_Composition of the Digestive Organs and of their Secretions—Chemical
Phenomena of Digestion_.
_Mucus._—The living membrane of the alimentary canal is moistened
with a liquid possessing many of the characteristics of vegetable
mucus, but containing nitrogen. It is a thick, tenacious substance,
which contains, dissolved in the water through which it is diffused,
the ordinary salts of the serum of the blood; it swells up with water
to a considerable mass, but without dissolving; it dissolves in
alkaline liquors, and is precipitated therefrom on the addition of an
acid and the tincture of galls; the mucus from different parts of the
mucus membrane is, however, by no means identical in properties.
The liquid secreted by the internal surface of the stomach—the
gastric juice—which exercises an important influence on digestion,
differs essentially in its character from mucus. When the stomach
is empty and contracted, it contains ordinary mucus; but if even
indigestible substances are introduced, and still more, after taking
proper food, a liquid is abundantly poured out, which is colorless
or pale yellow, and contains a very small quantity of solid matter
(two per cent.), which consists principally of inorganic salts
(common salts and sal ammoniac, with a trace of a salt of iron); it
is specially characterized by the presence of a notable quantity of
free muriatic acid, the proportions of which vary with the activity
of the digestive powers at the time. This gastric juice possesses
the remarkable property of softening down and dissolving fibrine and
albumen, and thus converts the masses of food into the uniform pulp
(chyme), from which the absorbing vessels of the small intestines
take up the nutritious elements.
If we form an artificial gastric juice by mixing together the
muriatic acid and salts in the right proportions, it is found to
be totally incapable of dissolving the materials of the food,
and, indeed to be quite inactive towards digestion. The organic
material of the gastric juice, although its quantity be so minute,
is, therefore, essential to its powers, and these may be perfectly
conferred upon the previously inactive, artificial juice, by the
addition of a little of the mucus of the stomach or by steeping
in the acid liquor, for a short time, a small portion of a mucous
membrane, and filtering the liquor; for this purpose it is not even
necessary to use the mucous membrane of the stomach, for that of the
bladder has been found to answer equally well. The substance which is
dissolved out of the membrane in these cases has been termed pepsine.
It has not been obtained in a truly isolated or pure form, but its
properties are very remarkable. For its full activity it requires
the presence of a free acid, as the artificial gastric juice becomes
much less active in dissolving food when neutralized by an alkali,
though it retains other properties, as that of coagulating milk-like
rennet. If the artificial juice be precipitated by nitrate of lead,
the precipitate washed, and then decomposed by sulphuret of hydrogen;
the solution thus obtained possesses all the digestive powers of
the juice. Hence, the pepsine and muriatic acid act together, by
combining with oxide of lead.
Pepsine appears to be completely decomposed by contact with alcohol
or boiling water; its powers are also destroyed by deodorizing
substances; the solution of albumen and fibrine in gastric juice
differs essentially from their solution in muriatic acid, as in the
former case the quantity of acid is very minute, in relation to the
quantity of material dissolved, and after solution the acid remains
quite uncombined.
The action of the stomach in digestion, appears, therefore, so far as
our actual knowledge extends, a purely catalytic fermentative action;
one in which the active excitant is an organic substance (pepsine),
secreted by the mucous surface, and whose properties are developed by
the presence of muriatic acid, which is secreted at the same time.
The new products into which the food, fibrine, albumen, gluten,
starch, oils, sugar, etc., are converted, and which collectively
constitute the white uniform pulp, termed by physiologists chyme,
have not been made the subject of accurate chemical researches.
In the mouth the mass of nutritive material is acted on by a liquid
which is secreted by the salivary glands, the saliva. It is alkaline,
and holds in solution not one per cent. of solid matter, which
contains some carbonate of soda and common salt, admixed mucous, and
a peculiar organic body, termed salivary matter.
This last substance is soluble in water; its solution is not
coagulated by heat, nor precipitated by tincture of galls, corrosive
sublimate, acetate of lead, nor by acids. The pancreas, so similar
in structure to the salivary glands, has a different secretion;
it contains no salivary matter, but albumen and some salts; it is
generally slightly acid.
_Composition of the Bile._—The precise part which this remarkable
secretion performs in the animal economy is not yet fully known; it
has been the subject of repeated and accurate chemical examination,
although, from the facility with which its elements are transferred
into other bodies, by the action of the reagents employed, every
succeeding analysis has led to different results.
_The Coloring matter of the Bile._—is present during health but in
small quantity, but in disease it sometimes accumulates so as to
form solid masses. When pure, it is a reddish-yellow powder, which
is scarcely soluble in water or in alcohol, but dissolves easily in
a solution of caustic potash. This solution is of a clear yellow
color, but when exposed to the air it becomes deep green, absorbing
oxygen. This change is remarkably produced by nitric acid, and it is
indeed the reaction by which the presence of the bile in the serum of
the blood, in the skin, in the urine, and eyes, etc., may be shown in
cases of jaundice.
