plainly indicates, that an identity of this kind
would be equally impossible in detail. Thus
it is not improbable, that the tissues of every
individual possess chemical peculiarities more
or less specific to himself. And it is all but
certain, that the various proximate principles
isolated by the chemist are not definite com-
binations of certain elements in equivalent
proportions as are the salts, acids, and alka-
lies of the inorganic world but rather ever-
varying mixtures. Those various forms of
protein which it is so convenient to distinguish
by the names of albumen, fibrin, and casein,
may indeed be separated from the tissues of
animals, and even of vegetables, by the same
rough processes ; and may therefore respec-
tively exhibit the closest resemblance in their
composition and properties. But an accu-
rate analysis would probably show, that the
organic substance represented by either of
these terms is never precisely identical in any
two specimens. It is the total of a number of
constituents, the result of a variety of pro-
cesses, the end of a serial metamorphosis :
rather than a definite and specific compound
of carbon, oxygen, hydrogen, and nitrogen.
And not only is there no identity in the
composition of the organism and the ingesta,
but it would seem that there are some tissues
of the body which have absolutely no repre-
sentative in the food : no kindred substance to
which their formation can possibly be referred.
Such are the various tissues that yield gela-
tine ; a substance which, though it appears
to escape assimilation when introduced into
the organism from without, is yet constantly
formed within it, from the metamorphoses of
other parts of its substance.
The chemistry of nutrition therefore implies
neither construction, on the one hand, nor
identity, on the other ; but something mid-
way between these two extremes. Its forces
occupy, so to speak, a debateable ground
between the prehension of old materials, and
the formation of new ones. And the food
submitted to its action is only required to
possess such a similarity of composition with
the body, as will concede these limited
changes, without implying any wider process
of metamorphosis.
Any exact definition of the degree of re-
semblance thus requisite, would be foreign to
our present object. Indeed, in the existing
state of our knowledge, it is impossible to
specify the precise nature of those metamor-
phoses, which accompany the digestive act,
and are bounded by the food and the organ-
ism as their respective beginning and end.
It is enough to indicate, that they appear to
be intermediate between the forces of che-
384-
mical affinity on the one hand, and homoge-
neous and heterogeneous adhesion on the
other; and that while they are sometimes *
akin to the formation of hydrates, they oc-
casionally resemble those still more recon-
dite phenomena which are concerned in the
production of isomeric or isomorphous com-
pounds : substances which, though identical
in their composition, offer striking differences
in their solubility, as well as in many of their
chemical properties and reactions.
This very limited convertibility of the main
components of the food, renders their variety
almost as essential, as though each different
tissue of the body had required the entry of
its corresponding substance from without. In
other words, within the range of the chemical
parallelism just mentioned, the organism de-
mands alimentary compounds containing all
the different ingredients necessary to cover its
own waste.
This fact receives a good illustration from
that selection which the instinct of most per-
sons would impel them to make. Left to
himself, Man always chuses a mixed diet,
composed of proper quantities of animal and
vegetable, liquid and solid, matter. Nay more,
that almost equally imperious instinct which
urges him to vary his diet, though often con-
fused with the morbid cravings of luxury, is
essentially nothing less than an expression of
the natural wants of a healthy organism.
Obscured, however, as these really natural
instincts often are by the stereotyped tastes
and habits of a highly artificial state of
society, we gain a far better insight into the
proper composition of food, by examining
that store of nutriment which, in the shape
of the yolk of the Bird's egg, or the milk of
the Mammal, Nature herself provides for the
maintenance of the young of these classes.
Of these two substances, the milk is justly
regarded as forming the very best example of
a proper food : both as regards the nature of
its several ingredients, and the proportions in
which they are mingled with each other.
