

of animals, similar to the one which was made by accident in the case of

St. Martin, that the secretion of gastric juice is under the control of

the nervous system and, furthermore, that it is under the control of

that part of the nervous system which we have already learned regulates

the functioning of smooth muscles and glands. Our present knowledge of

the nervous system would lead us to expect nothing else, but at the time

when these discoveries were made much less was known than now about how

the glands, as well as the smooth muscles, are controlled. This Russian

investigator showed that it is not necessary for food to enter the

stomach in order for gastric juice to be secreted. The enjoyment that

accompanies the taking of food into the mouth and so getting its taste

and flavor is sufficient to arouse nervous disturbances which excite the

gastric glands to activity. On the other hand, unfavorable emotions such

as anger, fright, or worry, prevent the discharge of these nervous

disturbances and so the glands remain inactive. Hand in hand with this,

as we saw in Chapter XII, there will be a complete absence of muscular

activity in the stomach; not only does the food fail to be churned, but

gastric juice fails to be poured out. No digestion goes on in the

stomach nor is the food passed on into the small intestine to be

digested there. We can expect nothing else than that indigestion will

come on. It appears that in human beings, with their high nervous

organization, simple weariness may suffice to hinder the outpouring of

the nervous disturbances upon which gastric secretion depends, so that

one may not be worried at mealtime, but may be simply overtired and

still fail of digesting the food. Fortunately we have means of avoiding

this; it has been shown that meat extracts have a definite chemical

effect upon the gastric glands, arousing them directly to activity. A

hot, well-flavored soup at the beginning of a meal is a great aid to

digestion, both because it tends to arouse us to active enjoyment of the

meal and also because of the chemical influence it exerts directly upon

our gastric glands. Cheerful conversation at table is an aid to

digestion, because it contributes to our enjoyment of our food. Heated

or angry discussion is to be avoided because of the danger that

unfavorable emotional disturbances may be aroused, hampering or even

putting a stop to the digestive activities.

We have here a powerful argument in favor of good cookery; some may be

inclined to think that food is food and that one kind is about as

nourishing as another. Not only theoretically, but in practice, food

that is attractive both in appearance and in flavor benefits the body

more than unattractive and badly cooked food. Of course, we must

recognize that no statement of this sort has absolutely general

application. The old proverb which says that “hunger is the best sauce”

continues true. Hard-working manual laborers and actively playing

children enjoy and digest food that is indifferently attractive. Brain

workers and people of moderate health are better for an attractively

prepared dietary.

We have now traced the food into the small intestine and will remember

that up to this point the actual act of digestion has not gone very far.

The enzyme of saliva has decomposed part of the starch, or perhaps all

of it, into a complex sugar; the enzyme of gastric juice has split some

of the very complex proteins into simpler ones. Of the final products of

digestion none has yet been formed; their formation is the work of the

small intestine. Into the upper end of it, as we have already seen,

pancreatic juice and bile are poured from the pancreas and liver

respectively. The pancreatic juice contains three enzymes; one of them

is identical with that of saliva and completes the decomposition of any

starch that the saliva has failed to act upon; the second is an enzyme

which acts upon proteins, differing from pepsin of gastric juice in that

it carries the splitting of proteins to completion; the third is the

enzyme which acts upon fats. Fats are the most troublesome of all the

foodstuffs from the digestive standpoint, because they are insoluble in

water. Not only do they themselves offer difficulty, but they smear over

the other food masses and make it hard for the digestive juices to get

at them. Although the enzyme for digesting fat is secreted by the

pancreas, its successful working depends on the presence of bile from

the liver. If there is an obstruction of the bile passages, so that bile

cannot be poured out into the small intestine, the digestion of fats

stops and, along with it to a considerable extent, all the other

digestive processes of the intestine. There is a familiar common

condition known as jaundice, in which the bile passages become stopped,

usually as the result of inflammation. When one is suffering from this

condition, it is very important that not much fat be included in the

diet; otherwise there is likely to be trouble with indigestion.

In addition to the pancreatic juice and bile there is a secretion known

as intestinal juice which is poured out by small glands scattered along

the wall of the small intestine. These secrete enzymes which convert the

various kinds of sugar into the particular sugar, glucose, which is the

one the body can use. The glands also secrete an enzyme which can carry

on to completion protein digestion. Thus there is ample insurance that

all the foodstuffs shall be made ready for the use of the body.

The next thing in order is for the digested foods to pass from the

digestive cavity into the blood. This process is known as absorption.

The first part of the alimentary tract from which absorption might occur

is the stomach, but, as we have just seen, the process of digestion is

not carried far enough in the stomach to fit the foods for going on into

the blood stream. Absorption from the stomach is, therefore, undesirable

rather than desirable. This is interesting because many condiments,

particularly the hot spices and mustard, irritate the stomach lining in

such a way as to promote absorption through it. Formerly this was looked

upon as an argument in favor of the use of these condiments, but we now

realize that it is rather an argument against them. The same applies to

alcohol. Alcohol irritates the lining of the stomach and produces

absorption through it. Unfortunately for those who desire a

physiological argument in favor of the use of alcohol at meals this is a

disadvantage rather than an advantage. The great region of absorption is

the small intestine, where the digestive process is completed and where,

therefore, absorption can properly go on. As to the method of absorption

not a great deal needs to be said; there is a very delicate membrane

lining the intestine, and just beneath this are the very numerous

capillaries through which the blood is flowing; also in the spaces

between the capillaries is the tissue fluid which drains into the

lymphatic system. The products of protein digestion and glucose pass

through the lining membrane of the intestine and through the capillary

walls directly into the blood stream and are carried along in it. The

digested fats, on the other hand, after passing through the intestinal

lining enter the tissue fluids instead of the blood. Incidentally it

should be noted that they are rebuilt into tiny fat droplets during the

process of absorption, so that what we find in the tissue fluids of the

intestine are true fats and not digestion products of fat. These tiny

fat droplets pass away from the intestine by way of the lymphatic

system. This, as we saw in the last chapter, drains finally through

large lymphatic vessels into the great veins just at the entrance of the

heart, and so the fat finally reaches the blood stream, but not so

directly as do glucose and the digestion products of protein.

There is a reason for this difference in the handling of fats and the

other foodstuffs which will be clear when we recall the point made in

the last chapter that all the blood which flows through the walls of the

intestine is collected in the portal vein and must pass again through

the capillaries of the liver before entering the circulation at large.

What happens is that all the digestion products, except the fats, pass

through the capillaries of the liver, but the fat gets into the blood

stream by another way and does not go through the liver en route; for

some reason or other it seems to be to the advantage of the body that

the fat should not be allowed to circulate through the liver

capillaries. It is, as we shall see in a later chapter, a distinct

advantage to have the glucose pass through the liver, and there is

probably also a good reason for having the digestion products of protein

take that course.

The digesting food is moved along the small intestine by contractions of

its muscular wall, which travel along slowly in the form of a wave, very

much as the wave of swallowing passes down the neck of a cow or horse.

These waves do not occur regularly but only now and then; what happens

is that several inches or two or three feet of the small intestine will

be filled with a food mass which stays for a while at that place,

digestion going on in it, and the digested food being absorbed through

the wall. Both the digestion and the absorption are aided by a kind of

churning motion made up of a series of contractions spaced about an inch

apart along the part of the intestine where the food mass lies. These

contractions form and disappear quite regularly at the rate of several a

minute. They have the effect of squeezing the food mass rhythmically,

but do not move it away from the place where it is lying. After this

churning has been going on for a while it subsides, leaving that part of

the intestine entirely quiet; then a contraction wave sets in at the end

of the food mass toward the stomach and pushes the mass bodily along the

intestine to a new section where the wave dies out and the churning

motion begins again. This process is repeated with every food mass as it

comes through from the stomach into the intestine until, finally,

usually three or four hours after the first food has commenced to come

into the intestine from the stomach, the whole mass has been propelled

the length of the small intestine and what is left has been passed on

through into the large intestine. The small intestine is about

twenty-five feet long in man; flesh-eating animals of about the same

size have intestines only about one-half as long; grazing animals the

same size, on the other hand, have intestines two or three times longer

than those of man; this intermediate length of the human intestine is

ordinarily looked upon as indicating that man is adapted to a mixed

diet, being neither strictly a flesh-eating nor strictly a

vegetable-eating animal.

