The so-called basement membrane upon which the epithelial cells rest

must not be regarded as a membrane in the physical sense. Rather is

it a basket-work which supports the cells, without in any degree

limiting their power of disgorging into the lymph-spaces of the villi

the substances which they have absorbed. Within the villus, connective

tissue forms a sponge-work, the spaces of which are filled with lymph,

in which considerable number of leucocytes roam, on the look-out, no

doubt, for any germs which may make their way between the epithelial

cells. In the centre of the villus is a lymphatic radicle—_i.e._, a

fusiform cul-de-sac—which is the dilated end of a lymph-vessel. It,

like all other lymph-vessels, is walled by flattened endothelial

scales. It communicates with the lymph-plexus beneath the mucous

membrane, which, again, communicates with a coarser plexus outside the

muscular coat. From the peri-intestinal plexus vessels lying in the

mesentery converge to the receptaculum chyli, the bulbous commencement

of the thoracic duct, which lies at the back of the abdomen in front

of the bodies of the vertebræ. The thoracic duct runs up the front

of the vertebral column, through the thorax, and then hooks over to

pour the fluid which it conveys into the great veins shortly before

they join the heart. After a meal containing fat the fluid in the

lymphatic vessels of the mesentery, the lacteals, has, as already

stated (p. 43), the appearance of milk. The fat absorbed by the

epithelium covering a villus is passed on into its lymph-space. From

this into the central lacteal receptacle, thence to the submucous

and peri-intestinal plexuses, the lacteal vessels of the mesentery,

the thoracic duct. Absorbed fat does not pass through the liver, but

is carried into the heart; thence through the lungs, and back to the

heart, which pumps it to all parts of the body. In addition to lacteal

radicle; the villus contains long capillary bloodvessels, and the

arteriole and venule in which they commence and end. These traverse

the lymph-spaces of the connective tissue, which contains, not only

the fat which the epithelial cells have passed into it, but the other

products of digestion also. None of the fat traverses the walls of the

bloodvessels; but the other products diffuse from the lymph, through

the walls of the vessels, into the blood. Many nerve-fibres are found

in the core of the villus on their way to epithelial cells, or to one

or two plain muscle-fibres which are disposed in the direction of its

long axis. For each villus is a little pump. By the contraction of the

muscle-fibres it is shortened, and the fluid in its lacteal radicle is

forced into the submucous vessels.

Two problems have to be considered: First, in what form and by what

mechanism are the several kinds of food absorbed? Secondly, what

becomes of them after they have been absorbed?

Clearly, the epithelial cell is the absorbing mechanism. It is not

a membrane governed by the laws which regulate diffusion of fluids

through membranes, but a living cell. There is hardly any limit to

its power of selecting the food which it ingests. It could, and very

possibly it does, ingest albumin and fats as such. Still, the elaborate

provision which is made for converting albumin into diffusible peptone,

and cane-sugar and maltose into easily diffusible dextrose, suggests

that substances which will pass through membranes are more readily

absorbed than substances which will not. We are justified in looking

upon absorption as a physical problem up to a certain point. But we

must not dwell too much on the physical aspects of the problem. If the

absorption of food were merely a process of diffusion, an enormous

quantity of water would be required to carry the diffusible products

of digestion into the villi. The passage of the foods is aided by the

selective activity of the epithelial cells. Peptonization greatly

facilitates the work of the epithelial cells, but it is not a condition

essential to absorption, so far as soluble proteins are concerned. It

is, however, essential that the proteins should be presented to the

epithelial cells in a soluble form. They could do nothing with the

solid fibres of meat, however much they might have been disintegrated

by mastication and by the action of hydrochloric acid. It is only

after digestion by pepsin and by trypsin that all the proteins of food

are brought into solution. Digestion is needed to reduce them to a

condition in which the epithelial cells can take them up.

Much thought has been devoted to the question of the form in which

fat is absorbed. Fat in the chemical sense—a pure fat, that is to

say—is a compound of a fatty acid and glycerin. Suet, lard, butter,

vegetable oils, etc., are mixtures of several fats. All consist of

glycerin united with fatty acids. The acids are stearic acid, palmitic

acid, oleic acid, and others of less importance. Fats are insoluble

in water; so also are the fatty acids. A fatty acid combined with an

alkali (in place of glycerin) is a soap. Soaps are soluble in water.

If milk is examined under the microscope, it is found to contain

droplets of fat, varying in size, but all minute. The larger droplets

tend to rise to the surface as cream, but the smaller droplets do

not run together. If milk from which the cream has been skimmed

is sterilized, it retains its normal appearance for an indefinite

time. Its fat remains in droplets. In technical language, milk is an

emulsion. Theoretically oil and water would make an emulsion, if the

droplets of oil were rendered sufficiently minute. Such a condition

has been almost obtained by agitating oil and water with powdered

glass. But the more viscous the medium through which oil globules

are distributed, the greater is the resistance to their fusion. If

oil which has become rancid—in which a certain quantity of fatty

acid has been liberated from the glycerin with which, in a neutral

fat, it is combined—is shaken with water containing carbonate of

soda, an emulsion is easily formed. The carbonate of soda and the

fatty acids form soaps. A solution of soap is sufficiently viscous to

keep the droplets of oil apart. Emulsification of fats occurs in the

intestine. It might be assumed that the epithelial cells ingest fat

in this finely divided state. But it must be remembered that, however

minute the droplets, they are enormously large as compared with the

molecules of peptones and sugar which the epithelial cells absorb.

It is unlikely that fat is absorbed in a manner so widely different

from that in which other foods enter the epithelial cells. Nor is

it necessary to make any such assumption. Pancreatic juice contains

a ferment which rapidly splits fats into their constituent fatty

acids and glycerin. In the presence of an alkali the fatty acids are

converted into soaps. In this soluble condition of soap and glycerin

the fats are probably absorbed. As soon as they have entered the cell,

the fatty acids and glycerin reunite to form fats, setting the alkali

free. The alkali is returned to the intestine, where it is available as

a solvent of further droplets of fat. The droplets of fat accumulate

in the epithelial cells. During active digestion they are also to be

seen in the connective-tissue cells, in the leucocytes, and in the

lymph inside the lacteal vessel. The epithelial cells extrude the oil

droplets, backwards, much in the same way as the cells of the mammary

glands extrude globules of milk. In herbivora, and in Man also so far

as we can judge, the contents of the small intestine are alkaline.

Conditions are therefore favourable for the formation of soap. But in

carnivora the contents are acid throughout the greater part of the

canal. Acid, it need hardly be stated, prevents saponification. Yet

carnivorous animals have an immense capacity for absorbing fat. Fatty

acids are soluble to a moderate extent in bile. It is possible that,

fats having been split into fatty acids and glycerin, the fatty acids

are carried into the cells in solution in bile. But if in carnivora

bile actively participates in the absorption of fat, there is no

reason opposed to its having the same function in Man; and, indeed,

all observations which have been made upon patients in whom the bile

was, for some reason, diverted from the intestine, and in animals in

which a fistula of the gall-bladder has been artificially produced,

show that in the absence of bile the absorption of fat is considerably

decreased. Yet there is no reason for thinking that bile is secreted

for the purpose of facilitating the absorption of fat. Just as much

bile is poured into the intestine of a cow which is feeding upon grass

as into the intestine of a pig or a dog when the animal is consuming

a very large quantity of fat. Nevertheless, it appears to be certain

that, not in carnivorous animals only, but also in herbivorous animals,

the assistance of bile is necessary for the satisfactory absorption of

fat. Doubtless the co-operation of bile and pancreatic juice is more

important to carnivora than it is to herbivorous animals, in which,

owing to the alkalinity of the contents of the intestine, all fatty

acids liberated by the action of pancreatic juice might be converted

into soluble soaps.

The problem of the form in which foods enter the absorbing cells is

intimately associated with the further problem of the form in which

they leave them. In the villus, and even within the epithelial cells,

fat appears abundantly as such. If, as we have reason for believing

to be the case, it enters in the form of soap and glycerin, the

re-formation of fat is an illustration of the synthetic power of the

tissues. For the purposes of the economy it is needed as fat, and not

as the constituents of fat. There is no reason for thinking that at

any stage in its future progress it is again split into fatty acid and

glycerin.