Milk. The alimentary properties of the
milk are due to the presence of a number of
proximate constituents, the more impor-
tant of which may be enumerated as fol-
lows. (1) A protein- compound, casein;
(2) a hydro-carbon or fat ; (3) a hydrate of
carbon or sugar; (4) certain salts; and
(5) the water in which the whole of these
materials are suspended or dissolved. Of
these five groups of substances, at least four
are indispensable ingredients of every proper
food. The hydrate of carbon and the hydro-
carbon are, to some extent, capable of forming
substitutes for each other. But with this ex-
ception, (an explanation of which will be
attempted by and by), the absence of any one
of these constituents, or even its presence in
insufficient quantity, suffices to destroy the
capacity of any particular food for maintaining
life ; so that an animal limited to such a diet
* Compare the remarks on the gastric juice at
p. 337.
STOMACH AND INTESTINE.
ultimately dies with appearances of inanition.
And a fortiori, the ingestion of but one of
these alimentary ingredients, such as albu-
men, fat, or sugar, is soon attended with
effects which still more closely resemble those
of starvation. Such a diet does indeed essen-
tially starve the entire organism, even while
it supplies some of the constituents of its
lost substance. For although the unchecked
waste of the remaining constituents of its
mass tells upon certain of its textures with
greater rapidity and energy than on others,
still it ultimately involves the whole in a
common destruction : a fact which need
little surprise us, when we recollect the
mixed composition of the simplest tissues, and
the intimate mutual dependence of the most
distant and isolated parts.
Constituents of food. 1. The first group,
consisting of what are called the protein-
compounds, includes a number of proxi-
mate principles, which are derived from both
the animal and vegetable kingdoms of nature.
The chief of these principles are albumen,
fibrin, and casein. By digestion in solution
of potash, and precipitation with an acid,
either of these yields a substance called pro-
tein: a name that alludes to the relation
this principle is supposed to bear to all the
compounds from which it is thus obtained.
It is regarded as their common starting-point
(TTjOfam ua, primaspartes teneo), and most essen-
tial component. And the slight differences
of composition offered by each particular pro-
tein-compound, are explained as chiefly due
to variations in the nature and amount of
certain collateral ingredients, the addition of
which to protein imparts the specific charac-
ters of albumen, fibrin, or the like. Hence
the various protein-compounds are supposed
to differ, not so much in elementary com-
position, as in certain characters which might
almost be termed morphological : namely,
outward form, physical properties, degree of
solubility, and the like.
The exact process by which one of these
protein-compounds undergoes conversion into
another is still a complete mystery. But that
such changes do constantly obtain, cannot be
doubted. And hence, while the quantity of
albumen in the animal body, and the constancy
with which it is present, assign to this proxi-
mate principle the leading position in the
above group of proteinous substances, it is on
its generic, and not its specific, properties
that our attention ought chiefly to be fixed.
The protein of the food may be regarded
as its most essential constituent. The reason
why such an importance is ascribed to it
becomes sufficiently evident, when we com-
pare its composition with that of the body
which it is intended to nourish. The highly
azotized constitution it possesses (C 54'7
+ H 6-8 + N 14-2 -f O 24-3 = 100) closely
approaches that of the solids of the organism
generally. And it shows an equally important
relation to most of the tissues in detail. It
forms a large constituent of the blood ; and
therefore of the plastic nutritional fluid that
STOMACH AND INTESTINE.
385
exsudes directly from this fluid. It is the main
component of the muscles, which execute
the various movements of the body. It is an
equally important ingredient in the tissues
of both the central and peripheric parts of the
nervous system. It is probably the- source of
the gelatinous * tissues ; which, in herbivorous
animals, can only be derived from a kind of
degradation or regression of the albuminous
substances. And, finally, its large amount in
the structures of the foetus proves that it is just
as important to the evolution and growth of
the animal, as it is to its maintenance. In
short, in protein and its various kindred sub-
stances, we recognize the principle, which
forms the material exponent of all the struc-
tures and functions, and is the chief sub-
stantive agent of the chemistry of life.
The quantity of protein necessary for the
proper maintenance of the healthy animal can
only be estimated from very indirect and ap-
proximate calculations.
In milk, the albuminous compounds are
chiefly represented by casein, which forms
about 3^ per cent, of its total quantity. But
we can scarcely guess how much milk is
daily consumed by the sucking animal, or
what proportion this amount bears to the
weight of its whole body. And we are
justified in assuming, that a large fraction of
the protein thus introduced into the system,
is applied to exigencies of growth and deve-
lopment which have little or no place in the
adult animal.