During the slow passage of the food mass through the small intestine the

processes of digestion and absorption are completed, so that very little

material enters the large intestine except the indigestible parts of the

original food. These are made up mostly of cellulose and similar

indigestible vegetable materials, but they include also indigestible

fragments of gristle and other meat remnants. It is also worthy of note

that the amount of water does not decrease much during the passage of

material through the small intestine; in other words the material that

enters the large intestine is about as liquid as that which passes from

the stomach in to the small intestine. This does not mean that there is

no absorption of water from the small intestine; in fact, water that we

drink is probably absorbed quite rapidly; what it does mean is that

enough water is secreted into the intestine from the blood in the lower

parts to keep the whole mass liquid; this is of course important as an

aid both to digestion and absorption.

The large intestine, as its name implies, is a tube of much larger

diameter than the small; it begins low down on the right-hand side,

passes up to about the level of the stomach, then across the body and

down on the left-hand side, making thus a sort of inverted “U” in the

abdomen; the space within the “U” is filled up by the loops and coils of

the small intestine. The first part of the large intestine is devoted to

the absorption of water; the result of this absorption is to leave the

contents of the large intestine in a semidry condition; absorption of

water goes on continuously, so that the longer the material remains in

the large bowel, the drier will it become. In the ordinary course of

events the material, as it is dried out, is passed along through the

large intestine and at intervals, which should not exceed twenty-four

hours, the accumulated material is discharged from the body.

Various facts about the functioning of the intestine which have seemed

to many people mysterious are easily explained on the basis of what we

have next to talk about, which is the presence both in the small and in

the large intestine of flourishing colonies of bacteria. It has perhaps

not occurred to most of us that we are in the position of involuntary

hosts to enormous numbers of these microscopic plants, but that is

exactly what we are. There is no possibility of preventing this, since

the combination of warmth, moisture, and abundant food, which is

afforded within the intestine, is the most favorable possible situation

for many kinds of bacteria, and no effort on our part would enable us to

be free from them. Fortunately, in the ordinary course of events, we are

not affected one way or the other by their presence. In the small

intestine their chief diet consists of the sugars which are produced as

the result of starch digestion or are eaten directly. In connection with

nourishing themselves from these sugars, they bring about what is known

as alcoholic fermentation; that is, some of the sugar is converted into

alcohol with a by-product of carbon dioxide. Exactly the same thing

happens when yeast develops in a mass of dough; in the latter case the

bubbles of carbon dioxide are what we are after, since they are what

makes the bread light, the alcohol being driven off by the heat of

cooking. In the alcoholic fermentation that goes on in our small

intestines the carbon dioxide is a waste product and is passed on out of

the body, but the alcohol is absorbed and used as fuel. Little has been

said thus far about the physiological effects of alcohol. At this point

all we care to say is that alcohol can be oxidized by the body tissues

with the liberation of energy and so is a perfectly good fuel food. In

the minute amounts in which it is absorbed from the small intestine it

is utilized completely and there is not the slightest reason to suppose

that it has any ill effects whatever. The objections to the use of

alcohol, which have led to its abolition as a beverage in this country,

depend on certain definite pernicious drug effects which it shows when

consumed in any but the smallest amounts, and which destroy the

usefulness of what would otherwise be a valuable article of human diet.

Fermentation within the small intestine ordinarily goes on entirely

unperceived by us. Certain foods, or the invasion of the intestinal

tract by certain species of bacteria, may change the fermentation in

such a way that irritating substances are produced which cause the

movements of the small intestine to be very greatly increased; its

contents are swept on through in very much less than the usual time and

we have the condition known as diarrhea.

Within the large intestine bacterial action goes on fully as vigorously

as in the small, and because of the smaller relative amounts of

digestible food and of water a much larger proportion of the intestinal

contents consists of the bodies of the bacteria and of the products of

their metabolism. Since the sugars have been completely absorbed out by

the time the food reaches the large intestine and the remaining

materials upon which the bacteria can feed are more largely of the

protein class, the bacterial action changes from fermentation to that

which we commonly describe as putrefaction. This type of action, instead

of giving rise to alcohol and carbon dioxide, produces a number of

highly offensive compounds; many of these can pass through the lining

membrane of the large intestine into the blood and so can circulate

around the body. Of late years we have realized that these products of

intestinal putrefaction are highly poisonous, especially if present in

the blood stream in any considerable amount. The term “auto-intoxication,”

which we run across frequently in health literature, means strictly the

poisoning of the body by the products of intestinal putrefaction. Of

course, the condition is much aggravated if putrefaction is allowed to

go on for too long a time. The ill feelings which result from

constipation and which in the life of very many people constitute a

really serious impairment of health are the direct result of poisoning

by these putrefaction products. The obvious remedy is the avoidance of

intestinal sluggishness; unfortunately this is easier in theory than it

often is in practice. Probably two conditions of modern life are chiefly

responsible: the first is our tendency to make the diet more and more

highly concentrated; that is, to leave out of it more and more the

indigestible parts. The result is that not enough indigestible material

enters the large intestine to make a sufficient bulk upon which the

intestinal muscle can work in moving the mass forward. While bulk is

accumulating, both absorption of water and putrefaction are going on,

until by the time a sufficient mass is present, it is so dry that the

muscles are not able to move it along and it has produced undue

quantities of poisons. Obviously the way to treat this condition is by

eating more indigestible material. For this purpose there is nothing

better than apples in their season. The old proverb “An apple a day

keeps the doctor away” has very sound common sense back of it. Apples

are not ideal for this purpose for all people, since they frequently

cause a distressing evolution of gas or even headache. Nearly everybody,

though, by experimenting, can find a time of day at which an apple can

be eaten without any digestive trouble whatever, and frequently with

considerable benefit. Apples are valuable simply for their bulk of

indigestible substance. The process of cooking converts most of this

into digestible material, so that for this particular purpose they must

be eaten raw. There are various other fruits such as figs, prunes, and

raisins which function similarly and in addition have a direct

stimulating effect upon the intestinal movements and so favor the

discharge of material. Some people can eat popcorn to good advantage,

although others suffer from gas distress if they do so.

The second condition of modern life which favors intestinal sluggishness

is the sedentary habit which so many of us, both men and women, have. A

vigorous outdoor existence is practically never complicated by

auto-intoxication. Anyone who can maintain habits of active exercise

will usually find himself troubled little by this condition. One further

point should be made, and this can scarcely be over-emphasized, since it

probably has as much to do as any other single factor in the avoidance

of auto-intoxication; this is the development of regular habits in the

matter of evacuation of the bowel. It is a general fact of the operation

of smooth muscle that it readily develops certain habits. We have

already seen an example of this in the behavior of the stomach in

connection with mealtime. As we have noted, the stomach lies flabby

between meals and enters into a state of tension just about the time

that we are in the habit of eating. This adjustment is made no matter

what particular habits individuals may have. Those who habitually eat

only two meals a day will have this tightening of the stomach twice a

day; others whose habit it is to eat five times a day will have a

similar tightening five times in the twenty-four hours. Similarly the

large intestine can establish a regular habit with regard to

evacuation. This is best done in childhood, so that parents by insisting

upon regularity in their children in this respect can usually assure

them of a lifetime in which there will be little trouble from

auto-intoxication. On the other hand, parents who are neglectful of

their children’s welfare in this respect are laying up for them a

lifetime of trouble and very much discomfort.