We cannot see absorbed proteins with the microscope, as we can see fat,

nor can we apply chemical tests which will distinguish between the

proteins which the cells contained before digestion commenced, and the

proteins which they have received as its result. Nevertheless, it is

certain that peptones are reconverted into proteins as soon as they are

absorbed. They are not to be found in blood or lymph. If the peptones

absorbed after a proteid meal remained as such after they passed

through the wall of the alimentary canal, they would produce various

undesirable results.

There is some difficulty in following droplets of fat across the space

which intervenes between the epithelium of a villus and its lacteal

radicle. It has been asserted that leucocytes act as carriers, catching

the droplets as they are extruded by the epithelial cells, and bearing

them into the radicle, where they set them free. Undoubtedly, many

leucocytes are present in the lymph-spaces of a villus. After a meal

they are found charged with fat. But it is hardly in accord with what

we know of the character of a leucocyte to suppose that it will let go

fat which it has once ingested into its own body-substance. A leucocyte

is not a disinterested organism. If fat droplets are floating across

from the epithelium to the lacteal, leucocytes are pretty certain to

steal some of them. But we know of no other case in which they give up

what they have stolen, unless it be something which is injurious to

their own health. Even then they usually cling to it, whether it be a

germ or a particle of soot, until their own dissolution sets it free.

Neither proteins nor sugar reach the lacteal radicle. Both these

substance are absorbed from the lymph in the tissue-spaces of the

villus by the blood-capillaries and venules which traverse them. The

veins of the intestine unite to form the portal vein, up which proteins

and sugar are carried to the liver, where they are stored, to be doled

out into the blood-stream as the tissues need them.

=Bacteria of the Alimentary Canal.=—The enzymes (ferments) of the

several digestive juices are not the only agents which modify the

constitution of the foods within the alimentary canal. Throughout the

whole of the tract conditions are in many respects favourable for the

growth of putrefactive organisms. Mouth, stomach, small and large

intestine, has each its special bacterial flora. It is doubtful whether

any of these organisms, with the single exception of the bacteria which

in herbivorous animals break up cellulose, are favourable to digestion.

That they are not necessary has been shown by an ingenious experiment

on new-born animals. Guinea-pigs born in an aseptic chamber, through

which filtered air was drawn, and fed every hour on sterilized milk,

throve and put on weight. When killed at the end of eight days, no

germs were present in their alimentary tracts. Yet in all animals

under ordinary conditions bacteria are present in great numbers, at

any rate, after the nursing period, and, for good or ill, produce

important fermentations. Only a single bacillus (_B. bifidus_), and

that a friendly germ, is, it is asserted, present in the intestines of

an infant at the breast; whereas a bottle-fed baby houses a variety of

parasites.

In the stomach, sugars are changed by the _Bacterium acidi lactici_

into lactic acid, which is further split into butyric acid, carbonic

acid gas, and hydrogen. Succinic acid and other substances are also

formed. This occurs in the first stage of gastric digestion. When a

considerable quantity of hydrochloric acid has been poured out, lactic

fermentation is stopped. The small amount of gaseous products formed

normally is of little consequence; but flatulence is a most annoying

symptom of indigestion. “Put your trust in Providence, and you will

feel more cheerful after luncheon,” Dr. Jowett is alleged to have

remarked to a despondent friend. The presence of food stimulates the

stomach to contraction. Accumulated gases are expelled. Hydrochloric

acid is secreted, and puts a stop to fermentation for a time. But if

the meal be too heavy or the mucous membrane in an irritable condition,

the contents of the stomach become unduly acid in the later stages of

digestion. Other bacteria then develop, leading to fresh trouble; more

gases accumulate, and the dyspeptic’s distress is greater than it was

before. Unfortunately, antiseptics, such as creosote, and carminatives,

such as oil of lavender, oil of peppermint, or alcohol, which for the

moment give relief, increase irritability, and consequently in the

long-run make matters worse. It is the fermentation of the later stages

of digestion which causes most annoyance. Admirable as was the Master

of Balliol’s advice, it hardly took account of the fact that bacteria

which cause flatulence, with its resultant feeling of oppression,

are derived for the most part from the imperfectly digested, and

therefore actively fermenting, remnants of food which were present in

the stomach when the meal was taken. It would be far beyond the scope

of this book to consider the pathology of dyspepsia; but the study of

normal conditions reveals the fact that some amount of fermentation

invariably occurs. The _Bacterium acidi lactici_ is always present in

the stomach. Normally its activity is arrested by the hydrochloric

acid of the gastric juice about twenty minutes after a meal. After this

no further multiplication of bacteria should occur. The presence of

bacteria which grow in a strongly acid medium usually indicates that

the stomach was not completely emptied before fresh food reached it. It

may be that the last meal was too large or the interval too short. If

the mucous membrane is in an unhealthy condition, its own secretions

afford material on which bacteria thrive. Nothing short of washing it

out with a stomach-pump will clean it up. The presence, at the time

of feeding, of food left over from the previous meal is likely to

perpetuate the unsatisfactory state of affairs. All the glands of the

alimentary tract exhibit a tendency to periodicity. Their efficiency

is greatest when activity follows a period of rest. If the stomach is

not able to expel its contents, it has not the opportunity of preparing

for fresh duties. Fat undergoes a certain amount of rancid fermentation

in the stomach. Proteins are not attacked by bacteria in the stomach

unless the condition of the organ is very unsatisfactory. The odour of

the products of their decomposition is then recognizable in the breath.

Bacteric fermentations in the small intestine are unimportant under

normal conditions, with the exception of the fermentation of cellulose.

Cellulose has the same empirical formula as starch. It is completely

insoluble, and is not affected by any of the digestive juices. The

greater part of the cellulose consumed by herbivora is, however, broken

up by bacteria into acetic and butyric acids, carbonic acid, and

marsh-gas. In Man also a small quantity is similarly destroyed.

In the large intestine the bacteric fermentations are not unlike those

which occur in the stomach, with, in addition, the destruction of

proteins, or of products of proteid digestion. The greater the quantity

of undigested food which reaches the large intestine, the greater is

the development of bacteria. When the stomach is dilated, the ascending

colon, and especially its cæcum, is usually dilated also. Bacteric

fermentation in the large intestine, with resulting flatulence, is

evidence of imperfect digestion, due either to an excess of food or to

weakness of the alimentary organs, or, as is more commonly the case,

to the combination of these two factors. The relation of fermentation

to alimentation can be shown by counting the microbes in a specimen of

the contents of the large intestine. In a particular case it fell from

65,000 per milligramme upon a mixed diet to 2,000 per milligramme upon

a diet of milk.

In the world at large bacteria perform many offices of the utmost

usefulness to other living things. They fix nitrogen in the soil,

sweeten polluted rivers, reduce animal and vegetable matter to a

condition in which it is available as plant-food. Their presence

within the alimentary canal is inevitable; but it is somewhat doubtful

whether, with the exception of the fermentation of cellulose, they do

the economy any service with which it could not dispense. As parasites

of the alimentary canal, some kinds are less desirable than others.

Recently a method of limiting their variety has been introduced

and advocated with much enthusiasm, as favourable to the hygiene

of the digestive tract. In countries in which the cows are driven,

in summer, to mountain pastures, the peasants of the plains live

during their absence largely upon milk brought down at intervals, and

allowed to turn sour. Sour milk, in Bulgaria, develops a bacterium of

extraordinary vigour. It can live in a medium containing as much as

10 per cent. of lactic acid, a concentration fatal to other forms of

_Bacterium acidi lactici_. It is easily cultivated, and when ingested

continues to multiply in the alimentary canal. So peculiarly lusty is

this bacterium that it makes life impossible for other germs. As it

dies out after two or three months, it seems unlikely that a man who

swallows the Bulgarian milk-germ runs a risk of inviting a repetition

of the tragedy which followed the acclimatization of the mongoose

in Jamaica. Its supremacy has been attributed to its capacity of

developing a concentration of lactic acid too high for the well-being

of other bacteria; but it is improbable that it has the opportunity of

doing this in the alimentary canal of a person living on a mixed diet.

The extinction of other bacteria (if they are extinguished) is more

likely to be due to an antagonism of a more subtle kind, at present

inexplicable, but not without parallel. The purifying influence of the

water of the Ganges has for ages been an article of faith. Pilgrims

from fever-stricken districts bathe in it, foul it, drink it, with the

corpses of their fellows floating down the stream. Recently it has been

shown that this belief is not without foundation. The water of the

Ganges at Benares contains bacteria which are as tigers among lesser

vermin. The germs of cholera and typhoid fever disappear from cultures

into which these overbearing microbes are introduced.

