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to do with the functions of living tissues; and it was largely
through their efforts and the labors of their followers that the
prevalent idea that vital processes are dominated by unique laws
was discarded and physiology was brought within the recognized
province of the chemist. So at about the time when the microscope
had taught that the cell is the really essential structure of the
living organism, the chemists had come to understand that every
function of the organism is really the expression of a chemical
change—that each cell is, in short, a miniature chemical
laboratory. And it was this combined point of view of anatomist
and chemist, this union of hitherto dissociated forces, that made
possible the inroads into the unexplored fields of physiology
that were effected towards the middle of the nineteenth century.
One of the first subjects reinvestigated and brought to proximal
solution was the long-mooted question of the digestion of foods.
Spallanzani and Hunter had shown in the previous century that
digestion is in some sort a solution of foods; but little advance
was made upon their work until 1824, when Prout detected the
presence of hydrochloric acid in the gastric juice. A decade
later Sprott and Boyd detected the existence of peculiar glands
in the gastric mucous membrane; and Cagniard la Tour and Schwann
independently discovered that the really active principle of the
gastric juice is a substance which was named pepsin, and which
was shown by Schwann to be active in the presence of hydrochloric
acid.
Almost coincidently, in 1836, it was discovered by Purkinje and
Pappenheim that another organ than the stomach—namely, the
pancreas—has a share in digestion, and in the course of the
ensuing decade it came to be known, through the efforts of
Eberle, Valentin, and Claude Bernard, that this organ is
all-important in the digestion of starchy and fatty foods. It was
found, too, that the liver and the intestinal glands have each an
important share in the work of preparing foods for absorption, as
also has the saliva—that, in short, a coalition of forces is
necessary for the digestion of all ordinary foods taken into the
stomach.
And the chemists soon discovered that in each one of the
essential digestive juices there is at least one substance having
certain resemblances to pepsin, though acting on different kinds
of food. The point of resemblance between all these essential
digestive agents is that each has the remarkable property of
acting on relatively enormous quantities of the substance which
it can digest without itself being destroyed or apparently even
altered. In virtue of this strange property, pepsin and the
allied substances were spoken of as ferments, but more recently
it is customary to distinguish them from such organized ferments
as yeast by designating them enzymes. The isolation of these
enzymes, and an appreciation of their mode of action, mark a long
step towards the solution of the riddle of digestion, but it must
be added that we are still quite in the dark as to the real
ultimate nature of their strange activity.
In a comprehensive view, the digestive organs, taken as a whole,
are a gateway between the outside world and the more intimate
cells of the organism. Another equally important gateway is
furnished by the lungs, and here also there was much obscurity
about the exact method of functioning at the time of the revival
of physiological chemistry. That oxygen is consumed and carbonic
acid given off during respiration the chemists of the age of
Priestley and Lavoisier had indeed made clear, but the mistaken
notion prevailed that it was in the lungs themselves that the
important burning of fuel occurs, of which carbonic acid is a
chief product. But now that attention had been called to the
importance of the ultimate cell, this misconception could not
long hold its ground, and as early as 1842 Liebig, in the course
of his studies of animal heat, became convinced that it is not in
the lungs, but in the ultimate tissues to which they are
tributary, that the true consumption of fuel takes place.
Reviving Lavoisier’s idea, with modifications and additions,
Liebig contended, and in the face of opposition finally
demonstrated, that the source of animal heat is really the
consumption of the fuel taken in through the stomach and the
lungs. He showed that all the activities of life are really the
product of energy liberated solely through destructive processes,
amounting, broadly speaking, to combustion occurring in the
ultimate cells of the organism. Here is his argument:
LIEBIG ON ANIMAL HEAT“The oxygen taken into the system is taken out again in the same
forms, whether in summer or in winter; hence we expire more
carbon in cold weather, and when the barometer is high, than we
do in warm weather; and we must consume more or less carbon in
our food in the same proportion; in Sweden more than in Sicily;
and in our more temperate climate a full eighth more in winter
than in summer.
“Even when we consume equal weights of food in cold and warm
countries, infinite wisdom has so arranged that the articles of
food in different climates are most unequal in the proportion of
carbon they contain. The fruits on which the natives of the South
prefer to feed do not in the fresh state contain more than twelve
per cent. of carbon, while the blubber and train-oil used by the
inhabitants of the arctic regions contain from sixty-six to
eighty per cent. of carbon.
“It is no difficult matter, in warm climates, to study moderation
in eating, and men can bear hunger for a long time under the
equator; but cold and hunger united very soon exhaust the body.
“The mutual action between the elements of the food and the
oxygen conveyed by the circulation of the blood to every part of
the body is the source of animal heat.
“All living creatures whose existence depends on the absorption
of oxygen possess within themselves a source of heat independent
of surrounding objects.