In concluding this part of our discussion we should note that artificial

stimulation to intestinal activity should be regarded as a measure of

last resort and under no circumstances as a habitual means of inducing

evacuation. Persons who allow themselves to become dependent upon

laxatives are laying up for themselves trouble, since these invariably

become less and less effective, making it necessary to increase the dose

and finally establishing a condition in which only the vigorous efforts

of a physician will restore the body to normal. A safe general rule is

that anyone who finds himself becoming dependent upon laxatives should

immediately put himself under competent medical care for the purpose of

restoring his system to normal functioning in which he will not be

dependent upon drugs.

CHAPTER XVI

THE SERVICE OF SUPPLY OF OXYGEN

Every cell must have oxygen for its metabolism. This it must get from

the tissue fluid upon which it fronts, and tissue fluid in turn must get

it from the circulating blood. The blood in turn has to get it from

somewhere and the place from which the blood gets it is called a lung or

a gill, according as the animal breathes air directly or gets its oxygen

out of the water. The purpose of the present chapter is to trace oxygen

from the air through the blood to the tissue fluids and so to the living

cells. We saw in the chapter on “Blood” that we have a special

substance, the hemoglobin, which helps in the transportation of oxygen.

We shall have occasion here to see how it does this. It is well to bear

in mind that for practical purposes we include the methods by which

carbon dioxide is gotten rid of along with the study of the supply of

oxygen, so that although this chapter carries the heading “The Service

of Supply of Oxygen,” we shall also study in it how the carbon dioxide

is carried away. This is a convenient way of dealing with the subject,

because the two gases are handled in very much the same manner; it is

also made necessary by the fact that the control of the apparatus by

which these gases are handled is so interwoven that the transportation

of one could not well be studied without giving attention also to the

transportation of the other.

The problem of the oxygen supply and of the removal of the carbon

dioxide is in theory very simple; the air contains a large percentage of

oxygen and a very small percentage (three parts in ten thousand) of

carbon dioxide. If the blood is exposed to air with nothing between but

a very delicate membrane, oxygen will diffuse from the air into the

blood until the blood has taken up all that it is capable of holding. As

the blood circulates around the body and comes to the tissue spaces

where there is little or no oxygen, because the living cells are

constantly taking it up and using it, the oxygen which previously

diffused into the blood will diffuse out into the tissue spaces. The

only special arrangements that have to be provided are a sufficiently

great surface of exposure to the air, so that no matter how rapidly the

blood may be flowing it shall be able to take up all the oxygen it can

hold, and an arrangement for making sure that the blood can hold and

carry as much oxygen as the tissues need. The first of these

requirements is met by the special construction of the lung or gill; the

second by having present in the blood stream a chemical substance

(hemoglobin) which automatically takes up large quantities of oxygen and

so insures that sufficient shall be transported.

In principle the structure of a gill corresponds with that of a lung;

since we are particularly interested in the working of our own bodies we

shall content ourselves with describing only a lung. We saw in the

chapter on the Circulation that the pulmonary artery which leads away

from the right side of the heart breaks up into a system of capillaries.

These capillaries are thousands in number and they are spread out over

the whole lung surface. The lung itself consists of a hollow bag with

very thin and very elastic walls connecting with the throat by means of

the windpipe. In reality the bag is double, for the windpipe splits at

its lower end into two tubes, known as the chief bronchial tubes, and

these subdivide repeatedly until their fine terminals end in the elastic

lung sacs themselves. We spoke of the lung as a bag; in reality it is a

system of thousands upon thousands of separate tiny bags. The structure

is comparable to that of a bunch of grapes, the stem representing the

chief bronchial tube, the smaller stems the subdivisions and the

individual grapes the lung sacs proper. The advantage of this

arrangement is, of course, in the very large surface which it gives;

every one of the individual lung sacs has its wall filled with

capillaries and there are so many of the tiny individual sacs that the

total surface over which the blood is spread is measured in hundreds of

square feet. We cannot imagine any other arrangement by which so large a

surface of exposure could be packed away into a cavity the size of the

human chest.

Of course, we see immediately one serious defect of this arrangement of

the lung surface; every one of the individual sacs is full of air and so

the blood vessels which line its wall have exposure to air, but between

the individual lung sac and the outside atmosphere is, first, the very

tiny bronchial tube with which the sac connects, then the somewhat

larger one into which that opens, then a still larger one, and so on

until we come by way of the chief bronchial tube and windpipe up to the

throat, and so through the mouth or nose to the outside. It is quite

evident that this system of passages does not permit of a very free

movement of air. We must realize also that the blood which flows

through the walls of the lung sacs must constantly take up oxygen from

the air within the sacs, if it is to meet the needs of the body tissues.

Simple diffusion through the narrow bronchial tubes could not possibly

bring oxygen into the lung sacs fast enough to supply the requirements

of the blood flowing through their walls. The situation is met by active

lung ventilation; that is, by forcibly changing the air in the lungs at

frequent intervals. The way in which this is done is, as we all know, by

breathing. Breathing is nothing but a bellows movement of the chest by

which air is alternately expelled and allowed to enter. It does not

require a very active lung ventilation to keep the air in the lung sacs

sufficiently supplied with oxygen under conditions of bodily quiet. Our

ordinary breathing movements are gentle, less than a quart of air is

breathed in and out again in every breath, and we breathe only fifteen

or sixteen times a minute. Of course, when there is vigorous functional

metabolism, as in brisk muscular exercise, the oxidation processes in

the tissues go on at a very much more rapid rate, and correspondingly

larger amounts of oxygen must be carried by the blood to meet the

demand. Under these circumstances there is an improvement in lung

ventilation, the movements of the chest are greater and also happen more

times in a minute.

The act of breathing is carried on by ordinary skeletal muscles. This is

the only act connected with bodily maintenance of which this is true.

Our other “vital” organs are operated by means of smooth muscles. On

account of this difference we have a certain degree of control over our

movements of breathing. As we all know, we can hold the breath for a

short time without difficulty, or can breathe more quickly or more

deeply any time we choose. In this respect breathing differs strikingly

from either the heart action or the movements of the digestive organs,

over which we have no direct control at all. Our control of the muscles

of breathing is, however, rather limited; we cannot hold the breath

indefinitely. This means that the nervous mechanism which causes the

muscles to contract will work in spite of the efforts of our will to

prevent it. The actual machinery is very much like that which has

already been talked about in connection with other “vital processes.” We

have a “center” in the brain stem from which the nervous discharges

come. This center is located immediately adjoining the vasoconstrictor

center about which we learned in Chapter XIV. Because of the location of

these two important centers in a single very small space, the spot where

they are was named by a French physiologist more than a hundred years

ago “the vital knot”; the point of this is that death can be induced

more quickly and with less actual tissue destruction here than anywhere

else in the body.

The center which controls breathing has been named the “respiratory

center.” It discharges automatically about fifteen or sixteen times a

minute, causing the muscles of breathing to contract and so the bellows

action of the chest to be carried on. Like the other centers in the

brain stem this one can be acted upon by nervous disturbances passing

into the brain stem from the sense organs. Perhaps the best example of

this is the modification of breathing that comes as the result of a dash

of cold water on the skin. Most of us have noticed that we give a sort

of gasp upon stepping suddenly under a cold shower or plunging into

cold water. It may not have occurred to us that this gasp is entirely

involuntary, but we can easily prove that it is by trying to breathe

with perfect regularity at the moment of stepping under a cold shower.

We shall easily convince ourselves that this modification of the

breathing is something over which we have no control. It is, as a matter

of fact, an excellent example of a simple reflex. Coughing and sneezing

are other reflexes in which sensory irritation of some sort acts upon

the respiratory center modifying its discharges. In addition to these

reflex changes in breathing we have also the familiar effects of

muscular exercise. We know that after even moderate exercise the

breathing is quickened somewhat, and after vigorous exercise it becomes

very rapid and deep, and that after very severe exertion, particularly

in an untrained person, the puffing and blowing is not only pronounced

but even distressful. We shall see presently how muscular exercise

brings these changes about, but before doing so it will be necessary for

us to take up the movement of the gases between the lungs and the

tissues, by way of the blood; oxygen from lungs to tissues, carbon

dioxide from tissues to lungs.