=Conditions Requisite for Normal Digestion.=—When M. Chevreul,

Professor of Chemistry at the Jardins des Plantes of Paris, attained

his hundredth year, an interviewer very naturally inquired of him,

“Have you always had a good digestion?” To this the still vigorous

Professor answered: “I really cannot say, for I have never noticed.”

So long as it is well used, the stomach is an unobtrusive organ. It is

tyrannical when it deems itself the victim of inconsiderate treatment.

A study of its physiology serves to show that it will work contentedly

only upon certain clearly defined terms, of which the following are

perhaps the most important: The stomach exacts due warning that its

services are wanted. The nerves of smell and taste must announce the

approach of food and guarantee its quality. “What may I eat?” asked

a large-framed, strenuous, eager, overworked barrister of a great

physician. “Eat, sir? You may eat whatever you like. But be quite

sure that you do like it.” Wise advice. The human race would not have

developed its strong preferences for certain kinds of food if all foods

were equally suitable to satisfy its needs. Taste is not a matter of

fashion. It is the expression of the experience of mankind. Fanciful

as civilization has made us, and easily as appetite is perverted, if

we are sure that we really like, and want, a food, we may trust that

our liking will guide us as safely as it guides a buffalo or a deer.

“Eat what you like.” Eating with liking carries with it the idea

of obtaining the maximum of satisfaction from the exercise of this

necessary function. Most things which are reckoned unwholesome are full

in flavour or rich in consistency. They satisfy the palate when spread

out very thin. It is poor economy to help oneself to caviare with a

table-spoon. In the second place, the stomach must be assured that the

teeth are doing their proper share of work. Among the many half-truths

which every year are exalted to the level of a revelation or a rule of

conduct is the doctrine of the “chewers”—persons who take no meals,

but industriously and almost continuously masticate nuts and biscuits.

Thirdly, the meal must not be so large that the stomach cannot deal

with it “at a sitting.” In from two to three hours the last of the food

should have passed through the pylorus, allowing the stomach to rest

before it is called into activity again. As proteins are practically

the only foods which are digested in the stomach, the work required of

this organ depends upon the quantity of proteins present amongst the

constituents of a meal. Meat is the food richest in proteins, although

bread, vegetables, milk, cheese also yield them. Some people can digest

three meat meals every day; but others, probably the majority, find

that it is unwise to take any considerable quantity of meat more than

once in twenty-four hours. It is only when the cells of the gastric

glands have accumulated a store of pepsinogen-granules that proteid

digestion is vigorously carried on. Fourthly, the food must be in a

form in which it does not irritate the stomach, provoking an outflow of

acid out of proportion to the pepsin which accompanies it. Experience

alone can teach the foods which are to be avoided on this account.

But speaking generally, it may be said that the stomach resents the

presence of substances which cannot be amalgamated into chyme. Its

task is the reduction of the mixture of foods which compose a meal to

the consistence of a smooth cream. Hot buttered toast or pie-crust are

made of wholesome constituents enough, but, fat being melted into the

starch, the fragments are impermeable to the gastric juice. They act

mechanically as irritants of the mucous membrane. Again, it may be said

that “pure” foods are apt to provoke acidity. Nothing could be more

wholesome than eggs or pounded meat or custard pudding; but taken by

themselves these articles of diet over-stimulate the mucous membrane.

They need to be diluted with starch-foods, or even with cellulose.

And this calls attention to the dietetic value of vegetables.

Vegetables, which consist chiefly of innutritious cellulose, distribute

the digestible constituents of a meal and increase its bulk, greatly

favouring its progress through the alimentary canal. Especially

in herbivora is it important that the bulk and looseness of the

food should be well maintained. Rabbits thrive on sugar, starch,

and albumin, mixed with such an absolutely indigestible substance

as horn-shavings. If the inert substance be omitted, they die of

intestinal inflammation, although fed on the same mixture of pure

foods. Other rules which govern digestion might be mentioned; and it

is needless to point out that, when the mechanism is deranged, steps

adapted to the particular malady must be taken to bring it back to a

normal condition. There is, however, one precaution upon which, in a

certain number of cases, it is impossible to lay too much stress. The

digestion of proteins is seldom carried out satisfactorily when much

sugar, and especially much cane-sugar, has been eaten at the same meal.

Excessive lactic fermentation prevents the proper peptonization of

meat. The chemistry of digestion is not sufficiently well understood

to enable the physiologist to say what is amiss; but probably

by-products of peptic digestion are produced. To many people this is of

little consequence; but to those who exhibit a gouty tendency it is,

unfortunately, a most serious matter. Civilized races are particularly

subject to the uric acid diathesis. In the course of nitrogenous

metabolism uric acid is formed in place of fully oxidized and easily

soluble neutral urea. Although the chemical sequence has not been

discovered as yet, there is no question but that imperfect gastric

digestion means the formation of uric acid, with all its lugubrious

results: malaise, neck-ache, emotional depression. Birds and reptiles

form uric acid as the end-product of nitrogenous metabolism, not urea.

So also do city-fathers, butchers, and others whose diet consists too

largely of meat. Many nervous, ill-nourished men and women tend to

do the same, however abstemious their meals. It is useless to tell

such persons to reduce the amount of proteins in their diet. Their

attempts at increasing the starch, sugar, and fat at the expense of

nitrogenous foods lead to dyspepsia, which makes matters worse. They

often find, however, that if they are careful to restrict to the

narrowest limits the amount of carbohydrates (especially sugar) which

they take in conjunction with meat, fish, eggs, or other proteid foods,

the formation of uric acid ceases. Sugar, bread, fruit, and other

carbohydrates, may be taken in abundance, and with great advantage, at

breakfast and lunch, without proteid food, if dinner consists of broth,

fish, meat, cheese, vegetables, with a minimum of bread.

=The History of the Foods after Absorption.=—All foods, with the

exception of inorganic salts and salts of various vegetable acids,

fall into three classes: (1) Proteins—substances of complex chemical

constitution, containing nitrogen; (2) carbohydrates—so called because

hydrogen and oxygen, in the proportions in which they enter into the

formation of water, are united with carbon; (3) fats. Proteins of

various kinds are consumed as food. The peptones produced from them by

digestion also vary. Yet very little is known as to the differences in

physiological value which distinguish the various kinds of protein when

absorbed into the fluids of the body (_cf._ p. 134). All carbohydrates

after digestion and absorption appear as dextrose. The various fats

preserve their individuality until they are taken up by the tissues.

When fixed in the tissues, they assume, except under somewhat abnormal

conditions, the composition characteristic of the fat of the animal

which has eaten them. If a dog which has been severely starved is fed

upon mutton-fat, it puts on in the first instance fat which resembles

that of a sheep rather than the normal fat of a dog. As soon, however,

as it is well nourished (which would never occur unless some protein

and carbohydrate were added to the mutton-fat), its fat assumes the

usual form.

For practical purposes we are obliged to speak of the three classes

of food—proteid, carbohydrate, and fatty—as if there were but one

member in each class. And we have abundant evidence that such a simple

classification is fully justified. The body has so large a power of

altering chemically the nature of the food which it absorbs that it

makes little difference in the further history of the food whether the

protein supplied to it be an albumin or a globulin; the fat, stearin,

palmitin, or olein; the carbohydrate, starch or sugar.

In earlier days it was customary to regard the body as the receiver of

a variety of foods which it could break down into simpler substances

by oxidation, but could not reconstruct. Plants were regarded as the

manufacturers of organic compounds, animals as the destroyers of the

complex substances made by plants. The union of molecules, synthesis,

was looked upon as the function of the vegetable kingdom. Animals

built into their tissues the products elaborated by plants; some of

these products they shook to pieces for the purpose of setting their

energy free; others slowly disintegrated as the result of tissue

“wear and tear.” Gradually it was realized that many chemical changes

occur in the body which cannot be viewed as merely exhibitions of

its analytical capacity. The tissues were recognized as laboratories

in which reactions occur which consist in something more than the

splitting of complex into simpler molecules. The instances earliest

understood were connected with the history of carbohydrates and fats.

