Popular Science Monthly/Volume 74/February 1909/An American Contribution to the History of the Physiology of Digestion

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MY interest has lately been aroused in reading a little American monograph published over a hundred years ago as a dissertation submitted for the degree of doctor of medicine to the faculty of the University of Pennsylvania. It is entitled: "An Experimental Inquiry into the Principles of Nutrition and the Digestive Processes," by John P. Young, of Maryland (submitted June 8, 1803).[1] The essay does not appear to have received notice from the writers of that period; nor was there, probably, more occasion for calling attention to this monograph than to the usual doctor's thesis of the present day. Dr. Young's contribution, nevertheless, seems noteworthy because, in examining the knowledge of digestion then current, he applies the test of experimental evidence obtained at first hand—a sort of critique less in vogue in his day than in ours. On the title page he quotes from Lavoisier: "We ought in every instance to submit our reasoning to the test of Experiment, and never to search for truth, but by the natural road of Experiment and Observation." The dissertation further possesses a value, aside from its intrinsic merit as a scientific inquiry, in giving some indication of the status of physiological studies in America at the opening of the nineteenth century and in the first medical college of this country. To appreciate Dr. Young's monograph in the light of those times one must indulge in a moment's retrospect.

The history of the physiology of digestion may conveniently be divided into three periods. The first of these embraces the earlier days of science until the publication of Haller's "Elementa Physiologise" (1757), when theory and debate still maintained the triumph of the "animal spirits" and the various conceptions of "vital principles." In the succeeding epoch Réaumur (1752), Stevens (1777) and Spallanzani (1783) put into practise the teaching of Bacon:

Non fingendum aut excogitandum,

sed quid natura faciat observandum.

Previous to this time various theories of digestion were based upon obscure ideas of trituration, concoction, fermentation and putrefaction or whatever these words might imply. Difficult as it is for us to-day to reproduce the point of view of men who were "struggling with the spiritualistic fermentations of van Helmont, on the one hand, and with the material effervescences of Sylvius, on the other," we can nevertheless appreciate the remark of William Hunter:
Some physiologists will have it, that the stomach is a mill, others, that it is a fermenting vat, others, again, that it is a stew-pan; but, in my view of the matter, it is neither a mill, a fermenting vat, nor a stew-pan; but a stomach, gentlemen, a stomach.

The third epoch in the study of the physiology of digestion coincides with the rise of modern chemistry and may, perhaps, be said to start with the discovery of free hydrochloric acid in the gastric juice by Prout and by Tiedemann and Gmelin in 1824, soon followed by the pioneer work of Dr. William Beaumont upon Alexis St. Martin. Foster writes:

It was left for the nineteenth century to throw a new light on the nature of the gastric changes and at the same time shew that what took place in the stomach was not the whole of digestion, but only the first of a series of profound changes taking place along nearly the whole length of the alimentary canal.

Let us bear in mind, then, that although the presence of a solvent fluid in the stomach had begun to be admitted in 1803, its nature and the mode of its operation were not understood until Beaumont's classic experiments (1833) on "the man with a lid on his stomach," as St. Martin was derisively called. Réaumur (1753) experimented on a buzzard, administering to it hollow metallic capsules perforated like a sieve and containing foods within. The possibility of mechanical crushing or trituration was thereby excluded; but when the tubes were regurgitated it was found that digestion (solution) of the food materials had nevertheless taken place. Some chemical action must have been exerted; and by placing sponges in the metallic tubes, Réaumur was able to express therefrom specimens of gastric fluid. He appreciated that it possessed properties antagonistic to putrefaction; and fragmentary as his observations may appear, he introduced a-new method into physiological research. To Spallanzani was left the extension of these investigations in most fruitful fields. He well recognized the antiseptic power of the gastric secretion. With regard to the nature of digestion Spallanzani concluded (1783) in these words:

None of the three forms of fermentation distinguished by chemists under the name of spirituous (alcoholic), acid, or putrid, have any place in digestion.