By lung ventilation the air in the tiny individual lung sacs is kept

supplied with oxygen and also measurably free from carbon dioxide. From

this air there is a continuous diffusion of oxygen into the blood. The

first oxygen that diffuses in may dissolve in the blood liquid just as

oxygen will dissolve in any water to which it is exposed, but as this

goes on the hemoglobin of the red corpuscles begins to take up oxygen,

forming a chemical compound to which is given the name of

_oxyhemoglobin_. If there is enough of the gas present, every molecule

of hemoglobin will take up oxygen to its full capacity. The amount that

will dissolve directly in blood is so slight that to all intents and

purposes the ability of the blood to carry oxygen depends on the extent

to which hemoglobin can combine with it. It is important to emphasize

this, because it means that there is a definite limit to the amount of

oxygen that the blood can carry, a limit which is reached as soon as the

hemoglobin is saturated. Hemoglobin has so great a power of combining

with oxygen that the moderate lung ventilation which ordinary quiet

breathing gives suffices usually to saturate it. Now and then we

encounter statements which give the impression that there is a virtue in

deep breathing in improving the amount of oxygen which becomes available

for our tissues. As a matter of fact, this is not the case; ordinary

quiet breathing when the body is at rest saturates the blood with

oxygen, which means that it is carrying its full cargo, and evidently no

amount of deep breathing can make it do more than that. We should not be

understood as intimating that deep breathing is not a valuable exercise;

the point which we wish to emphasize is that its value does not lie in

affording an increased supply of oxygen.

Oxyhemoglobin is of a bright scarlet color; hemoglobin itself, not

combined with oxygen, is a very dark purplish color; partial

combinations are brighter and brighter as they contain more oxygen, so

that an expert can judge of the degree to which any specimen of

hemoglobin is combined with oxygen by noting its color in comparison

with fully saturated hemoglobin. The combination of hemoglobin with

oxygen takes place as the blood is passing through the capillaries of

the lungs; therefore the blood which leaves the lungs has the bright

scarlet color characteristic of oxyhemoglobin. This blood is called

arterial blood, the reason is that it is the kind that is found in the

arteries of the body in general. It happens that it makes its first

appearance in the pulmonary vein, by which it is conveyed from the lungs

to the left side of the heart, so that the expression arterial blood

does not mean anything in particular except to describe blood in which

the hemoglobin is saturated with oxygen. This blood is pumped out by the

left side of the heart to all the parts of the body; in its passage

through the capillaries it is in a region where there is active

utilization of oxygen by the living cells. These are steadily taking up

oxygen from the tissue fluids about them, so the blood in the

capillaries, which is carrying an abundant supply of oxygen, is brought

in contact with tissue fluids containing little or none, with only the

delicate wall of the capillaries between. Under these circumstances

rapid diffusion of oxygen from the blood into the tissue fluids takes

place and accompanying this there is a breakdown of oxyhemoglobin, so

that not only most of the oxygen which was dissolved in the blood passes

out, but also a considerable part of that which was formerly in

combination with hemoglobin. Under ordinary circumstances only from

one-fourth to one-third of the oxyhemoglobin decomposes during the rapid

passage of the blood through the capillaries; thus the blood that goes

on into the veins will still be carrying two-thirds or more of the total

oxygen cargo. The color of venous blood will be darker than that of

arterial blood, because it contains a good deal less oxyhemoglobin, but

it is nowhere near so dark as is blood in which all the oxyhemoglobin

has been decomposed. This is well recognized in melodramatic fiction

where the wounds of persons who have met death by strangulation are

described as oozing black blood. It is an actual fact that blood from

which all the oxygen has been withdrawn is so much darker than ordinary

venous blood that it gives the impression of being black, although it is

really a dark purple.

If we should be moved to inquire why so small a fraction of its whole

store of oxygen is given up by the blood to the tissues ordinarily, we

shall find the answer in remembering that the demand of the body for

oxygen is extremely variable; every increase in functional metabolism

means an increase in the amount of fuel that is oxidized and therefore

an increase in the amount of oxygen that is required. Actual

measurements have shown that in very vigorous muscular exercise the

oxygen consumption may be approximately ten times as great as in

complete rest. In order that this very greatly increased metabolism may

be carried on it must be possible for the blood to deliver approximately

ten times as much oxygen to the active tissues as it delivers to them

when quiet. There are just two ways in which this can be done; one is by

a more complete decomposition of the oxyhemoglobin by which all its

oxygen is set free; the other is by a more rapid movement of the blood.

It is by a combination of these two that the oxygen requirements of the

body in times of vigorous metabolism are taken care of. As has already

been said, the heart rate is just about doubled in vigorous exercise.

There has also been shown to be some increase in the amount of blood

that it pumps with every beat. The result is that more than twice as

much blood leaves the heart in a minute under these circumstances as in

time of rest. The oxyhemoglobin is also completely decomposed when the

tissues are active, and these two facts together are sufficient to

account for the great increase in the oxygen supply.

Hand in hand with the increased consumption of oxygen, there is of

course an increased production of carbon dioxide and of water, since the

oxidation of fuel substances produces these waste products. The cells

are always pouring both out into the tissue fluids, but to a very much

greater extent when they are actively functioning. We need make no

effort to keep track of the water, since it merely adds itself to the

water already present, and we shall consider later how the water

supplies of the body are handled. The carbon dioxide, however, must be

gotten rid of, and the mechanism for getting rid of it must work

efficiently, otherwise metabolism itself will be hampered, since it is a

familiar law of chemical action that if the products of an action are

allowed to accumulate they interfere with its further progress. The

method of getting rid of carbon dioxide is by simple diffusion from the

cells into the tissue fluids and from the tissue fluids into the blood.

Carbon dioxide is many times as soluble as oxygen, so that a great deal

more of it can be handled by merely dissolving. This is not sufficient,

however, to take care of all the carbon dioxide; the remainder must go

into chemical combination with some substances that are in the blood.

There is no single conspicuous material for carrying carbon dioxide like

the hemoglobin which transports the oxygen. There are, however, a number

of compounds in the blood which are able to combine with carbon dioxide,

among them the blood proteins of which much was made in Chapter XIII.

The carbon dioxide distributes itself among these various substances and

so is transported. It should be noted that the blood does not become

saturated with carbon dioxide as it does with oxygen. Arterial blood

ordinarily carries practically all the oxygen it is able to take up;

venous blood on the other hand probably never comes anywhere near being

as fully charged with carbon dioxide as it is able to be.

During the passage of the blood through the capillaries of the lungs an

outward diffusion of carbon dioxide into the air in the lung sacs is

going on simultaneously with the inward movement of oxygen from this air

into the blood. The diffusion is never so complete as to deprive the

blood of all its carbon dioxide; there is in fact only a little less of

it in arterial blood than in venous, although the diffusion is

sufficiently rapid so that as much carbon dioxide as is produced in the

whole body in a minute is passed out into the air of the lung sacs in

the same time. The effect of this outward diffusion is naturally to

increase the amount of carbon dioxide in the air of the lung sacs, and

if this increase is allowed to go on unhampered, there will presently be

so much carbon dioxide there as to stop further outward diffusion, and

so to put an end to the escape of carbon dioxide from the blood. This is

avoided by lung ventilation. Every time a breath is drawn some air that

is almost free from carbon dioxide enters the lung spaces to replace the

carbon dioxide-laden air that was expelled at the previous exhalation.