In the disease diabetes an enormous quantity of sugar is excreted,

amounting in extreme cases to between 1 and 2 pounds _per diem_. When

carbohydrates are present in the food, the amount of sugar excreted in

diabetes is greater than it is when they are withheld; on an almost

exclusively proteid diet the amount of sugar excreted far exceeds the

amount of carbohydrates in the food. Another illustration of the power

of making sugar possessed by the animal economy is afforded by a dog

fed upon lean meat, and nothing else. Sugar is found in its blood,

and a store of carbohydrate (glycogen) in its liver. The formation

of fat is an instance of constructive metabolism. There is abundant

evidence that the quantity of fat produced may greatly exceed the

quantity contained in the food. Animals are fattened for the market

on a diet which contains less fat than that which accumulates in

their bodies. When nursing her young, an animal may secrete in her

milk much more fat than she obtains as such in food. It was a great

mistake to suppose that the body is dependent upon its tradesmen for

fat and sugar. It can make either of these substances out of a mixed

diet in which it is relatively deficient. It must, however, be a mixed

diet. An animal cannot live exclusively on fat or exclusively on

carbohydrate. It is impossible, therefore, for us to determine whether,

if given the one alone, it can turn it into the other. Chemists were

very unwilling to credit the body with the power of performing even

the simpler of these transformations—the conversion of carbohydrate

into fat. Proteins are essential constituents of a fattening diet.

Their immensely complex molecule has always afforded a tempting field

for arithmetical ingenuity. It is easy to remove from it the atoms

needed for the composition of fat, and yet to leave such groups of

atoms as might reasonably be supposed to constitute its “nitrogenous

moiety.” The hypothesis that the metabolic capacity of the body is

limited to analytical processes justified the supposition that, when

more fat is laid on than the food contains, the balance comes from

proteid substances, which split into nitrogenous and fatty moieties.

It has been shown, however, that an animal during fattening may put

on more fat than is contained as such in the food, or obtainable from

its diet, even though all the atoms of carbon and hydrogen in its

proteid food were devoted to its formation. The balance must come

from carbohydrates. Perhaps a still more striking illustration of

constructive capacity is the power of making glycerin. If a dog receive

fatty acids in its diet, it accumulates normal fats. The glycerin

which, united with fatty acids, constitutes the fat, was not contained

in its food. Starch and sugar are sources of fat. As yet there is no

evidence that fat can be converted into sugar.

The chemistry of the nitrogen-containing compounds appears to present

more difficult problems. Plants build up proteins. Is the animal’s

relation to these substances limited to their disintegration? Do

proteins inevitably descend from step to step until they reach

urea? There are reasons for thinking that, even when dealing with

nitrogenous substances, the metabolic power of the body is not

exclusively analytical. The liver can make urea from ammonia-salts,

such as lactate, or even carbonate, of ammonia—substances more stable,

and therefore in the chemical sense simpler, than urea. This is an

indication, though a faint one, that the body has a constructive

capacity, a power of producing more complex from simpler substances,

even in the case of nitrogenous compounds. Beef-tea, mutton broth,

meat-extracts have long been regarded as foods of value when the

power of assimilation is low. Chemists point out that the nitrogenous

substances which these decoctions contain are so near the bottom of the

ladder that the energy set free by their further oxidation to urea is

scarcely worth consideration. They admit that their ready availability

renders them useful as restoratives, but they deny them the status of

foods, on the assumption that their further progress must be downward.

As was stated when the conversion of peptones into leucin and tyrosin

was described, evidence is beginning to accumulate which shows that

within certain limits, at present impossible to define, the system can

reconstruct its proteins from amides and other simple products of their

degradation.

The animal economy receives, and after due digestive preparation

absorbs, three classes of food—nitrogenous, fatty, and carbohydrate.

If either of the two latter kinds be deficient in the diet, the body

can to a certain extent produce it from the other two. What is the

special value of each kind of food? What use is made of it? Before

attempting to answer these questions, we must endeavour to trace the

further history of the foods after they have traversed the wall of the

alimentary canal.

After leaving the stomach and intestines, the foods follow two

different routes. Proteins and carbohydrates are carried by the portal

vein to the liver. Fats are carried by the thoracic duct to the general

circulation. An excess of fat is found in the blood in all parts of the

body after a meal rich in fat. The eventual destination and fate of

fatty foods is unknown. Under certain circumstances they are added to

the fatty deposits in connective tissue; but if no additional fat is

being laid down, they go to other tissues, in which they are oxidized

into carbonic acid and water. When the amount absorbed is excessive,

a certain quantity of fat may be stored in the liver. In the cells of

this organ it is housed for a time, in order that it may be distributed

to the tissues after they have used up the supplies which first reach

them through the general blood-stream.

Proteins are completely lost to sight after they are absorbed into the

blood. They take part, of course, in the formation of growing tissue,

blood-corpuscles, skin, hair, nails. It is also common to speak of them

as making good the wear and tear of active tissues, although it is

very doubtful whether we can legitimately speak of the wear and tear

of tissues. The protoplasm which does the work of the body is not worn

out in the same way as the materials of which a machine is made. There

is no friction to rub it down. Proteins, like other foods, are used up

as sources of muscular energy and heat. Eventually they are reduced to

urea, carbonic acid, and water. Chemists naturally seek for substances

intermediate in constitution between proteins and urea. They assume

that the degradation of proteins will occur in regular steps; complex,

partially oxidized, nitrogenous compounds being formed first—in

the muscles, for example—to be further oxidized in the glands. The

existence in all organs of nitrogenous “extractives,” which can be

separated out when the organ is subjected to chemical analysis, seems

to justify the search for stages; but hitherto this search has been

singularly unsuccessful. Urea is the final product. It is not found

in muscle, nor, indeed, in any tissue other than the liver, which, as

already said, has the power of making it, even from salts of ammonia.

It is therefore clear that if proteins are destroyed in muscle and

other tissues, and if all urea is made by the liver, the antecedents

of urea must be carried from the muscles to this organ. The substance

which is most characteristic of muscular metabolism is lactic acid.

It is not impossible that all the nitrogenous portion of the complex

proteid molecule is reduced to ammonia (NH₃), which may be regarded as

the simplest of all nitrogenous compounds, and that this, combined with

lactic acid (C₃H₆O₃) as lactate of ammonia (NH₄C₃H₅O₃), is carried by

the general circulation to the liver, where it is converted into urea.

A considerable amount of lactate of ammonia may be injected into a vein

without any of it overflowing through the kidneys. It is all reduced

to the condition of urea, water, and carbonic acid. If the liver is

so diseased as to be functionless, or if by operative measures it is

thrown out of action, salts of ammonia are excreted by the kidneys

instead of urea. In birds and reptiles uric acid takes the place of

urea. Their livers yield uric acid on analysis. If lactate of ammonia

be injected into their blood, it is converted into uric acid, so long

as the liver is intact.

We know nothing of the forms assumed by the proteins absorbed into the

blood, of the organs in which they are stored, or of the higher terms

of the series of substances through which they pass before they are

finally excreted as urea, water, and carbonic acid. No nitrogenous

compounds are found in lymph or blood which can be pointed out with

confidence as the products of tissue wear and tear. When considering

the sources of muscular energy, we shall have something more to say

regarding the part that proteins play in the economy.

If there is great difficulty in following fats and proteins after their

absorption, it is quite otherwise when we come to deal with sugar.

Carbohydrates are the great sources of energy. Muscular work may be

generated by the oxidation of either of the three classes of foods,

but undoubtedly the carbohydrate glycogen is its most constant source.