His well-conceived experiments in which animals swallowed meat attached to strings by which it could be withdrawn from time to time, and the ways in which gastric fluid was removed by squeezing out sponges swallowed and withdrawn, are familiar. The impression which these researches left is well emphasized by Beumont. He wrote (1833):

Suffice it to say that the theories of Concoction, Putrefaction, Trituration, Fermentation and Maceration, have been prostrated in the dust before the lights of science, and the deductions of experiment. It was reserved for Spallanzani to overthrow all these unfounded hypotheses, and to erect upon their ruins, a theory which will stand the test of scientific examination and experiment. He established a theory of chemical solution, and taught that chymification was owing to the solvent action of a fluid, secreted by the stomach, and operating as a true menstruum of alimentary substances. To this fluid he gave the name of gastric juice. . . . By far the most respectable and intelligent physiologists have now settled down in the belief that chymification is effected in the stomach, by a specific solvent, secreted by that organ, called, after spallanzani, the Gastric Juice. From the difficulty, however, of obtaining and submitting this fluid to the test of experiment, and the diversity of results in the examination of such as has been obtained, no very satisfactory conclusions have been arrived at. The presence of an active solvent is rather an admission—a conclusion from the effect to the cause.

Spallanzani failed to understand the acid character of solvent gastric juice. Even as late as 1825 Leuret and Lassaigne, in a memoir honored by the Académie des Sciences, declined to accept Front's evidence of the existence of hydrochloric acid in the gastric secretion. This deserves notice with reference to the experiments of Dr. Young which will be described later.

Young's essay opens with a review of the Nutrientia, the views of Dr. Cullen being subjected to criticism. This famous Edinburgh teacher[2] referred "the principal of nutrientia to vegetables; and that they derive this property from their acid, sugar and oil." Taking these up in order, Young rejects acid as a true nutrient, with these words:

The doctor (Cullen) appears to have founded his opinion on the idea, that all vegetable substances, when taken into the stomach, undergo a fermentation, whereby an acid is evolved; and "as this entirely disappears with the progress of the aliment, without being again evident in the mass of blood," so he supposed it undoubtedly entered into the composition of the animal fluid. That an acetous fermentation takes place in the human stomach in a healthy state, we entirely reject, as will appear in what follows; and if this opinion be well founded, we obviate the principal argument favouring the idea, of an acid being nutritious. Acescent vegetables we can not doubt as affording nourishment, but this is not to be referred to their acid, but to their sugar and oil.

Young overthrows Cullen's assumption that "sugar is not alimentary in its pure saline state, but only when combined with an oleaginous matter," by citing the ease of the West Indies negroes who grow fat on sugar at certain seasons when they are at work on the cane. The absorption and need of water and "calcareous earth" is also discussed. The author reaches the conclusion that water not alone supplies the waste of fluids, but also goes to form the solids of the body. He says:

Dr. Fordyce informs us he put a gold-fish in a glass vessel, and supplied it with spring water; the fish lived in this manner for fifteen months, grew to more than double the size it was when first confined, and threw out much feculent matter. Lest it should be supposed the fish lived on substances held in the water by solution, he used distilled water and impregnated it with the air of the atmosphere, and put other gold-fish in the water thus treated, and kept them six months,[3] during which time they threw out feculent matter, and thrived as before mentioned.

In referring to the "action of the mind" on the secretion of saliva Young makes the following comment in a foot-note:

Is not the secretion of the saliva and gastric juice synchronous? It is highly probable from long habit, the actions of these two sets of vessels become associated; hence, when the stomach and its vessels are irritated, as in nausea, there is always a flow of saliva, though nothing stimulating has been applied to the mouth. The excitement of the vessels of the one seems to keep pace with that of the other; when the nausea is so great that vomiting is just at hand, the flow of the saliva is proportionally increased; and when we make an unsuccessful effort to vomit, we generally throw out a mouthful of saliva.