The description of gas transportation that we have just given opens the

way for an account of the control of breathing. From what has just been

said it should be clear that the amount of carbon dioxide in the blood

corresponds closely with the amount that is in the air of the lung sacs.

As the percentage of carbon dioxide in this air goes up, outward

diffusion becomes less free, and so the amount of carbon dioxide in the

blood will have to increase. The tissues are all the time producing and

pouring out carbon dioxide, and so there will be a steady increase in

the amount of carbon dioxide in the blood. This applies to the arterial

blood as well as to the venous, since, as we saw a moment ago, there is

nearly as much carbon dioxide in the former as in the latter. This is

the fact which is utilized in the body for operating the breathing

machinery. The respiratory center in the brain stem is susceptible to

carbon dioxide; the more of this gas there is in the blood, the more

tendency there will be for the center to discharge. There is a certain

level of carbon dioxide below which it is entirely inactive, but when

this level is passed nervous discharges begin and become more and more

powerful as the amount of carbon dioxide in the blood goes up. Now we

can see what makes us breathe. Let us imagine that there is not very

much carbon dioxide in our blood, but that the tissues are constantly

producing it and giving it off. Since we are supposing the amount is not

enough to excite the respiratory center, there will be no movements of

breathing. There will be a steady increase of the amount of carbon

dioxide in the blood and at the same time a corresponding increase of

the amount of carbon dioxide in the air sacs of the lungs; presently

there will be enough in the blood to arouse the respiratory center to

discharge. This will cause a breath to be drawn; the effect of this will

be to sweep out much of the accumulated carbon dioxide from the lung

sacs; this in turn enables more rapid diffusion of carbon dioxide from

the blood to occur, and so the amount of it in the blood may fall below

the level at which the respiratory center is made active. In a moment,

of course, the continued outpouring of carbon dioxide from the tissues

will raise the level again to the point of exciting the respiratory

center, and so we will have a rhythmically recurring discharge of that

center causing a rhythmic drawing of breath.

According to the account just given the activity of the respiratory

center is determined exclusively by the carbon dioxide in the blood; it

could be so regulated, but, as a matter of fact, in all higher animals,

including man, the carbon dioxide control of the respiratory center is

interwoven with a complicated nervous control whose effect is to make us

breathe more often in a minute, but to make the individual breaths

shallower than they would be if the control of breathing were

exclusively by means of carbon dioxide. The net result in lung

ventilation is exactly the same, but the rapid shallow breaths are

advantageous in that they avoid large fluctuations in the amount of

carbon dioxide in the blood, while they do serve fully to provide

sufficient oxygen.

The rate and vigor of breathing are ordinarily adjusted automatically to

the amount of carbon dioxide in the blood stream, but, as we know, we

can, of our own will, breathe quite differently. Let us see what will

happen if while we are sitting quietly we begin to breathe deeply and

rapidly, overventilating the lungs. So far as oxygen is concerned, this

will make no difference at all, since, as we have already seen, the

ordinary automatic breathing keeps the blood charged with all the oxygen

it can hold. What overventilation does is to sweep out the carbon

dioxide from the lung sacs more rapidly than usual and this permits of a

correspondingly more rapid outward diffusion of carbon dioxide from the

blood. The result will be that carbon dioxide will leave the blood

faster than it is being poured into it from the tissues, and so the

total amount of the gas in the body will be cut down. The first effect

of this we would expect to be the removal of the automatic stimulation

of the respiratory center, so that, after a period of excessive

breathing, one would not at once resume breathing spontaneously. This,

as a matter of fact, is the case; anyone can easily prove on himself, by

breathing deeply and rapidly for a minute or two, that the automatic

control of breathing is temporarily suspended immediately after. It

follows naturally that one can hold the breath a good deal longer if the

lungs are overventilated for a short time just before the attempt is

made. This also can be easily proved. Prolonged overventilation of the

lungs has, likewise, a number of other effects, all of which are due to

cutting down the total amount of carbon dioxide in the body. The most

conspicuous is a feeling of dizziness or light-headedness that comes on.

If pushed to excess, there is a very definite feeling as though one were

about to soar away into space, and this is followed by unconsciousness.

Certain religious cults in India have interpreted this sensation

resulting from deep breathing as an actual severance of soul from body,

and maintain that during the time of unconsciousness the spirit really

floats freely in space. Without venturing any statement as to the

relation between the soul and the body during either consciousness or

unconsciousness, we would point out that these bodily sensations are

definitely due to the very simple fact that there is less carbon dioxide

in the blood than is normal on account of the overventilation of the

lungs, and just as soon as the metabolism that goes on all of the time

in the tissues pours out enough carbon dioxide to bring the amount up to

normal, consciousness will return and the ordinary condition of affairs

will be resumed.

Although this finishes what we have to say about the movements of gases

into and out of the body, the general subject cannot be completed

without a word concerning the conditions that should be maintained in

the air immediately surrounding us. This makes up the topic of

_ventilation_. We all know that some air is much more fit to breathe

than other; until very recently, however, our ideas as to the conditions

which make air fit or unfit to breathe have been hazy or entirely

erroneous. Fortunately, of late years, the subject of ventilation has

been actively investigated and we now have a satisfactory knowledge of

its laws.

There are, of course, two things that must be true of any air that is to

be breathed; these are that it must contain enough oxygen and must not

contain too much carbon dioxide. So far as the oxygen supply is

concerned we may state that only with the greatest difficulty are

conditions reached in which there is not enough oxygen in the air. As we

all know, the air becomes thinner the higher we go above the surface of

the earth; both mountain climbers and aviators have attained heights at

which the amount of oxygen in the air is only about one-third that of

ordinary air and have been able to obtain enough oxygen for their

bodily needs even under those extreme conditions. It is quite evident

that a room could scarcely be so poorly ventilated as to bring the

oxygen supply down below this figure, so that no attention need be paid

to the oxygen supply in working out practical methods of ventilation.

Air which contains carbon dioxide to the extent of four per cent could

not be breathed because the carbon dioxide being produced in the body

would not diffuse out fast enough into an atmosphere containing that

amount of carbon dioxide to keep the body alive. This again is a

percentage of carbon dioxide that is practically never reached. Probably

the most famous case in history of death from poor ventilation is the

“Black Hole of Calcutta,” a dungeon room about twenty feet square with

only two small windows, in which one hundred and fifty British soldiers

were imprisoned over one night; all but twenty-three of these died, but

it is doubtful whether their death was actually due either to deficiency

of oxygen or to excess of carbon dioxide. This is because there were

enough other factors which would make the air unbreathable to bring on

death before either of these could come into play. The modern science of

ventilation concerns itself with these other factors; chief among them

is the factor of moisture. As we shall see in the next chapter our

bodies are constantly giving off from the lungs and by evaporation from

the sweat glands water vapor into the air. This causes the humidity to

go up rapidly in rooms where people are congregated, and particularly so

where there are many people present. Also everyone gives off a great

deal of heat. We now know that the feeling of closeness which we ascribe

to a poorly ventilated room is due to the combination of warmth and

moisture. We also know that the discomfort which comes from being in

such rooms is due to the same causes. Actual vitiation of the air is

much less disagreeable than is the accumulation of heat and moisture. In

theory, of course, the best ventilation is secured by keeping rooms

flooded with outdoor air. In practice, however, this does not always

work out; for example, in many cities the air is so laden with dust and

smoke as to be bad for everybody and even dangerous for sick people.

Before such air is breathed the smoke and dirt should be gotten out of

it. This is done sometimes by forcing it through fine mesh cloth bags,

or the most modern scheme is by passing it through a thin screen of

water and so washing the dirt and smoke out of it. The second practical

difficulty with flooding rooms with outside air is the expense in cold

weather of warming the large volumes that would be required. For this

reason it has been found feasible in churches and public halls that are

occupied only occasionally to use the same air over and over by keeping

down the temperature and moisture. Of course, this cuts down very

greatly the expense of heating.