Assuming an exact maintenance of the
adult organism, without increase or decrease,
we might expect that an examination of
its various azotized excretions would teach
us how much nitrogen had been discharged
from the system within a given time : and
hence that, by comparing this quantity with
the known elementary composition of protein,
we might be enabled to calculate how large a
quantity of the azotizedf constituent of the
food ought to be added to the system, in order
to replace its daily loss.
But here we are met by a difficulty con-
nected with the process of nutrition itself:
\\ith that chain of events of which food and
waste constitute only the extreme links. The
amount of nitrogen given oflfby the body does
not depend solely upon the quantity excreted
by its \vaste, but also varies in close corre-
spondence with the quantity taken in its
food. It is therefore greater in carnivorous,
and less in herbivorous, animals.
Hence the true or essential waste of the
organism, in respect of this constituent, can
only be determined from an analysis of the ex-
cretions of animals which have been kept for a
day or two, either without food, or on a diet
altogether devoid of nitrogen. In both cases
the results are the same. The nitrogen of
* Ignorant as we are, both of the nature of this
metamorphosis, and of the various stages through
which it is conducted, there are reasons for con-
jecturing that the formation of the chondrin radicle
generally precedes that of the substance which yields
gelatin by boiling.
Supp.
the egesta drops to a certain minimum ; at
which it remains for a considerable period.
The quantity of nitrogen evolved by the
lungs and skin is at any rate so small, as
scarcely to form an important element of cal-
culation. And even the larger quantity
excreted in the biliary resin, hardly deserves
notice. It is in the uric acid, and above all
in the urea, of the renal secretion, that this
element is chiefly dismissed from the body as
an effete compound. And hence it is from
the urea found in such experiments that we
may best deduce the probable rate of daily
waste in the albuminous tissues ; and the cor-
responding quantity of protein which there-
fore has to be supplied in the daily food.
From observations of this kind on the
human subject, we may infer that, in Man,
the albumen of the adult organism under-
goes a loss of about 2 ounces daily ; a quan-
tity which corresponds to scarcely more than
T5 Voth of the weight of the body. While if
we suppose that a new-born infant, weighing
six or seven pounds, consumes daily about ten
or twelve ounces of milk, containing 3* per
cent, of casein, the quantity of protein thus
introduced into its alimentary canal would
amount to the larger proportion of about ^th
of its total bodily mass.
The larger proportion of albumen thus
consumed by the infant probably depends
upon at least two causes. As a smaller*
animal, it is subject to a more energetic waste
of substance. And as a growing animal,
it not only lays aside in its body a constant
surplus of its income over its waste; but
possibly undergoes a more active metamor-
phosis, that still further increases the propor-
tion of its effete materials.
But, apart from the influence of age or
size, there is no donbt that a careful compari-
son of the azotized ingesta and egesta would
always show a marked disproportion between
the two. There are indeed obvious reasons,
why the nitrogenous constituent of any
suitable food should always greatly exceed
that quantity which is required by the strict
exigencies of the organism. A part of the
casein which is contained in the milk taken
by the sucking-child, is often found to pass
through the alimentary canal without being
absorbed into the blood. And in the case of
many other varieties of food, the insoluble
state of the protein-compounds actually
present affords a still greater obstacle to their
absorption. In a proper mixed diet, how-
ever, we may detect some approximation be-
tween the presumable gain and loss. Thus the
daily rations of the British soldier on home
service include little more than 5 ounces of
albuminous substance ; a quantity which is
therefore little more than double the amount
* From researches by Frerichs, Lehmann, Bidder,
Schmidt, Boussingault, Valentin, and others, we
may estimate the daily waste of albuminous com-
pounds, relatively to the whole body, in the under-
mentioned animals, as follows: Rabbit, T ] th;
Cat, T } g th; Dog, 3 ^th; Horse, ^th.
c c
386
STOMACH AND INTESTINE.
of this material, which the waste of his body
probably dismisses from his system within
the same period of time.
