A Course of Lectures

ON

beautiful discovery of Denis, that fibrine is converted into albumen when dissolved in a solution of nitrate of potash. What is still more curious, with regard to this fact, is, that this conversion can only be effected with the fibrine of venous

OF

LIVING BODIES.

DELIVERED IN THE UNIVERSITY OF PISA. BY PROFESSOR MATTEUCCI, F.R.S. (Translated, for THE LANCET, by S. J. GOODFELLOW, M.D. Lond., late Physician to the Cumberland Infirmary.)

LECTURE V.

Digestion.

THE existence and preservation of an animal depend upon the introduction, at regular intervals, into its body, of certain peculiar substances, termed aliments. These substances, for the most part solid, undergo, in the digestive apparatus, a series of modifications, by means of which they are separated into fecal matters, which are expelled, and into others, which are mixed with the blood, into which they ultimately become transformed. The final end of digestion is the preservation of the integrity of the organism, by restoring to the blood the proximate principles of which it is constantly being deprived by the act of nutrition. Reason leads us to believe that all the parts of the organism are transformed and renewed more or less rapidly; and experimental physiology furnishes us with a certain number of experiments leading to this conclusion, which it would be very desirable to see varied and extended. To divide and make soluble the alimentary substances, in order to facilitate their absorption, summarily expresses what occurs during digestion. Nothing more, therefore, is witnessed in this function, beyond that which is purely physical, than the modification of the condition of the aliment.

Before entering upon the physico-chemical phenomena of digestion, I would briefly state some generalities with regard to this function.

All alimentary substances may be arranged, so far as their chemical composition is considered, under three well characterized categories: in the first are comprised, neutral azotised substances, as albumen, fibrine, and caseine; in the second, fatty matters; and under the third, gum, starch, and sugar, the composition of which may be represented by water and carbon. Experiment has shown that alimentary substances coming under the two last heads are insufficient for the alimentation of an animal, and that it is necessary that they should always be joined to those belonging to the first.

We shall further see what share the alimentary substances comprised in these several categories exercise respectively in the functions of the animal economy.

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With regard to the substances of the first class, I cannot pass over the important discoveries lately made by Mulder and Liebig-that albumen, fibrine, and caseine, are identical in their composition; in all three, the proportion of carbon to azote is as eight equivalents of the first to one of the second: they seem to differ from each other only in the small quantities of phosphorus and sulphur which accompany them; these taken away, there remains a principle common to all, which Mulder has termed proteine, and the formula of which, according to Liebig, is, C, Ha Ng O. We must, then, consider these substances, although endowed with physical properties so different, as isomeric, and only as modifications of proteine. Another important fact, discovered by Dumas and Liebig, is, that vegetable albumen and animal albumen are identical; also, that in corn flour there exists a substance analogous to caseine, and that in gluten there is found a resemblance to animal fibrine. There is not, then, any essential difference between the aliments of herbivorous and carnivorous animals, except that the first take them from plants, and the second, from other animals.

And since the composition of the blood, as well as that of the greatest number of animal and vegetable tissues, is analogous to that of the neutral organic substances which I have mentioned, since they are found, when forming part of the animal organism, without any change of chemical composition, and only contract, during nutrition, a new form, it is natural and just to admit that, in the act of digestion, the neutral azotised alimentary substances simply enter the blood in the state of solution, and without undergoing any other alteration. The isomerism of these substances is equally shown in the No. 1244.

the nitre, nor transformed into albumen. Scherer exposed fibrine of venous blood to an atmosphere of oxygen, and saw the oxygen converted into carbonic acid, and the fibrine lose the property of being changed intoalbumen in a solution of nitre. Some physiological experiments have long since proved that the digestion of similar alimentary substances is a purely physical act, and that it is effected independently of the living organism. Not one of you is ignorant of the celebrated experiments of our countryman Spallanzani, that flesh, gluten, and coagulated albumen, introduced into the stomach in perforated metallic tubes, become dissolved and digested as if they were free in the stomach. This solution is effected, as we have seen, by one of those actions which we described in the first lecture, termed catalytic, or actions of contact.

The recent experiments of Melsens, and particularly those of Bernard and Barreswil, have shown that the gastric juice contains a free acid, which will be the lactic, holding in solution a peculiar substance, called pepsine, which has been obtained tolerably pure. It is this substance which Payen has recently investigated, and which he has called gasterase. The acidity of this gastric juice is more or less, according to the quality of the aliments; while fasting, the acidity in the stomach is less strong. It increases by contact with aliments, and is strongest when they are composed of fibrine, albumen, &c. Here I have, in some glasses, an infusion of pepsine, to which I have added a few drops of hydrochloric acid. In one of these little glasses I have put some coagulated albumen; in another, some fibrine. These, so prepared, have been kept for ten or twelve hours in an atmosphere heated to 86° Fahr., and the albumen and fibrine have already disappeared to a great extent; there remains only some slight traces, already transparent on the edges, which will also shortly disappear altogether. If I neutralize the acid, and evaporate the solution, I can easily reproduce the albumen and fibrine, which have not undergone any change in their nature, and have only been dissolved by their contact with the acid infusion of pepsine. This substance acts, then, in dissolving fibrine and albumen as a body endowed with catalytic properties, and it is by an action of contact that their solution is effected. It is only in the stomach, or by certain glands which are situated in its mucous membrane, that the acid solution of pepsine or the gastric juice is separated. I have kept some pieces of small and large intestine in a solution slightly acidulated with hydrochloric acid; but it never acquired the solvent property; it only became gastric juice by contact with the membrane of the stomach.

The property with which the pepsine is endowed constantly requires the presence of a free mineral or organic acid. We have just seen how the catalytic action of this substance is modified when dissolved in an alkaline solution. I should here mention that pepsine loses its properties, and becomes insoluble, if it be heated to 50° centigrade, (122° Fahr.)

The neutral azotised substances, dissolved in the stomach by the liquid acid, or by the catalytic action of the pepsine, penetrate into the blood, through the walls of the capillary blood vessels of the stomach, solely by imbibition. Water and coloured alcoholic drinks, introduced into the stomach, are also absorbed. They do not pass this viscus, and are not found in the chyle, and yet they find their way into the blood. Bouchardat and Sandras fed some animals with fibrine, coloured with saffron or cochineal, but could never discover the colouring matter in their chyle. Again: animals that had been fed upon fibrine, and others that had been kept for some time fasting, and then killed, always furnished an identical chyle. The matter found in the intestines did not differ, except that in animals fed upon fibrine a small quantity of that substance remained in the stomach only partially dissolved. We know also, by the celebrated discoveries of Tiedemann and Gmelin, that the quantity of fibrine found in the lymph and the chyle, after a long fast, is not less than that which is contained in it immediately after digestion. The results are the same when coagulated albumen, gluten, or caseous matter, is used instead of fibrine. The digestion of these neutral azotised substances consists, then, in their simple solution by an action of contact, and the absorption of this solution, which takes place principally in the stomach.

Nothing is, therefore, more physical than this part of digestion. The mastication of elements impregnated with a slightly

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