There is one other source of harmful effect from bad air besides the

high humidity and undue warmth; this is the presence in it of ammonia

and other poisonous compounds that are given off from the bodies of

people. It used to be believed that organic poisons were exhaled from

the lungs with every breath, but we now know that the amount of these,

if any are present, is too small to be important in comparison with the

very much larger amounts that come off from the evaporating sweat, from

decaying teeth, and from the digestive tract; there is no doubt that in

any assemblage of people the air will be vitiated by organic poisons

from these sources. The more cleanly the individuals are, the less will

be the contamination. It is generally believed, although perhaps not

absolutely proven, that the bad health found in sweatshops and crowded

slums generally is due largely to chronic poisoning from the constant

breathing of effluvia from the unwashed bodies and clothing of the

inhabitants. The obvious remedy is insistence upon personal cleanliness,

although this does not lessen the desirability of breathing as pure air

as can be gotten. The point to be emphasized is that where personal

cleanliness prevails, the closeness of rooms is chiefly due to excessive

moisture ordinarily accompanied by too high a temperature. Ventilation

measures should be carried out with this in mind.

CHAPTER XVII

THE SERVICE OF REMOVAL OF WASTE

The metabolism that goes on in all our body cells results in the

formation of waste products. The chief of these, carbon dioxide, is

taken care of by means which were described in the last chapter. Besides

this gaseous substance there are produced a number of compounds which

are soluble in water and which are poured out from the cells into the

tissue spaces surrounding them and which pass thence into the blood

stream. The special places in the body wherein these substances leave

the blood stream make up the apparatus for the removal of waste. This

apparatus must be able to take out of the blood stream all the waste

products of cell metabolism except carbon dioxide, also any materials

which may have been absorbed into the blood stream from the digestive

tract, but not used by the tissues, and finally all the accumulations of

water. The discharge of water makes up a topic by itself which will be

considered after a word has been said about the apparatus by which the

removal of wastes is carried on. Sharing in this function we have first

the kidneys, second the liver, and third the sweat glands.

The kidneys are the most important of the organs for removing waste.

They are a pair of bean-shaped bodies lying in the small of the back.

Each

[Illustration: DIAGRAM OF KIDNEY

(From Martin’s “Human Body”)]

one is made up of thousands of tiny tubes; each of these tubes starts as

a little sac in which is a knot of blood vessels which is really a tiny

filter, filtering out from the blood water and the inorganic salts that

are dissolved in it.

All the other materials that are in the blood are held back by the

filter, so that nothing escapes from it in these places except water and

salts. Further along each of the tiny tubes is a section which is

surrounded by a network of fine blood vessels where two things take

place; the first of these is the taking out from the blood of the

various kinds of waste products; the second is the absorbing back into

the blood of part of the water which filtered out through the knot at

the tip of the tube. Beyond the point where these things happen the tube

does nothing except to convey the finished kidney secretion, or urine,

down to the outlet where opens a very large tube, the _ureter_, by which

the kidney secretion is carried down to the bladder. The formation of

urine is made up then of three stages; first the filtration of water and

salts out of the blood, second the escape from the blood into the kidney

tube of the various waste products of cell activity, and third, the

reabsorption into the blood of part of the water. It is estimated that

about four times as much water filters out from the thousands of tiny

filters at the tips of the kidney tubes as comes out at their ends to be

carried down to the bladder. We do not understand exactly why this

should happen; one result of it is to make the urine contain about four

times as much salt as the blood; the water that filters out at the

beginning of the kidney tube carries with it just the amount of salt

that is dissolved; after the water is reabsorbed the salt is left behind

in the kidney tube, so that all the salt that filters out stays in the

urine, but only about a quarter of the water. The discharge of waste

products is easier to understand than the way in which the water is

handled; all organic substances which happen to be in the blood and

which do not belong there pass out from the blood during its passage

through the fine network surrounding the kidney tubes. Of course we do

not know just how this is carried on; there must be some special

features about the protoplasm in this part of the kidney to cause these

substances to pass out from the blood here and nowhere else. Not only

waste products of cellular activity, but also all or nearly all the

organic accessories of the diet and all drugs taken as medicine leave

the blood in this region.

An interesting thing about the removal of water from the blood by the

kidneys is the relation it shows to the activity of the sweat glands.

The total amount of water gotten rid of in any day is the amount taken

in in drinking during the same time. The amount we drink is controlled

chiefly by the sense of thirst, assuming that we are in a place where

water can be had. The amount of water that passes out in the form of

sweat is not under our control; in a later chapter we shall see how the

formation of sweat is regulated. There is also a constant and fairly

steady loss of water from the lungs, since every breath that we exhale

carries with it all the water vapor which it can hold. What the kidneys

have to do is to get rid of the water that is not passed out either from

the sweat glands or the lungs. The amount of urine that is formed in a

day is, therefore, extremely variable; on days that are warm and when a

great deal of exercise is taken there will be so very much sweat formed

that in spite of copious drinking comparatively little water will be

discharged from the kidneys. On the other hand on cold days in which not

much exercise is taken there will be a large amount of urine formed. It

is interesting to note that some substances like asparagus, and to a

less extent coffee, cause a large increase in the amount of urine. We do

not know certainly whether these act by increasing filtration from the

tips of the kidney tubes or by diminishing the reabsorption in the part

farther along, but so far as our information goes either or both may

occur.

The kidney is not the only organ by which wastes are removed; the liver

has a part in this as well. The action of the liver is not, however,

simply to assist the kidney; there are certain special substances which

are taken care of by the liver and which are never gotten rid of through

the kidneys. The first of these are the broken-down blood pigments. We

have already seen that after the red corpuscles become worn out they are

decomposed in the spleen and the hemoglobin carried to the liver where

the iron and other usable parts are saved and the useless remainder is

passed on into the bile to be conveyed out of the body. Bile is green in

vegetable-eating animals and golden brown in flesh-eating; the

difference depends on the so-called bile pigments which are the waste

remains of hemoglobin. The difference in color is the result of a slight

chemical difference in the pigments which in turn depends on the diet,

so that in man the bile is either green or golden brown according to

whether the diet is vegetarian or chiefly of flesh. The liver also takes

out from the blood a waste substance which has the formidable name of

_cholesterin_. The interesting thing about this substance is that unless

the bile is altogether normal chemically it will not be dissolved and so

will be thrown down in the form of little grains or lumps. So long as

these grains are small they can pass down the bile duct to the small

intestine and be carried out of the body, but after they become bunched

together into lumps it is difficult or even impossible for them to be

carried down the narrow bile duct. These lumps make up what are commonly

called gallstones. The acute pain known as gallstone colic is due to the

stretching of the bile duct by the passage of one or more of these

stones down it. At the present time no way is known to prevent the

formation of gallstones in persons in whom it starts, nor of softening

them after they have once been formed; the only relief is through

surgery, which, fortunately, is entirely adequate, so that thousands of

persons are living in great comfort to-day who would be dead or living

in acute suffering if the surgeon had not been able to remove the

accumulation of gallstones from the passages in their livers.

The sweat glands, as we have already seen, make up part of the service

of the removal of waste in that they help to carry away water from the

body. To a very slight extent, but much slighter than is commonly

believed, they get rid also of waste products of cell activity; for

example ammonia, which is one such product, can frequently be detected

in the sweat by its odor; practically, however, the importance of sweat

glands is not in their discharge of waste organic substances, but only

in their discharge of water. In this they are playing a very important

part in the regulation of body temperature, so that the further account

of the sweat glands as well as of the skin of which they form a part is

left for the chapter in which the regulation of temperature is

described.