Is it far-fetched to recall in this connection the comparable psychic secretion which has been described in recent years for both saliva and gastric juice and the probability of a common stimulus for the production of each?

Let us now consider more particularly Dr. Young's observations on the processes in the stomach. He assumes that sufficient evidence was already at hand from experiments on animals to permit plausible, if not conclusive, inferences concerning our own digestion. He writes:

It would be unnecessary to recite particular experiments, to prove the solvent property of the gastric fluid, this being admitted on all hands. . . . The effects of solution are most remarkable in such animals as swallow their food without mastication; we will, therefore, relate a few experiments made on some of these.

Our common large bull-frog (Rana ocellata) was chosen in order to observe the effects of the gastric fluid, as they swallow all their prey whole. They have a large membranous stomach, which when distended, occupies the whole anterior part of the abdomen: the œsophagus is very wide, so that their food can be examined at pleasure. Two of a very large size were procured, and their stomachs were found to be greatly distended with food: being desirous of seeing what was their natural aliment, and the effects of their digestive power upon it, by means of a pair of forceps, one of their stomachs was easily emptied of its contents; and to my surprise, and that of others who witnessed the fact, it was found to contain a common sized spring frog, and afforded a fine opportunity to see the effects of their gastric liquor. The whole external surface of the frog was acted upon, the muscles having, superficially, quite lost their texture; some parts of the backbone were bare, the spinous processes of which were quite soft. Upon introducing a forceps, a second time, the hinder parts of a second frog were found, which shewed the effects of their fluids in a still greater degree: the muscles of the thigh were reduced to a complete jelly, though still retaining their form; some parts of the bones that were covered with flesh were quite soft and flexible. Upon extracting the contents of the stomach of the second frog, it was found to contain a field mouse, about a third larger than our common mouse: its whole surface was quite soft, having entirely lost its texture; the fore legs were nearly disconnected from its body, the bones of which were soft; the bones of other parts of the body were also examined; they were all soft. But what was most surprising, the teeth of this animal did not escape; the incisors were, as Dr. Jacobs witnessed, soft and flexible, having the appearance of a piece of half dried tendon. Neither the frog nor the mouse had any acid or putrid smell. It appeared very evident from the preceding experiment that the fluids of these animals acted upon bones; but in order to ascertain whether they could dissolve them completely down, the following experiment was performed. The head and all the bones of the mouse were cleared of their flesh, and forced into the empty stomach of one of the frogs; he was then put into a jar of water. In two days, the bones were all discharged in the form of a mortar; by rubbing it between the fingers, small pieces of bone were distinguishable. This will serve to shew us the powerful action of an apparently inert fluid on an animal matter, sparing not bones, nor even the teeth of animals. Being desirous of knowing the length of time they would require to dissolve down a small frog, the following experiment was performed. A packthread was tied to the hind legs of a living spring frog; its head was then put into the mouth of one of the large frogs; as soon as he felt it move it was swallowed greedily. In five hours it was drawn up by means of the thread; the skin and external surface of the muscles were tender. It was again introduced; in the space of seven hours, it was drawn up a second time; the abdominal muscles were now dissolved, and the intestines had protruded; the bones of the feet were soft, and separable from the leg by the least force; in a word, the whole was a complete dissolved mass. It was swallowed a third time, and attempted to be drawn up in six hours afterwards; but it had so far lost its texture that the two legs, to which the thread was tied, could only be brought up; the bones of these were soft and flexible, as before mentioned. Many experiments of this kind were made to see the effects of their gastric menstruum: in many cases, after giving them small frogs, the trunk and head of these animals were drawn out of their stomachs complete skeletons, but the bones were always soft, and felt like tender cartilage. In all the half-digested substances which were at different times taken from their stomachs, as frogs, veal, beef, etc., an acid was constantly found present: they were seldom examined before two hours after being swallowed; at this short interval when their surfaces were touched with litmus paper, it was turned red. Snakes, like the large frogs, also swallow their food without mastication: many experiments were therefore also made on them, by forcing frogs, lizards, etc., into their stomachs, to see the effects of solution: they agreed in every respect with what has been said of frogs, like them perfectly dissolving down entire animals. The only difference between them was, that the solution of snakes went on only about half as fast as that of the large frogs.