2. The next group of alimentary substances
is that of the fats, the composition of which
has led to their receiving the generic name of
hydro- carbons. They are found in both
animal and vegetable food. In the milk, they
are represented by its butter ; the quantity of
which amounts, on an average, to about 3|
per cent.
The great variety of different alimentary
substances of this kind is such as to pre-
clude even their enumeration. The most
important are stearin, elain, and margarin.
The composition of these three fats may be
generally stated as almost corresponding to
single equivalents of carbon and hydrogen:
or, more exactly, to ten atoms of each of
these elements, minus one of hydrogen, and
plus one of oxygen (C 10 H 9 Oj ; or C 7
The uses sustained by these constitu-
ents of the food in the organism are easy
to indicate, but difficult to specify. The
protection afforded by the fat of the body to
its temperature, and to the mechanical safety
of its internal structures, might perhaps be ac-
complished without involving any rapid waste
and replacement of the material by which it
is afforded. But the vast quantity of fatty
matter which enters into the composition of
the nervous system, and the primary import-
ance of this delicate and energetic organ to
the maintenance of life, entitle us to infer, that
its functions imply such a rapid metamorphosis
of its substance, as can only be sustained by the
continual supply of new materials to replace
those rendered effete. And the numerical
phenomena of nutrition further show, that
the process of respiration is constantly dis-
missing from the body an amount of carbonic
acid, the proportion of which to the azotized
egesta proves that it must have been derived
more or less directly from an oxidation of the
fatty, as well as of the albuminous, tissues.
The quantity of fatty matter contained in the
healthy organism strongly confirms these
views ; and thus helps to account for its
dietetic importance. For, including all their
varieties in the tissues just alluded to, we
can hardly estimate the total hydro-car-
bons of the human body at less than ^th or
-^th of its weight. And since they scarcely
form e^-cyth part of the blood, it follows, that
even assuming this nutrient fluid equal to
th of the corporeal weight, its fatty con-
stituent amounts to little more than ^ tn or
g^oth of the fat which is deposited in the
central andperipheric structures of the nervous
system, and stored up in the adipose cells
of other parts of the body. Such an estimate
further entitles us to conjecture, not only that
the quantity of fat taken up at any one time
by the digestive organs is limited to a very
small one ; but also, that it either undergoes
some important metamorphosis before reach-
ing the general mass of the blood, or is very
rapidly eliminated from this fluid.
3. The hydrates of carbon form a class of
nutritional substances, the elementary com-
position of which is still more exactly indi-
cated by their name. In other words, they
consist of carbon, united with hydrogen and
oxygen in those equivalent proportions of
these two elements which are necessary for
the formation of water (C ]2 H 12 O J2 ). This
group is a very large one : and includes, not
only the various forms of cane, grape, and
milk sugar, but a number of kindred sub-
stances; such as dextrin, gum, cellulose,
inosit, and, especially, starch. All of these
organic principles, however various their
physical properties, have nevertheless the
same chemical composition. And many of
them are easily converted into grape sugar;
either by the excitement of a limited meta-
morphosis by an azotized ferment, or by expo-
sure to the action of dilute acids.
The sugary ingredient of the milk forms
about 5^ per cent, of its quantity ; and is the
only representative of the hydrates of
carbon which it contains.
The average amount of the substances
belonging to this and the preceding group
of alimentary constituents will of course
vary greatly in the different kinds of food.
Speaking generally, however, these two
groups may be stated to predominate by
turns in the food derived from the two
kingdoms of nature. Thus while the hydro-
carbons are chiefly derived from the fat of
animal food ; the hydrates of carbon belong
even more exclusively to the starch and sugar
of vegetable food. But, in strictness, no
such marked difference can actually be made
out between the two kinds of food in this re-
spect. The milk, the liver, and even the blood of
the animal, all contain sugar : while inosit, a
substance closely allied to sugar, forms an im-
portant constituent of its various muscles. And
not only do many plants contain large quan-
tities of oily matter stored up in various parts
of their tissues, but even the seeds of the ce-
realia, which form the best vegetable diet,
present an amount of fat ranging from '2 to
2 per cent.