Provision is therefore made for the storing of glycogen in the liver,

and the distribution to the muscles of a regular supply. After a meal

the portal blood, on its way from the intestines to the liver, contains

a higher percentage of sugar than the blood in the hepatic vein or

in any other vessel. If sections of liver be examined after feeding,

and compared with those obtained after a period of starvation, it is

found that the cells of the well fed liver contain glancing masses

of a substance which takes a port-wine colour with iodine. This is

glycogen, or animal starch. It has the same empirical formula as starch

(C₆H₁₀O₅)ₙ. In the dry state it is a greyish powder, which, unlike

starch, forms an opalescent solution in cold water. Like starch, it is

non-diffusible. In the animal kingdom it stands to sugar in the same

relation as starch to sugar in plants. If a sheep be killed while it is

feeding in the paddock, and its liver removed and weighed, it will be

found that it is from one-third to one-half heavier than the liver of

a sheep of the same weight obtained from a butcher; for butchers have

the stupid practice of starving animals before they kill them. It was

long ago discovered that it is unnecessary to feed an animal for a day

or two before it is killed, and this option has been elevated into a

prohibition. A tradition has grown up that it is undesirable to give

food for some time before killing. Not only will the liver of a sheep

killed during active digestion be found to be heavier than that of a

starved sheep, but it will also prove more succulent; for it is loaded

with sugar (into which glycogen is rapidly converted after death),

as well as with proteins and fats, which are withdrawn from it when

the animal fasts. It appears that the liver cannot secure the whole

of the sugar which is absorbed after a full meal. Some of it passes

into the general circulation, and is stored in the muscles; but the

liver always maintains a considerable reserve. Even after prolonged

deprivation of food, it holds on to a certain quantity, especially in

carnivora. Glycogen is found in the liver of a dog after a long period

of starvation. The muscles lose during activity the glycogen which they

contain when at rest.

It has already been pointed out that the body is not entirely dependent

upon external agencies for the production of the sugar which it needs.

When the supply is inadequate, it manufactures glycogen for itself

out of the other constituents of the diet. It can, indeed, make it at

the expense of its own proteins. If a dog which has been caused to do

muscular work, without a sufficiency of carbohydrate food, until (as

judged from a control experiment) all glycogen has disappeared from its

liver, be placed under the influence of a narcotic drug, which arrests

the activity of its muscles, glycogen reappears.

=Dietetics.=—Even those who are most ignorant of the science of

physiology flatter themselves that they have one piece of information:

“The whole of the body is renewed once in every seven years.” I cannot

trace the origin of this sapient apothegm, which for generations

has passed current. If seven weeks or seventy years were the period

allowed for the renewal of the tissues, the statement would be equally

near the truth. Judging from the rate at which they are destroyed,

it is unlikely that blood-corpuscles live for more than five or six

weeks. Hairs are shed about two years after they first appear above

the surface. On attaining this age a hair drops off and a new one

takes its place. The superficial cells of the skin are shed in great

numbers every day, and their place taken by younger cells which come

up from the deeper layers. The cells of many glands would seem to

have a comparatively short term of life. On the other hand, some

tissue-elements are far more permanent. By the time a child is a year

old all its nerve-cells are in position. They last as long as the

individual lives. When the statement with regard to the renewal of the

tissues is understood as meaning, not that the cells are destroyed and

replaced by new ones, but that within a period of seven years all the

molecules which enter into their protoplasm are extruded from the body

and replaced by molecules received as food, the assertion verges on the

transcendental. It is unlikely that we shall ever obtain data against

which it can be checked.

The essential part of every living cell is its sponge-work of

protoplasm. “Bioplasm” is perhaps a better term to use when we are

speaking of protoplasm as a structure, since it does not suggest any

prejudice with regard to its chemical constitution. Within the meshes

of the bioplasm are nutrient materials, as yet unused, and worked up

products in various stages. It has always been taken for granted that

when treating of nutrition, we have to consider the repair of the

bioplasm, as well as the provision of raw material which it can convert

into the specific products of the cell. Suppose that the cell belongs

to the class of supporting tissues; let it be a cell of cartilage,

for example. The bioplasm manufactures a collagenous substance which

remains in and around its meshwork. If it be an epidermal cell, it

forms horny substance. If a secreting cell, it accumulates secernable

products. If a muscle-cell, it develops a large quantity of material,

which by a change in form produces movement. In this last case we

suppose that the energy set free as muscular force is due to oxidation.

More stable bodies take the place of a less stable substance. After

contraction the relatively complex contractile material is renewed from

the foods stored in the muscle-cell; or if it be not, in the ordinary

sense of the word, destroyed, if it has merely parted with certain

oxidizable constituents, it obtains a fresh supply of such constituents

from the foods which the muscle-cell contains. Even in the case of

cartilage or epidermis, we imagine that, since the matrix is “alive,”

it is always undergoing molecular change, and consequently always

requiring food. The fact that every tissue, however inert, dies when,

owing to the blocking of the bloodvessels which irrigate the part, its

supply of nutriment is cut off, justifies this belief that all living

tissue is undergoing change.

When we make up a balance-sheet of the body as a whole, placing to

the debit side the food which it receives, and to its credit side the

work done in external movement and in the production of heat, we again

find reason for believing that every part of every cell is constantly

undergoing change.

The balance-sheet of the body can be drawn out in either of two ways.

We can estimate the quantities of nitrogen, carbon, hydrogen, and

oxygen supplied to it in the several foods, and compare them with the

amounts of each of these four elements given off in urea, carbonic

acid, and water, making, of course, a note of the body’s balance in

hand at the beginning and at the end of the period of observation. Or,

we may estimate the amount of potential energy contained in the food,

and ascertain the use to which this energy is put in doing external

work, in maintaining the temperature of the body, and in warming the

breath and other excreta.

If we are making up the balance-sheet of a fully-grown man, we may

take for granted that he is not making fresh tissue. During the

period throughout which he is under observation, care is taken to

avoid altering the conditions of his life in such a manner as to

lead him to develop additional muscle. If he gains in weight while

under observation, he is putting on fat. If he loses in weight, he is

sacrificing fat.

The whole of the nitrogen taken in leaves the body in urea, unless,

as we have said, growth of tissue is taking place. The body has not

the same temptation to store nitrogen as it has to store carbon.

Consequently, it is very sensitive to any deficiency of nitrogen in

the diet. If food does not contain as much protein as is needed,

the deficit is made up at the expense of the tissues. It does not

necessarily follow that under these circumstances a man loses in

weight. He may be putting on fat, although losing in strength owing to

waste of muscle. For observations upon the income and expenditure of

the body to be of any value, a condition of “nitrogenous equilibrium”

must be established. The nitrogen taken in must equal in amount the

nitrogen given out.

Very exact determinations of income and expenditure may be made by

placing an animal, or even a man, in a box through which air is drawn.

A record is made of the volume of air drawn through the box. The

percentages of water vapour and carbonic acid which the air contains

are estimated before it enters and after it leaves. The solid food

consumed and the urea excreted are also measured.

If it is desired to measure the amount of heat given off, an animal may

be placed in a calorimeter.

Even when most passive, the subject under examination, whether an

animal or a man, is expending energy in keeping the body warm, in

movements of respiration, and in shifting position. If it is desired to

ascertain the relation of oxidation to external work, it is easy to

devise a form of resistance, such as the turning of a wheel, or the

lifting of a weight which can be measured.

In testing diets, it suffices to make sure that nitrogenous equilibrium

is maintained, and then to estimate the gain or loss in weight and the

output of energy in external work.

=The Relative Value of Foods.=—Dried proteins contain about 15 per

cent. nitrogen, 54 per cent. carbon, 7 per cent. hydrogen, 22 per cent.

oxygen, a little sulphur, and frequently some phosphorus. A large

proportion of their carbon and hydrogen is available for combustion.

Fats contain 75 per cent. of carbon, and a considerable quantity of

hydrogen available for combustion; carbohydrates, 40 per cent. of

carbon, with hydrogen and oxygen in the proportions in which they

occur in water. If 1 gramme of protein is oxidized to the condition

of urea, carbonic acid, and water, sufficient heat is liberated to

raise the temperature of 4,100 grammes of water 1 degree centigrade.

Its calorific value is therefore expressed as 4,100 calories, the unit

of measurement—a calorie—being the amount of heat needed to raise 1

gramme of water 1°. The calorific value of 1 gramme of fat is 9,300

calories; of 1 gramme of starch, 4,100 calories. Thus, the energy

potential in protein and in starch is the same; that in fat more than

twice as great as that in either of the other foods.

=A Normal Diet.=—Nitrogenous equilibrium and body-weight can be

maintained and work done on diets which vary widely in percentage

composition. This is a question which we shall consider at greater

length later on. In the meantime, for the sake of illustration, it is

necessary to formulate a diet which is fairly representative of the

selection of foods made by a man of average weight—say 70 kilogrammes

(145 pounds)—who desires to do a moderate day’s work in comfort. It

has been found to amount to about 100 grammes of protein, 100 grammes

of fat, 240 grammes of carbohydrate, all measured dry and as pure

foods. If the several elements of such a diet be multiplied by the

figures which represent their calorific value, it will be found that

the man is supplied with 2,324,000 calories. The illustration that we

have chosen is the diet of a professional man who is not engaged in

hard physical work. The pure foods would be found to the amounts stated

in 17 ounces lean meat, 4 ounces butter, and 17 ounces bread. The

day’s diet would, of course, be much more varied than this, but it is

simpler to express it in these terms.