The gastric fluid of man and that of frogs and snakes agree perfectly in their action on flesh, as the experiments of Spallanzani prove that the first of these powerfully dissolves meat out of the body. As the menstruum of the two latter animals acted so uniformly on bones, it appeared highly probable the fluid of our own stomach would also. To ascertain this, the condyles of the thigh bone of a chicken, weighing eleven grains, were swallowed; the bone remained a considerable time in the stomach, as was supposed from some uneasy sensations that were occasionally experienced for between two and three days; the fourth day it was discharged, reduced to a shell, weighing only three grains. Thus far the digestion of man and these animals perfectly agree, in solution being the first step towards the conversion of food into chyle; but they differ in some particulars, and probably by attending to these, they may be of use to us. First. They are cold-blooded animals: heat is a powerful agent in all solutions, and the experiments of Spallanzani prove it greatly assists the action of the gastric liquor out of the stomach.

Secondly. They do not masticate their food.

These two inconveniences are obviated, by these animals never drinking when their digestion is going on, so that their fluid acts in its undiluted state; whereas in man, it is always diluted, as he seldom eats without drinking. That this was the case with these animals I had clear proof; for although I examined the contents of their stomachs so often, in no one case could I find any fluid more than a jelly-like substance, appearing to be made up of gastric juice and dissolved flesh. Supposing, however, that the pressure used in bringing up the food of the frogs might have forced the more fluid parts into the duodenum, I resolved to ascertain the fact in another way; this was easily done. A teaspoon could readily be passed into their stomachs, and with this the dissolved food could all be brought up; it was always, however, of the consistence above mentioned. During the time these experiments were made, they were constantly kept in large jars of water. The attention to this circumstance by these animals, which swallow their prey entire, is a necessary part in their digestion, as they require a very powerful menstruum, so as to dissolve not only entire muscles, but also bones. The inference we would draw from it would be, to attend occasionally to what necessity urges them to observe constantly. Thus when our stomachs are weak, or we are troubled with dyspeptic symptoms, like them we ought to avoid much diluting our gastric juice; so that although it were secreted not perfectly healthy, yet having the advantage of acting in its uncombined state, solution and digestion may go on, when it otherwise would not, with the common quantity of drink. Indeed our stomachs in this respect act a kind part to us; for when we make our first dish on broth it seldom relishes much solid aliment after it; hence soups are the first dish at the table of the temperate, and the last at that of the epicure.

Both Spallanzani and Réaumur believed that vegetable food is less easily digested by certain animals than meat. Young reinvestigated this question on frogs. He found that when peas, beans, wheat and bread enclosed in linen bags were introduced into the stomach, all but the bread were still entire at the end of thirty hours; but when the peas and beans were well bruised before introduction they were dissolved. The author concludes that the living principle in the seeds resists digestion. In harmony with this view he found that seeds would germinate when retained in the stomach. An entertaining story is cited from the Italian anatomist Morgani.

He informs us that a young lady living entirely on vegetables (it being lent), was seized with a violent affection of her stomach, and great emaciation ensued. Different medicines were used, but without the least alleviation of her symptoms. At length a violent vomiting commenced, and to the astonishment of all present, she threw up a small plant, with perfect leaves and roots! This at first sight might be looked upon as approaching the marvellous; yet why should we doubt it? The authority of our author is as respectable as any other of our profession; and we have just seen that seeds will vegetate when retained a sufficient length of time in the stomach. The probability here was, that the young lady had swallowed the seed of some small plant, without destroying its texture by mastication; which being retained in the stomach, and exposed to heat and moisture, vegetation progressed.