CHAPTER XVIII

MORE ABOUT THE USE OF FOOD BY THE BODY

We have now brought our description of the various things that happen in

our bodies up to the point where we may begin to make some kind of

summary of them; particularly in respect to what goes on within our

individual cells. We have seen how some of our cells are muscle cells by

which we make motions; others are the cells of sense organs by which we

get the necessary information for guiding our activities; still others

are in the nervous system and carry on the adjustments by which we act

in accordance with the indications of our surroundings and also of our

past experience; finally we have the cells which manufacture materials,

as in the various digestive glands or in the glands which make hormones.

We have also looked into the kind of materials which these cells

require, where they come from, and how they are prepared in the

digestive tract, and we have seen how these prepared materials and

oxygen are conveyed to the different parts of the body where the cells

can get them and finally how the waste products which all these cells

give off are gotten rid of. We are now to turn to the process of

metabolism itself as it takes place within the cells. In Chapters III

and IV we looked into the use of food for power development and for

repairing the wastage of protoplasm, as well as for the making of new

protoplasm in growth. We have seen also that the body contains much

nonliving material, as in the bones and teeth, which must come from the

food and which must be put in place as the result of metabolism on the

part of living cells.

The first thing which we wish to take up here is the use of protein in

the repair of protoplasmic wastage and in growth. We saw in Chapter III

that protein is manufactured originally by the living cells of green

plants. We have also seen that protein is the only material that can

repair protoplasmic wastage or that can make new protoplasm. We have

omitted to say thus far that the most important place from which we can

obtain protein is from living protoplasm. It is true that most seeds

store up within themselves nonliving protein to be used by the young

sprout as it forms, and seeds make up a large part of our diet; but

except in grain and other seed foods we obtain our supplies of protein

by eating protoplasm. This protein is to be used by us for repairing our

protoplasmic wastage or, in parts of us that are growing, for making new

protoplasm. We have already seen that the protein which we eat must go

through a process of digestion before we can use it for these purposes,

and our present task is to explain just why this is necessary and to

show how the protein is actually used in our bodies.

A thing about protein which fits it specially to be the chief material

of living protoplasm is that it is very much the same sort of substance

wherever we find it and yet can differ enough to account for the

differences that exist among animals and plants. In spite of the fact

that protoplasm analyzes about the same, no matter where it comes from,

we are bound to believe that the difference between a dog and an oak

tree is at bottom a chemical difference; they are unlike because the

protoplasm of one is not the same substance chemically as the protoplasm

of the other, and the difference is a difference in the proteins. We can

come even nearer home than that and say that the differences between the

races of mankind are probably chemical differences between their

proteins. Human protein is undoubtedly different from the protein of

beef or pork or mutton. What we have then is a substance which can be at

the same time similar and different; also since we can make human

protein out of beef protein or any other kind which we happen to eat it

must be fairly easy to change one into the other.

Protein is about the most complex substance that we know anything about;

it is made up of a number of organic acids combined chemically. These

organic acids all contain nitrogen, which puts them into a class to

which is given the name of _amino acids_. To the chemist the name amino

acid shows a certain kind of chemical formation; to us it need mean no

more than an organic acid which contains nitrogen. The proteins which

are in our bodies are as complex as any that exist and some of them are

made up of as many as eighteen different amino acids. The same eighteen

acids are present in the proteins of all the higher animals. When we eat

lean beef or pork we get exactly the same eighteen amino acids that are

in our own proteins, but not put together in precisely the same way.

What we have to do with these proteins is to break them up into the

amino acids of which they are composed and then put these together again

in the combination which makes up human protein. The breaking up of the

proteins is carried on in the digestive organs; we have said a good deal

about it in Chapter XV; what were called in that chapter digestion

products of proteins we now see are amino acids. These are taken up into

the blood stream, carried around the body to the tissue fluids and by

the living cells taken up to be built into human protein. Exactly the

same thing happens to any proteins that we eat.

One of the great differences between animal protein and plant protein is

that the percentages of the different amino acids are very different.

Some of the amino acids that make up a large proportion of animal

protein are very scantily represented in plant protein. To make human

protein we must not only have all the amino acid ingredients, but we

must also have enough of every one. On a purely vegetarian diet to get

enough of those amino acids which are scantily present in plants we have

to eat a large surplus of those amino acids which are specially

abundant. In this respect plants as providers of amino acids are less

economical than animals, because animal proteins have more nearly the

same proportions of the different amino acids as do our own human

proteins. Of course, as we will see at once, in theory cannibalism is

the most economical way of getting protein; if we were to eat human

protein we would have exactly the correct proportion of the different

amino acids and so could get along with a minimum amount. This is not to

be interpreted as an argument for the practice of cannibalism among

human beings, although we may as well face the fact that there is no

physiological or dietary reason for avoiding the practice. In some of

the lower animals, particularly in rats, cannibalism is a regular part

of the life habit. Rats do not have the instinct of storing up food

supplies as do squirrels and some other kinds of animals. When food is

abundant they multiply very rapidly and then when food becomes scarce

the stronger feed upon the weaker. It is largely for this reason that

the unsanitary and extremely expensive rat nuisance is so hard to abate.

The amino acids that are circulating in the blood stream after every

meal are primarily to be used for repairing protoplasmic wastage; also

they serve for the manufacture of new protoplasm, provided growth is

going on. In theory an adult who is through with all his growth except

in the skin and one or two other minor tissues should be able to get

along with just the amount of protein which will make good his

protoplasmic wastage. Since protein is an expensive food and likely to

be hard to get in times of scarcity, the question of how much protein

should be eaten is of great practical importance. There are several ways

of studying the problem; one is by the actual study of diets to find out

how much protein people do habitually eat; another is by finding out how

much the daily protoplasmic wastage amounts to. If no more protein is

being eaten than is necessary for the protoplasmic wastage, these two

figures should be about the same. The way of finding out how much

protoplasmic wastage there is is to go on a diet which contains abundant

starch and fat for energy supplies, but no protein or amino acids. When

one is on that kind of diet he knows that he will not have to burn up

any of his own tissues to supply him with the energy for his metabolism;

whatever breakdown of protoplasm occurs on such a diet is the natural

wastage of the body and not the result of using tissues for fuel, as is

the case in complete starvation. It is fairly easy to keep track of the

decomposition of protein in the body, because protein contains nitrogen

and the nitrogen is given off almost wholly in the waste products that

are passed out from the kidneys. By collecting the urine and analyzing

it for nitrogen the amount of protoplasmic breakdown in the body can be

determined, provided no nitrogen-containing compounds were taken in with

the food. Otherwise, of course, one could not be sure that nitrogen

appearing in the urine had actually come from the wastage of protoplasm.

The fact is that when an average-sized human being goes on a diet which

contains no protein, but is ample in other respects, he loses daily from

his body about an ounce of protein. This is proven by the occurrence in

the urine of an amount of nitrogen which stands for that much protein.

The person may be gaining or losing weight meanwhile; if his consumption

of fats, starches, and sugars is excessive, he may deposit some fat, in

which case he might gain weight in spite of the loss of some of his

actual living protoplasm. Usually though, in experiments of this kind,

there is a steady loss of weight made up of the ounce of protein and of

three or four ounces of water. We have to remember that living

protoplasm is three-fourths or more water, so that whenever any

protoplasm wastes away, some water will be lost as well as protein. It

is a very interesting fact that this protoplasmic wastage goes on

steadily at the rate of about an ounce a day whether the body is active

or inactive; this means that the wastage is a matter of the basic

metabolism and not of the functional metabolism. The former goes on all

the time day and night, in sleep and in waking, and in connection with

it the living protoplasm shows this small amount of wastage. Functional

metabolism does not, at least under ordinary conditions, increase the

amount. Speaking of the body as though it were a machine, we would say

that it rusts out just as fast as it wears out. This is one of the

features in which the living machine differs from most mechanical

devices of human manufacture.