Such a diet would hardly answer the requirements of a man doing hard

muscular work. Experience shows that he would expect to receive a more

liberal supply of energy, and that to obtain it he would increase

slightly his allowance of proteins, and very considerably increase

the quantity of carbohydrates that he consumed. The diet of European

workmen is remarkably constant in the relative amounts of its several

constituents, no matter what their nationality or the exact form of

their work may be: Proteins, about 135 grammes; fats, 80 grammes;

carbohydrates, 500 to 700 grammes—giving a supply of energy equal to

3,500 to 4,000 kilo-calories.

Speaking generally, carbohydrates are the source of muscular force,

and fats of heat. In warm climates men work on carbohydrates. The

’rickshaw men of Japan are said to eat only rice on working days, and

to reserve fish for days of leisure. The Japanese, as is well known,

consume extremely little fat. The Esquimaux and other inhabitants of

high latitudes eat immense quantities of fat. Proteins constitute the

luxurious element of a diet. Not only are they more attractive to

most palates, and therefore preferred by persons whose dietary is not

severely regulated by price, but the body prefers them. It works with

greater alacrity when supplied with more protein than, in a strictly

physiological sense, it needs.

The supply of food must exceed the apparent demand. The most efficient

of motors cannot convert more than 15 per cent. of the energy potential

in its fuel into work. If a man endeavours to obtain a better result

than this from his muscular system, if he tries to make his machine do

more than 15 units of work for every 100 units of energy with which he

supplies it, he does it at the expense of his own tissues. First he

loses in weight, owing to the consumption of fat; then the excess of

nitrogen discharged over nitrogen consumed shows that he is burning

up the proteins of his own tissues. It is needless to add that the

weakness which results puts a stop to excessive work. Muscles, as we

shall find when we consider the relation of their output of work to the

energy supplied to them, can produce a much better result than the best

of engines; but we are speaking of the body as a whole, which wastes

energy in the movements of respiration, masticating food, shifting

position, maintaining the body temperature, etc.

Health may be maintained and work done on diets which depart widely

from the one which we have selected as a standard. Darwin found the

Gauchos of South America living exclusively on meat. Nansen and

Johannsen, when seeking the North Pole, lived for months on meat and

blubber. Millions of the inhabitants of India abstain from meat and

meat-fat, their diet consisting of rice, buttermilk, and a little

fruit. In the case of all persons with whom the price of food is an

important consideration, carbohydrates are preferred to proteins and

fats. Oatmeal is very much cheaper per unit of energy than meat. A

man may be a meat-eater or a vegetarian, although he is probably

unwise in overlooking the obvious teaching of his teeth and digestive

organs, which are those of an omnivorous animal. His prehistoric human

ancestors lived chiefly on the harvest of their spears and tomahawks.

If we insist upon looking back still farther, we discern a cleavage of

the race into the arboreal fruit-eaters, which still retain pre-human

characters, and the more enterprising and energetic troglodyte hunters

from whom the human race was evolved.

A man may vary his diet within wide limits. Innumerable considerations

lead certain individuals to desire to depart from the diet which we

have termed “normal”—_i.e._, typical of inhabitants of the temperate

zone. One man rebels against the expense of living; he would fain

reduce the quantity and the cost of food. Another, having to traverse

regions in which food is scarce, wishes to ascertain the lightest, and

therefore the most portable, combination of its essential elements.

A third—and he belongs to a much larger class—tormented with

indigestion or harassed by gout, asks, “Why must I consume things which

give the stomach trouble, or produce disagreeable and incapacitating

after-effects?” Many circumstances prompt to experiments in diet.

Much latitude is undoubtedly allowed. But there are limits within

which alone health can be maintained and work done. It is of great

interest to ascertain exactly how wide these limits are; and especially

important is it to find out the lower limit, the minimum of food, and

the minimum of each particular kind of food, which will enable the

human machine to work. The problems involved are somewhat complicated.

If it were possible to live on a single food, it would be as easy to

ascertain the irreducible minimum as it is to find out with how much

coal or with how much petrol an engine can be made to turn a wheel. But

to support the body several different kinds of food are indispensable.

It is therefore necessary to determine, not only the minimum quantity

of the combined foods, but also the minimum amount of each kind of

food, and the effect upon the total of variations in the relative

amount of each of its several factors. The problem is complicated, but

certain limits are impassably defined. In the first place, with regard

to the total amount, the work which the body does cannot under any

circumstances be reduced below a certain level. The food consumed must

provide a supply of energy equal, at the least, to the performance of

the minimum of work. The body must receive each day food of due caloric

value. Then with regard to the amount of each several constituent.

Many considerations lead us to wish to increase one of them or to

diminish another. Some food is cheap, and other food is dear. Economic

reasons are in favour of the cheaper food. Even ethical considerations

are not without weight. We have, perhaps, a prejudice against

sacrificing life to supply the pot. We have doubts as to whether our

system can properly digest, metabolize, and excrete meat. We need an

unambiguous answer to the question, To what extent can nitrogen-foods

be replaced by carbon-foods, and _vice versa_? A cell, as already

said, consists of a framework of bioplasm bathed in cell-juice which

contains nutrient substances and manufactured products. The bioplasm

is alive; the proteins, carbohydrates, and fats of the cell-juice are

the materials with which it is nourished, and upon which it works.

Some physiologists incline to the view that non-living substances must

enter into the bioplasm before they undergo metabolism. They consider

that the molecules of the non-living substance must at the time when

they undergo a chemical change be physically and chemically a part of

the living substance. Others take the opposite view: that the living

substance does not undergo change, but brings about changes in the

non-living substance which is in contact with it, enclosed within its

meshes. This is a problem which is not likely to be solved, nor is

its solution of great importance in relation to the question which we

are discussing. Whichever of the two views be justified, we have to

distinguish between the bioplasm of the cell—the machine—and its raw

materials and manufactured products. The question to which we want an

answer is the following: Must the bioplasm undergo change? There seems

to be no reason in the nature of things why it should. It is not, as we

have already pointed out, subject to wear and tear. A perfect machine

would in the absence of friction, which rubs down its steel and brass,

continue to turn out its products so long as it was supplied with raw

materials and the energy needed to manufacture them. We could imagine

the bioplasm as indestructible, receiving energy from a portion of the

foods, and expending this energy in the production of chemical change

in the remainder. We could imagine that when once the tissues had

attained their full growth they would require no more protein for their

own nutrition; they would be occupied in producing heat and motion from

the non-nitrogenous foods. But observation shows clearly that this is

not the case. The force which energizes the bioplasm, enabling it to

evoke metabolism in non-living substance, is obtained at the cost of

its own destruction. The bioplasm wastes unless constantly supplied

with proteid food.

Under ordinary circumstances the amount of urea excreted varies

directly as the quantity of nitrogen contained in the food. Since

urea contains 45 per cent. of nitrogen, and protein 15 per cent.,

every gramme of urea excreted represents 3 grammes of dry protein

consumed; or, in terms of nitrogen, every gramme of nitrogen excreted

represents 6·25 grammes of protein consumed. If all food is withheld,

the excretion of nitrogen falls, but it never reaches zero. Many

observations have been made on fasting men. On the second day of

fasting the nitrogen excreted falls to about 13 grammes, representing

80 grammes of protein used up. It is generally thought that by the

second day all “floating proteins” are exhausted, and that therefore

nitrogenous metabolism is reduced, as it were, to a business basis.