Vegetable and animal foods alike are, then, capable of solution by the gastric fluid, provided that their "organization or vital principle be previously destroyed." One could thus believe the further evidence that "a respectable gentleman" had seen two polar bears "that had subsisted on vegetable food alone, from the time that they were taken from their mother's breasts; and that they were more than half grown, and very fat." On the other hand, he cites the case of the Italian naturalist who "by dint of hunger learnt a pigeon to eat meat of which it became so excessively fond, that it preferred it to every other kind of food, even to wheat, which in their natural state, they eat before anything else."

Will simple solution by the powerful action of the gastric fluid explain the conversion of "aliment into chyle?" asks Dr. Young. Many earlier teachers had assumed that activities which we now know to be associated with microorganisms play a part. The warmth and moisture of the body would facilitate this fermentation and putrefaction. Our author writes:

Chemists divide fermentation into three kinds, the vinous, acetous and putrefactive; the product of the first is vinous spirit, or alcohol; of the second, acetous acid, or vinegar; of the third, ammoniac, or volatile alkali.

In order to ascertain whether a vinous fermentation could take place in the human stomach the following experiment was performed. My friend, Mr. Mitchell, avoided his usual breakfast, in the place of which he took, between the hours of eight and ten, twelve ounces of sugar. Nothing more was taken until one o'clock. Having the power to ruminate, it was at this hour thrown up; the mass was sweet: upon being put to rest no intestine motion or disengagement of air was to be perceived. It was then submitted to distillation: a limpid fluid passed over into the receiver, which was sweetish, but had none of the properties of a vinous spirit. Carbonic acid gas is constantly evolved during the vinous fermentation; Mr. Mitchell, therefore, paid particular attention to this, as long as the sugar was on his stomach; but there was not the least eructation of air during the whole period the experiment was going on. If ever a vinous fermentation took place in the stomach, we expected to have found it in this experiment; as this viscus was plentifully supplied with saccharine matter, which passes so readily to this state; but as nothing of the kind occurred, we conclude the vinous fermentation has nothing to do with the digestive process.

Young's experiments on frogs had already taught him, in confirmation of Spallanzani, that the gastric juice resists decay and it "even restored putrid substances to their original sweetness." Here is an additional experiment upon himself:

On an empty stomach I made a light dinner, on chicken pye, and drank simple water: in half an hour, by irritating my fauces, it was thrown up; at this time it was plentifully supplied with gastric fluid, as well as saliva, as the quantity of food was but small. It was then exposed in a tumbler to a heat equal to the human temperature. For the space of nine hours there was not the least intestine motion nor any disengagement of air. As digestion is performed sooner than this period, it was not attended to any longer.

Young convinced himself of the acid character of the gastric fluid and attempted to identify the acid present.

A piece of fresh veal was introduced into the empty stomach of one of the large frogs: in two hours it was examined; the surface was a little tender; upon being touched with litmus paper it was turned red. Here digestion was progressing quite regular, yet an acid was present. It appeared impossible at the same time to conceive the meat could become sour in so very short a time, and in so very low a temperature; it was therefore conjectured, the acid was to be referred not to the meat, but to the gastric juice, which the following experiments confirmed us in. A frog was kept starving for two days; a piece of litmus paper was then forced into its empty stomach by means of a pair of forceps; upon being drawn out, it was covered with gastric juice, and the litmus turned red. The naked gastric juice was afterwards often examined, by bringing it out of their stomachs with a teaspoon, and constantly found to be slightly acid. Being thus fully persuaded the acid, in the digested food of frogs, did not arise from a fermentation, but was to be referred to their gastric juice, we were led by analogy to suppose the acid of our own stomachs was to be attributed to the same origin: but this analogical reasoning might be called mere probability; the following experiment was therefore performed. Early in the morning, my stomach being empty, I irritated my fauces with a view of throwing up some gastric juice: though many efforts were made, none could be vomited. The following day I took some meat on an empty stomach: in half an hour afterwards, by irritating my fauces, the meat was thrown up, and with it some gastric fluid: upon being tested, an acid was very evidently present. Here no one can suppose the acid was to be referred to the meat. We have little hesitation, therefore, in saying that the acid so constantly found in the stomach of man, and almost, probably, all animals, is to be referred to their gastric fluid.