The purposes fulfilled by these hydrates of
carbon in the animal economy, offer a marked
contrast to those subserved by the two pre-
vious groups. The protein compounds form
what is eminently the basis of the organism ;
the plasma from which are developed the
blood and the tissues. They are thus his-
togenetic and hcemagenetic, as the phrase is.
The fatty matters of the body not only form
a large constituent of the active nervous sub-
stance, but are also retained and stored up
in the more inert and passive form of adi-
pose tissue. While the grape-sugar, into
which the various hydrates of carbon are all
finally converted, appears never to assume
any permanent form in the body, but to be
always rapidly eliminated from the blood. In
what shape, or after what metamorphoses, it
leaves this fluid, is at present uncertain. It
is, however, probable, that like the hydro-
carbons, these hydrates of carbon are essen-
STOMACH AND INTESTINE.
387
tially a species of fuel for that process of
calorific combustion,which pervades the whole
body, and which discharges its resulting car-
bonic acid by means of the respiratory func-
tion. And Liebig has adduced numerical
data from the fattening of animals, which lead
him to suppose, that these substances are
also capable of undergoing a process of de-
oxidation, that converts them into fat, and
thus enables them to augment the adipose
tissue. But this view rests on very insuf-
ficient foundations*: and is curiously con-
trasted with that oxidationf of hydro-carbons
into sugar, which the researches of various
recent observers seem to indicate as one of
the chief functions of the liver .J
4. The importance of the ivater of the food
is such as justly entitles this liquid to the
rank of a fourth alimentary constituent.
For it forms about four-fifths of the entire
corporeal mass : and undergoes, at the va-
rious excretory surfaces of the skin, the lungs,
and the kidneys, a continual expenditure ;
the replacement of which is obviously neces-
sary to the maintenance of the proper com-
position of the body.
The way in which this large aqueous con-
stituent facilitates the action of the various
organs is not very difficult to conjecture.
Their merely physical properties of hardness,
flexibility, and the like, often seem chiefly
determined by the quantity of the watery in-
gredient which they contain. And their far
more recondite vital properties seem quite as
immediately under its influence. Thus not
only do its solvent powers appear to be
eminently useful in furthering the minute
division, and the local transfer, of various
organic substances, but we are justified in
conjecturing that it gives a more specific
chemical assistance to many of those pro-
cesses of metamorphosis which are so inti-
mately connected with life. In both of these
respects, it would seem to afford a special aid
to the function of digestion. While that act
* The increase of fatty matter supposed to have
been derived from these hydrates was calculated by
subtracting the fat added in the vegetable food
from the increase of the animal's weight ; this sur-
plus being set down as due to augmented adipose
tissue. Hence any error in estimating the fatty con-
stituent of this food, on the one hand or any neglect
to calculate the watery and proteinous constituents
of the increased adipose tissue, on the other would
partially account for the difference observed. And
it seems not unlikely that both of these inaccuracies
actually occurred in these observations.
f Assuming that such a metamorphosis really-
obtained, it would not be difficult to explain most of
Liebig's results. For it is surely not impossible,
that the presence of an excess of sugar in the liver
might diminish the energy of this act; in other
words, that an excess of the product might lessen
the activity of the process. Thus the copious in-
gestion of sugar might check its formation, and
diminish the metamorphosis of the fat supplied to
the liver in the portal blood. And this retention
of the fatty fonn might not only affect the hydro-
carbons of the food, but also those which are pos-
sibly developed in the organism from its own pro-
teinous constituents.
% Compare the remarks on the liver, at p. 401.
of absorption, which conveys the dissolved
contents of the alimentary canal into the
surrounding veins, is greatly facilitated by
the heightened diffusive energy which the low
specific" gravity of water enables it to impart
to the fluids with which it has been mixed.
And finally, the use of water in relation to
the opposite extreme of nutrition namely,
to excretion may be well exemplified by the
urine, in which a highly poisonous product
of life is continually washed out of the system,
through the instrumentality of a stream of