_Chyle and Lymph._—The nutritive materials extracted from the food by
the absorbing vessels of the intestines, is thrown into the thoracic
duct, where it meets with another fluid, which is transmitted to
the same vessel from all parts of the body by the colorless veins
or lymphatics. The fluid from the intestines is termed chyle; that
from the body is generally termed lymph. It is the mixture of these
that has alone been examined, for the vessels which carry either
separately are too minute to allow of the extraction of their
contents in a pure form.
When taken from the thoracic duct, a few hours after a meal, when,
probably, the chylous element prevails, it is whitish, opaque,
liquid, like milk, with generally a reddish shade; a short time after
separation from the body, it coagulates; the clot is at first pale,
but it soon becomes light crimson red; the milkiness of the serum is
due to the presence of oil; it contains albumen, and coagulates by
heat; except that it is more dilute, and that the hematosine is for
the most part absent, the chyle and lymph have the same composition
as the blood. It appears to vary, however, with the nature of the
food, as Dr. Prout found the chyle of persons fed on vegetables to
contain a much smaller quantity of albumen than when they had had
animal food.
Dr. Prout also indicates in chyle the presence of a substance which
he terms incipient albumen, which is not coagulated by heat, except
after the addition of acetic acid; the properties of this form of
albumen, however, are not fully known.
_Constitution of the Urine in Health and Disease._—The nature of this
secretion has at all times been an object of considerable interest to
the chemist, from the indications which changes in its composition
give of diseases of important organs and from the number and interest
of the different organic substances it contains. As in almost all
other branches of animal chemistry, Burzelius first determined its
composition, and lately Lecaner has ascertained with great care
the limits to which the proportions of its ingredients may vary in
health, and this established a correct basis of comparison for urine
in the various conditions of diseases.
_Of the Urine in Disease and after Death—Urinary Calculi._—To the
chemist, the indications of disease of the urinary and digestive
organs, formed by changes in the composition of urine, are most
valuable. The majority of the substances which are taken into the
circulation, but are incapable of assimilation to our organs, are
thrown off by this secretion, and hence a variety of medicinal
substances may be traced to it after having been ingested,
sometimes quite unaltered, at others modified in their natures.
Thus if alkaline salts of organic acids be taken into the stomach,
the organic material is oxidized, probably during the action of
respiration, while the alkali passes into the urine in the state
of carbonate. If, however, the organic acid be taken uncombined,
it escapes decomposition, and, passing into the urine, produces an
abundant precipitate of salts of lime. In the case of the tartaric
acid and oxalic acids, some organic bodies, as aspharagine and the
oil of turpentine, are decomposed, and the products which they form
are execreted, giving to the urine peculiar odors; in the latter case
like that of violets.
The majority of coloring matters are thrown out of the system by this
secretion, while others are not so given off.
The mineral acids—alcohol, camphor and most metallic salts—do not
pass into the urine to any sensible degree.
_Urine in Diabetes._—The most remarkable change in the nature of
urine occurs in Diabetes Mellitus, it is voided in great quantity; it
is found to contain a great quantity of grape, sugar, and very little
urea.
It was supposed that in this disease urea ceased to be formed by
the system, and was replaced by sugar; but it has been shown that,
although the quantity of urea is very small in any one specimen
of urine, yet the total quantity is so much increased that in
twenty-four hours the natural quantity of urea is secreted; the
secretion of sugar being an act of faulty digestion, and totally
unconnected with the urea. These results have been fully confirmed by
experience.
The diabetic urine sometimes contains albumen, which arises from
complications of other forms of disease.
All that has been said in the former chapter about the solid and
fluid constituents of the human body may, at first sight, and to a
great many, seem to be superfluous and out of place in a work of
this kind. It is true that the different modes of preserving bodies,
as explained in this book, do not require this long dissertation on
animal chemistry in order to be understood; still, when we consider
that the chemicals used in these different processes have an object
to accomplish, it must be granted that a thorough knowledge of the
constituents of the body, their composition and chemical proportion,
will, to a great extent, explain the reason why these same chemicals
are used in preference to others.
The secondary object, which is not less important, consists in the
fact that a thorough knowledge of the animal chemistry of the human
organism is most necessary to understand the different changes which
take place in the formation of the different juices and tissues of
the body, when they enter into combination with the chemicals, the
object of which is to render them imputrescible.
However, the study of these combinations affords a simple and clear
explanation of the means resorted to in order to preserve bodies.