Although the loss of protein due to wastage is only about an ounce a

day, nobody can get along on a diet which contains no more than that

amount. Between three and four ounces of protein is the average daily

consumption of adults in this country. We should not forget that our

diet consists of meat, bread, vegetables, fruits, etc., which are

mixtures of proteins with the other food materials and with a large

percentage of water, so that in order to get three or four ounces of

protein we have to eat four or five times that weight of ordinary

foodstuffs. There has been much debate as to whether it is necessary or

even desirable for adults to eat three or four times as much protein as

the body requires for making up its wastage. The decision will have to

rest in part on what use the body makes of the surplus. Since from time

immemorial human beings have habitually eaten every day this large

surplus, it is evident that they have been wasting enormous amounts of

good food or else that some use is made of it even though it does not

serve its purpose of repairing the body waste. The surplus materials are

present in the body in the form of amino acids, since what the cells do

in repairing their wastage is to take up from the whole quantity of

amino acids in the tissue fluids as much as they require for making good

their loss. The mixture of amino acids that is left over will make

perfectly good fuel provided the nitrogen that is in it is gotten rid

of, and this is what happens in the body. All the amino acids in excess

of the amount needed for restoring the tissues are decomposed in such a

way that the nitrogen is abstracted in the form of ammonia and the

substance that is left, which is a starchlike compound, joins with the

other starch products and fats to be burned in the body as a source of

energy. We do not know certainly which tissues have the power of

decomposing the surplus amino acids. At the present time it is believed

that all or nearly all of them can do it, so that as they take up from

the tissue fluids the particular amino acids which they need for making

good their own wastage they take up also the surplus which they

decompose, utilizing the starchlike part for fuel and turning the

ammonia back into the body fluid as a waste product.

Ammonia is a very poisonous substance and it quickly poisons the body if

allowed to accumulate in the tissue fluids. This is prevented by the

action of the liver in changing the poisonous ammonia into a harmless

substance known as urea. This urea is carried by the blood stream from

the liver to the kidneys where it is passed out to become the chief

organic substance in urine. The more protein one eats the more surplus

amino acids will there be, and so the more urea will be formed and

passed out of the body. Flesh-eating animals and men (Eskimos for an

example of the latter) eat a very large surplus of proteins, the fuel

for their metabolism being furnished almost altogether either from the

usable remains of the decomposed amino acids or the fats that were in

the flesh they ate. Some people have been inclined to believe the

production of so much ammonia and its subsequent conversion into

correspondingly large amounts of urea to be injurious. As a matter of

fact, there is no particular reason for thinking this to be the case; it

is part of the duty of the liver to change all the ammonia that comes to

it to urea and of the kidneys to pass out all the urea that comes to it;

so long as these organs are healthy they are able to fulfill these

duties effectively, so this does not seem to be a good reason for

cutting down the percentage of meat in the diet. It is generally

believed that meat has special effects upon the nervous system, such as

to incite to cruelty and bloodthirstiness. There is no real scientific

proof as to whether this is true or not. The scientific fact is that man

is fitted for a mixed diet, neither exclusively of flesh nor exclusively

vegetarian. He has lived for thousands of years on that kind of diet and

can apparently go on for thousands of years more. We need to remember

that the various dietary fads which come into great prominence from time

to time are rarely based on a well-established scientific foundation nor

have any of them any long experience back of them. On the other hand,

the common mixed diet which all of us eat in accordance with custom and

the dictates of our appetites has the sanction of thousands of years of

successful maintenance of the human race. It is quite true that one can

get along on almost any kind of a diet provided it contains enough

protein to make good the daily body wastage and enough fuel material to

provide for the demands of metabolism. Anyone who is disposed to adopt

for himself a dietary fad will rarely suffer seriously from it; on the

other hand, those who prefer to eat as our fore-fathers have eaten need

not feel conscience-stricken because there is agitation against the

commonly accepted diet.

While on the topic of diet, a word should be said about cooking. In a

previous chapter the advantage of good cookery as an aid to digestion

was emphasized. We would add here merely the comment that in defending

the ancestral diet we do not intend to imply that their cookery was

always what it should have been. Over most of America there has

prevailed from pioneer days a habit of frying food in preference to

other means of cooking it. Our hardy pioneer ancestors throve on fried

meats; an outdoor life of muscular toil makes almost any kind of cookery

both acceptable and digestible. As labor-saving machines tend more and

more to diminish the amount of muscular labor that most of us do, we

find it harder and harder to maintain good digestion on fried foods. The

objection to frying is simple; fats are the hardest of all foods to

digest and fried foods are smeared all over with fat. It is only logical

to expect fried foods to be harder to digest than other kinds; it is

undeniable that the flavor of many fried foods is so agreeable that we

would be unwilling to omit them from the diet altogether. What is realty

objectionable is the practice of smearing all the food with fat in the

process of cooking it.

We have finished what we have to say about the use of food for the

repair of bodily wastage. While we are on the topic, a few words about

the use of food in growth will come in well, since the growth process is

closely related to the process of tissue repair. The chief difference

between them is that the process of growth comes to an end in all but a

few of our body tissues as soon as we become adults. The tissues which

go on growing are the layers of the skin just under the surface, the

reproductive tissues, and the blood-corpuscle-forming tissues.

Connective tissue has the power to grow at any time during life,

although it does not actually keep on growing as does the skin. Whenever

an injury is suffered which actually destroys muscular tissue or the

deep layers of the skin, there is no growth of new tissue to take the

place of either. Repair is made by a growth of connective tissue to fill

up the space. The result is the formation of a scar. If the edges of the

injury can be brought together skillfully enough, the outer layers of

the skin which do have the power of growth may bridge across the space

so that no scar will result.

A second thing to be mentioned about growth is a discovery which has

attracted a great deal of attention of late years; we will realize of

course that the chief thing in the making of new protoplasm is the

building together of protein out of amino acids. It is evident that for

the manufacture of new protein all the eighteen amino acids must be

present in sufficient proportion to give enough of each. There are some

plant proteins which lack one or two of the amino acids that are present

in human proteins and when a growing animal goes on a diet in which

these are the only proteins present it at once stops growing. Most of

the experiments proving this have been done on white rats, and it has

been found possible to keep a rat for more than a year at the size it

was when only a few weeks old simply by feeding it proteins in which one

or two amino acids were lacking. The fact that the animal lived during

this time proves that his protoplasmic wastage was made good and

therefore that there are proteins which can replace the body wastage but

cannot manufacture new protein. There is only one conclusion to be drawn

from this, namely that the daily wastage which the protein suffers does

not include all of it; some amino acids when once built in are there for

good; others, on the other hand, are constantly being lost in the

process of wastage, and these are the ones which must be replaced. There

is a common protein, gelatin, which is used a great deal for food, but

which by itself will not serve either for repair or growth because it

not only lacks some of the amino acids that are in living protoplasm,

but also some that are lost in the process of wastage. Gelatin is useful

as a food, therefore, only in combination with other proteins that

contain the necessary amino acids; or after the nitrogen has been taken

out, it becomes a good fuel.

One interesting question that was settled by the experiments described

above was whether the ability to grow is exclusively a matter of youth;

as we know, under ordinary conditions only young animals grow. When they

reach a certain age, they become mature and thereafter no increase in

size takes place. In the case of these white rats which were kept for

more than a year at the size of partially grown rats it was found that

as soon as their diet was changed to one in which all the necessary

amino acids were present they would begin at once to grow and grow to

full size, when they would stop growing just as they would have when

young. This proves that growth is not a matter of age, but is a matter

of achieving a certain size, and is controlled by factors which we do

not at present understand. An animal that has not been able to attain

[Illustration: Photo, Keystone View Co.

THE LUNCH HOUR IN A FRENCH SCHOOL WHERE EXERCISE, OUT-OF-DOOR LIFE,

SUNSHINE AND FRESH AIR ARE CONSIDERED ESSENTIAL]

[Illustration: Copyright, Paul Thompson

THE DIET KITCHEN ON A U. S. HOSPITAL SHIP]