So long as the supply of food is abundant, the body has a luxurious

habit of using proteins in preference to non-nitrogenous food. But

after a day’s starvation there is no longer any fancy metabolism,

no consumption of proteins as fuel when cheaper fats and sugar would

answer equally well. In the case of Succi, who fasted for thirty days,

the nitrogen excreted fell to 6·7 grammes on the tenth day, to 4·3

grammes on the twentieth, and to 3·2 grammes on the last day. Clearly,

we have to make a distinction, when all food is cut off, between

the oxidation of the protein which, failing all other material, is

withdrawn from the tissues for the purpose of supplying the force

absolutely necessary to maintain respiration and such other movements

as are inevitable, and to keep up the temperature of the body—force

which under other circumstances might be supplied by non-nitrogenous

food—and the oxidation to which bioplasm is inevitably subject,

so long as it is alive. The oxidation of bioplasm under ordinary

circumstances of course supplies force; but it does not follow that

this is sufficient to maintain the respiratory movements and the

contraction of the heart. When a herbivorous animal is starved, it not

infrequently excretes more urea at the commencement of the starvation

period than it was excreting when well fed. Its activities did not come

to a standstill when carbohydrate food was cut off. For a time they

were maintained at the expense of its own tissues. On the other hand,

the results obtained from the observation of the man who went without

food for thirty days show that Nature is able to economize force by

reducing the metabolism of living substance below the normal. It might

be supposed that the irreducible metabolism could be ascertained by

giving a nitrogen-starved animal non-nitrogenous food, but it is found

that this scarcely affects the tissue-waste. Becoming more active,

the tissues, while saved from the necessity of supplying fuel for the

production of heat and motion, suffer more waste. Again, it might be

expected that if to an animal which had been starved for a few days,

until its urea had fallen to the starvation limit, exactly sufficient

protein were given to supply this amount, the tissues would be saved.

It is found, on the contrary, that nearly twice as much urea is

excreted as before. If the quantity of protein be steadily increased,

equilibrium is at last established, but not until the amount of

nitrogen in the protein given is two and a half times as great as the

amount excreted during the starvation period. Additional food at once

gives rise to additional waste. The tissues which during the period of

scarcity had reduced their oxidation to a minimum become more active at

the first hint of returning plenty.

This last experiment illustrates a general law. An increase of

proteid food within certain limits increases the metabolic activity

of the tissues—provokes them to extravagance. It is possible, by

adding protein to a mixed diet which sufficed for the maintenance of

body-weight and nitrogenous equilibrium, to bring about a nitrogen

deficit and to reduce the body-weight. Or, if the body is gaining in

weight, owing to the accumulation of fat, the substitution of protein

for carbohydrate (weight for weight, since their caloric values are

the same) will lead to its reduction. It is difficult to avoid the

use of fanciful language in accounting for these results. The animal

economy is like an over-careful housekeeper, who, when meat is scarce,

doles out porridge also with a thrifty hand. When meat is plentiful

she is prodigal with every article of diet. Protein is the most costly

of foods. Any indication that it is scarce leads to a shutting-down

of activity. On the other hand, no other food is so readily absorbed

(unless the digestive organs be protein-sick); none is so quickly

incorporated in the bioplasm; none is so easy to decompose. When fed

with protein the machinery hums. The insatiable appetite for beef and

eggs which overtakes a man of sedentary habits after a long morning in

a boat or on a bicycle does not indicate that his muscular tissue is

suffering from wear and tear. It does not prove that he is setting free

energy by oxidizing proteid food. It shows that he is asking certain

tissues which are accustomed to a quiet life to exhibit prodigious

energy. They will not shake off their customary sloth unless he

stimulates them with sumptuous fare. At the end of a week he finds

that proteins are not the best fuel for steady work. If he consumes

sufficient to supply all the energy needed by his muscles, he is

hampered by a quantity of nitrogenous residues which have to be reduced

to urea and eliminated by the kidneys. He goes back approximately to

his old regimen, so far as proteins are concerned, and consumes more

carbohydrates for the supply of the force which his increased muscular

activity demands.

It is possible to live on meat alone, but the quantity required is very

great, involving the digestive organs, the liver, and the kidneys in an

excessive amount of work. On the other hand, it is possible to reduce

the consumption of proteins to a minimum by substituting for them fats

and carbohydrates. But, again, after the proper balance is disturbed,

the substitution ceases to be a simple problem in arithmetic. The

carbon-food has to be increased out of all proportion to the protein

which it replaces. If a dog which is being fed on a diet natural

to it—chiefly meat—is in a condition of nitrogenous equilibrium,

carbohydrate may be substituted for some of the meat. But from the very

beginning it is found that, if nitrogenous equilibrium is not to be

disturbed (if the dog is not to be induced to consume its own tissues),

a weight of carbohydrate must be given considerably greater than the

weight of the protein withdrawn. The disproportion increases as the

experiment proceeds, until perhaps 12 to 15 grammes of carbohydrate

have to be substituted for every gramme of protein. The proteid food

has now come down to 1·5 gramme per kilogramme of the animal’s weight.

Owing to the increase of carbohydrate, the caloric value of the total

food, nitrogenous and non-nitrogenous, is several times as great as

the animal requires. The surplus is oxidized without any equivalent in

work. At about this point the experiment is brought to an end, owing

to the failure of the digestive organs to deal with so large a mass of

food.

The value of gelatin as an article of diet is of interest in this

connection. Gelatin is not, strictly speaking, a protein, and it cannot

be built up into the tissues. It does not prevent, nor even delay,

starvation. Yet up to a certain point it can be used as a substitute

for proteid food. In the observation just referred to, protein might

be withdrawn at any stage, without disturbing nitrogenous equilibrium,

by substituting about 2 grammes of gelatin for every gramme of protein

withdrawn. It spares protein, although it does not take its place.

It is said that the minimum of protein necessary for the maintenance

of nitrogenous equilibrium may be reduced to about one-half by the

substitution of gelatin. This has been interpreted as indicating

that when we have reduced the oxidation of nitrogenous substance to

its smallest amount the nitrogen comes from two sources in about

equal proportions—(_a_) the bioplasm; (_b_) the food-proteins in

contact with it. It is inferred that gelatin, although it cannot be

built up into bioplasm, may take the place of proteins present in the

cell-juice. It appears to be impossible to starve the cell until it

consists of a bioplasm framework bathed in nitrogen-free cell-juice.

As the non-living proteins of cell-juice are removed, they are, if no

nitrogenous food be given, renewed by the breaking down of bioplasm.

When gelatin is absorbed, it takes its place in the cell-juice, and

the breaking down of bioplasm is no longer necessary. When digestion

is impaired, or vitality lowered, decoctions of meat which contain

extractives of low calorific value, useless, without synthesis (_cf._

p. 144), for the purposes of tissue-repair, may to a certain extent

save tissue-waste. In the same way, gelatin, which is very rapidly

digested in the stomach, may cover the consumption of proteins,

although it cannot take their place.

To sum up: The requisite daily income of energy must come from both

nitrogenous and non-nitrogenous food. It is impossible to reduce the

nitrogenous factor below a certain minimum. From this minimum upwards,

until a certain level is reached, every additional unit of nitrogenous

food enables the system to dispense with more than its equivalent of

non-nitrogenous food. When the proper balance of foods is attained,

there is no waste either of labour involved in digestion, or of labour

involved in metabolism and excretion.

=The Liver.=—The liver weighs from 3 to 3½ pounds. It lies beneath

the diaphragm, more on the right side than on the left. Its posterior

border, which rests against the last three ribs (separated from them by

the diaphragm), is about 3 inches thick. Its anterior border is thin,

and keeps close along the line of the ribs. If the organ is neither

unduly enlarged nor squeezed out of its place owing to the use of a

tight corset, it does not project below the ribs, save where it crosses

the space between the rib-cartilages below the end of the breast-bone.

[Illustration]

[Illustration: FIG. 7.—DIAGRAM OF A LOBULE OF THE LIVER DIVIDED

VERTICALLY THROUGH ITS AXIS.

In its centre is a space, the intralobular vein,

through which the blood falls into a branch of

the hepatic vein, on its way to the heart. An

interlobular branch of the portal vein, which

brings the blood from the digestive organs, pours

it by many smaller vessels over the surface of

the lobule. It filters into the lobule through

innumerable pseudo-capillary vessels, or spaces,

between the radiating columns of liver-cells.

Arterial blood is brought to the lobule by a

twig of the hepatic artery. Bile is drained away

from it by an affluent of the hepatic duct. In

the lower part of the diagram seven liver-cells

are shown, forming a divided column, magnified

about 300 diameters. The cells are loaded with

glycogen, and contain minute globules of fat.