“This truth applies to all animals, and extends besides to the
germination of seeds, to the flowering of plants, and to the
maturation of fruits. It is only in those parts of the body to
which arterial blood, and with it the oxygen absorbed in
respiration, is conveyed that heat is produced. Hair, wool, or
feathers do not possess an elevated temperature. This high
temperature of the animal body, or, as it may be called,
disengagement of heat, is uniformly and under all circumstances
the result of the combination of combustible substance with
oxygen.
“In whatever way carbon may combine with oxygen, the act of
combination cannot take place without the disengagement of heat.
It is a matter of indifference whether the combination takes
place rapidly or slowly, at a high or at a low temperature; the
amount of heat liberated is a constant quantity. The carbon of
the food, which is converted into carbonic acid within the body,
must give out exactly as much heat as if it had been directly
burned in the air or in oxygen gas; the only difference is that
the amount of heat produced is diffused over unequal times. In
oxygen the combustion is more rapid and the heat more intense; in
air it is slower, the temperature is not so high, but it
continues longer.
“It is obvious that the amount of heat liberated must increase or
diminish with the amount of oxygen introduced in equal times by
respiration. Those animals which respire frequently, and
consequently consume much oxygen, possess a higher temperature
than others which, with a body of equal size to be heated, take
into the system less oxygen. The temperature of a child (102
degrees) is higher than that of an adult (99.5 degrees). That of
birds (104 to 105.4 degrees) is higher than that of quadrupeds
(98.5 to 100.4 degrees), or than that of fishes or amphibia,
whose proper temperature is from 3.7 to 2.6 degrees higher than
that of the medium in which they live. All animals, strictly
speaking, are warm-blooded; but in those only which possess lungs
is the temperature of the body independent of the surrounding
medium.
“The most trustworthy observations prove that in all climates, in
the temperate zones as well as at the equator or the poles, the
temperature of the body in man, and of what are commonly called
warm-blooded animals, is invariably the same; yet how different
are the circumstances in which they live.
“The animal body is a heated mass, which bears the same relation
to surrounding objects as any other heated mass. It receives heat
when the surrounding objects are hotter, it loses heat when they
are colder than itself. We know that the rapidity of cooling
increases with the difference between the heated body and that of
the surrounding medium—that is, the colder the surrounding
medium the shorter the time required for the cooling of the
heated body. How unequal, then, must be the loss of heat of a man
at Palermo, where the actual temperature is nearly equal to that
of the body, and in the polar regions, where the external
temperature is from 70 to 90 degrees lower.
“Yet notwithstanding this extremely unequal loss of heat,
experience has shown that the blood of an inhabitant of the
arctic circle has a temperature as high as that of the native of
the South, who lives in so different a medium. This fact, when
its true significance is perceived, proves that the heat given
off to the surrounding medium is restored within the body with
great rapidity. This compensation takes place more rapidly in
winter than in summer, at the pole than at the equator.
“Now in different climates the quantity of oxygen introduced into
the system of respiration, as has been already shown, varies
according to the temperature of the external air; the quantity of
inspired oxygen increases with the loss of heat by external
cooling, and the quantity of carbon or hydrogen necessary to
combine with this oxygen must be increased in like ratio. It is
evident that the supply of heat lost by cooling is effected by
the mutual action of the elements of the food and the inspired
oxygen, which combine together. To make use of a familiar, but
not on that account a less just illustration, the animal body
acts, in this respect, as a furnace, which we supply with fuel.
It signifies nothing what intermediate forms food may assume,
what changes it may undergo in the body, the last change is
uniformly the conversion of carbon into carbonic acid and of its
hydrogen into water; the unassimilated nitrogen of the food,
along with the unburned or unoxidized carbon, is expelled in the
excretions. In order to keep up in a furnace a constant
temperature, we must vary the supply of fuel according to the
external temperature—that is, according to the supply of oxygen.
“In the animal body the food is the fuel; with a proper supply of
oxygen we obtain the heat given out during its oxidation or
combustion.”[3]
BLOOD CORPUSCLES, MUSCLES, AND GLANDS
Further researches showed that the carriers of oxygen, from the
time of its absorption in the lungs till its liberation in the
ultimate tissues, are the red corpuscles, whose function had been
supposed to be the mechanical one of mixing of the blood. It
transpired that the red corpuscles are composed chiefly of a
substance which Kuhne first isolated in crystalline form in 1865,
and which was named haemoglobin—a substance which has a
marvellous affinity for oxygen, seizing on it eagerly at the
lungs vet giving it up with equal readiness when coursing among
the remote cells of the body. When freighted with oxygen it
becomes oxyhaemoglobin and is red in color; when freed from its
oxygen it takes
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