A History of Science, vol 4 - Henry Smith Williams (epub e ink reader .txt) 📗

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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