Red blood-corpuscles and two leucocytes are seen

between the columns of liver-cells. One of the

leucocytes has ingested two blood-corpuscles.]

The liver is supplied with blood by the hepatic artery. This vessel is

small for so large an organ. Although responsible for the nutrition

of the liver, it does not bring it the materials which are stored in

its cells. A much larger supply of blood is derived from the portal

vein, which breaks into capillaries, or, to speak more accurately,

into sinuses, or pseudo-capillaries, in the liver. The blood, whether

conveyed to the organ by the hepatic artery or by the portal vein, is

drained away by the hepatic veins. The plan of structure of the liver

is best understood when viewed with reference to the hepatic veins.

These, if traced backwards, are found to break up into fairly straight

vessels, each of which has a large number of lateral branches. Each

of the lateral branches is in the centre of a mass of cells, which

are packed round it in radiating columns. These masses, which have a

diameter of from 1 to 2 millimetres, are termed “lobules.” By mutual

pressure the lobules are squeezed into a pentagonal or hexagonal form.

The vein in the centre of the lobule is the intralobular vein. Turn now

to the portal vein; this is seen to break up into branches which run

between the lobules, and are therefore termed “interlobular veins.”

The branches of the hepatic artery also run between the lobules, as do

the radicles of the bile-duct. Each lobule is a liver in miniature.

The blood of the portal vein, which has come from the spleen, in

which red blood-corpuscles are destroyed, and from the stomach and

intestines, from which it has absorbed the products of digestion, is

poured over the surface of the lobule, to be filtered through into

its central intralobular vein. In its passage from the interlobular

veins (and branches of the hepatic artery) to the intralobular vein

the blood is confined to radiating capillary channels; but since these

merely prevent the escape of red blood-corpuscles without imposing any

restrictions upon the exudation of blood-plasma, the portal blood is to

all intents and purposes filtered through the columns of liver-cells.

The body-substance of the liver-cells is soft, destitute of envelope,

and capable, when free on the (warmed) stage of a microscope, of

changing in form, somewhat after the manner of a leucocyte. Such

cells have a great capacity for taking up the products of digestion.

Possibly they take up and store fats and proteins, but undoubtedly it

is their chief business to absorb sugar which accumulates as glycogen

in their substance. The glycogen is handed out to the hepatic blood

as required. The pigment which results from the disintegration of red

blood-corpuscles in the spleen is secreted, along with the bile-salts,

into minute channels, or canaliculi, which groove the flat surfaces of

adjacent liver-cells. These canaliculi converge to the bile-ducts. The

liver is therefore at the same time the storehouse of sugar which it

takes up from the blood when it is in excess, and passes out to the

blood when it is deficient, and an excretory organ which eliminates the

refuse of hæmoglobin. The iron derived from hæmoglobin it stores, and

returns to the blood.

Another function of the liver has been referred to already. It is the

organ, and, as far as we know, the only organ, in which urea is made

in mammals, and uric acid in birds. If the liver of a freshly killed

animal be excised and a stream of blood passed through it, the blood

which leaves the organ contains urea. If a salt of ammonia, even the

carbonate, be added to the blood, it is converted by the liver into

urea. When a bird’s liver is made the subject of the same experiment,

uric acid appears instead of urea. The liver can convert many

nitrogenous substances into urea, but it seems probable that, normally,

the salt with which it has chiefly to deal is lactate of ammonia (_cf._

p. 146).

A few words must be added with regard to the functions of the liver

during prenatal life, obscure though these functions are. The liver

develops very early, and attains a relatively enormous size. At the

third month it weighs as much as the whole of the rest of the body

(_cf._ p. 34). Yet it cannot, one must suppose, have to do much of the

work which falls to its share in postnatal life. Food is reaching the

embryo in a constant stream, and not as the result of intermittent

meals. The embryo has no need to store glycogen; nor does its liver,

on analysis, yield much of this substance. In the embryo glycogen is

widely distributed throughout the tissues, not specially accumulated

in the liver. No digestion is occurring in the alimentary canal. Bile

is not needed to aid the hydrolysis and absorption of fats. A small

quantity of cholesterin and less lecithin is being eliminated, but not

much bile is needed to facilitate this process.

A process which is proceeding at a great rate in the embryo, in

various situations, is the formation of red blood-corpuscles. In

this the liver takes part. But its duty in regard to blood-formation

is not sufficiently onerous to account for its size. The formation

of blood-corpuscles in the liver is observed with difficulty in

microscopic sections. It is therefore impossible to speak with

certainty as to the extent to which it is going on, but it may be

safely asserted that this function by itself cannot be held to account

for the great size of the organ in embryonic life. What other office it

fills at this period is a question which still awaits an answer.

There is no more curious chapter in medical history than the story of

the views held at various periods with regard to the functions of the

liver. From being a mere mass of “parenchyma” serving as packing for

the abdominal viscera, it was elevated to the rank of Grand Purifier

of the “humours” of the body. Next, its excessive activity became

the cause of that form of dyspepsia known as “biliousness.” Still

later its want of activity was its chief vice. A “sluggish” liver was

held responsible for mental perversity and moral dulness. Calomel,

podophyllin, and other drugs were used as whips to stir it up; and the

increased secretions of the alimentary canal were mistaken for bile.

Poor patient organ! It is the still-room of the body, in which the

day’s supplies are stored, and from which they are served out, without

haste and without delay. And it makes urea. What else it does we have

yet to find out; and it is not impossible that when physiologists

have quite shaken themselves free from the explanations based upon

conjecture, which their predecessors have handed down, they may

discover that it has other duties which are not obvious, but of great

importance.

FOOTNOTE:

[2] Notice the distinction between filtration and dialysis. If water

containing soluble and insoluble substances is placed in a porous jar,

the water and the soluble substances pass through the pores of the jar.

The rate of flow varies as the pressure. If water containing soluble

substances is placed in a bladder, and the bladder is suspended in a

vessel of water, some of the substances which it may contain—white

of egg, for example—are non-diffusible; others will pass from the

water inside the bladder to the water which surrounds it. But every

diffusible substance has its own osmotic value. Some pass through the

membrane rapidly, soon establishing a condition of equilibrium in the

two fluids; others take a long time. Further, if the water on one side

of the membrane contains a certain percentage weight of a salt, the

molecules of which are large—say sulphate of magnesia—and the water

on the other side the same percentage weight of a salt of smaller

molecule—say chloride of sodium—water containing the salt of smaller

molecule will pass into the water containing the salt of heavier

molecule with a certain force. If, to start with, the two solutions

are at the same level, the level of the solution containing the less

diffusible salt, sulphate of magnesia, will at the commencement of

the experiment rise. It is therefore said to exert a greater osmotic

pressure than the more diffusible salt—chloride of sodium. Equilibrium

will not be established until the fluid on one side of the membrane

contains the same _number_ of molecules per unit volume as the fluid

on the other side. If the molecules of magnesic sulphate are pictured

as oranges, and the molecules of sodic chloride as nuts, it will be

understood that equilibrium is not established until the oranges and

nuts to the pint on one side equal in number the oranges and nuts

to the pint on the other. When these principles are applied to the

passage of water containing products of digestion through the wall of

the alimentary canal, it is evident that, if we understand all the

conditions, the process cannot be explained as merely an exhibition of

osmosis. Take the simplest illustration. When blood-serum is placed in

the intestine it is absorbed. If it were in a dialyser, there would

be equilibrium between the serum inside the intestine and the lymph

on the outside. There would be no osmosis. Or, again, supposing water

containing 2% of common salt is placed in the intestine, we find that

both salt and water pass through into the lymph. In a dialyser water

would pass from the lymph (which contains salts equal to about 0·9%

of sodic chloride) through the membrane into the stronger solution.

A salt-solution needs to be very concentrated to cause water to take

the reverse course through the intestinal wall, and so to act as a

purgative. When we study absorption from the alimentary canal, we find

that its wall, if it wants a salt or any other substance, sets the

laws of osmosis at defiance. If the salt is not wanted, the ordinary

phenomena of osmosis are exhibited. Sulphate of magnesia (Epsom salt)

would be deleterious if absorbed. The intestinal wall behaves towards

it like a dead membrane. The salt retains the water in which it is

dissolved: possibly water passes out of the lymph into the solution

of the salt. The contents of the intestines are in consequence unduly

liquid. The salt acts as a purge.