Young's friend, Mr. Mitchell, "being in good health and having the power to ruminate," collected gastric fluid for him. The analysis of the filtered fluid was performed by precipitating with acetate of lead. The precipitate was treated with muriatic acid "which decomposed it, a very white powder remaining at the bottom, and a fluid above." From analogy with the behavior of urine similarly treated the author concluded:

Though great accuracy and many varied experiments are required to ascertain certainly the presence of an unknown acid, yet we are disposed to believe any person who had witnessed the great similarity in the comparative precipitations just mentioned would have pronounced the same explanation was to be applied to both, or that the acid in the filtered fluid was the phosphoric.

Additional evidence of the presence of phosphoric acid was believed to be derived from the behavior of the fluid towards solutions of mercury or silver in nitric acid and towards lime water.

The supposed finding of a mineral acid led Young to comment upon the efficiency of metallic iron recommended by Italian physicians as a tonic, its solution being thereby explained. For, he asks, "does not the uniform effects of iron in its metallic state prove that an acid is always present in the stomach?"

The solvent property of the gastric juice on bones and teeth suggested the possibility of its use as a solvent for stone in the bladder.

A calculus was obtained from Dr. Jacobs of a very firm texture weighing exactly fifty grains. It was introduced into the stomach of one of the large frogs. In two days it was taken out for examination: at first sight it was evident solution had taken place, for the gastric juice which adhered to it was coloured with some of the dissolved stone: it was found to weigh forty-five grains. It was forced into the stomach a second time, where it remained for two days; it now weighed thirty-eight grains: from this, it appears, it is well worthy of more attention. When introduced into the bladder, with the heat of the human body, we have little doubt the gastric juice of frogs would act upon calculi with much effect. The fluid is easily procured, and without the necessity, as in other animals, of sacrificing a life every time we wish to obtain it: by means of a teaspoon it is readily brought up from their stomachs.

With the theory of fermentation rejected, the author proceeds to attempt an explanation of the digestive function.

Aliment is dissolved by the gastric menstruum; it then passes into the duodenum and meets with bile and pancreatic liquor; after being united with these, a heterogeneous mass is formed called chyme, and from this the lacteals secrete chyle. We are Jed to believe this to be the true doctrine, because, as before observed, simple solution will not explain the phenomenon of digestion; nor will the mixture of this dissolved mass, with bile and pancreatic liquor, change it into chyle; for we know chyle is formed when both these fluids are wanting: thus nutrition goes on when the biliary ducts are obstructed, and also when the pancreas is schirrous. That the absorbents have a secreting or digestive power, we learn from the following. Dr. Wistar informs us of a remarkable case, which occurred under his own observation, of a person who was supported for many weeks, by nourishing enemata, alone. Here it can not be said there was bile, gastric and pancreatic liquors to assimilate the injected fluid into chyle; yet chyle was formed and the system nourished. If the lacteals acted the part of simple absorbing, or capillary tubes, their contained fluids ought to partake of the sensible properties of the mass from which they are absorbed. But the reverse of this is the case: chyle has always the same taste, however different the sensible properties of the contents of the intestines may be, whether they are acid, bitter, etc. We draw a strong argument in truth of this opinion, by turning to the vegetable kingdom, throughout the whole of which the digestive process is seated in the absorbents. Water is to them what the fluids of the primæviæ are to the digestion of man: it dissolves their food, which being exposed to their vessels is taken up; but the fluid thus taken up can not be imitated by any mixture of earth and water, any more than we can imitate chyle by combining aliments with the fluids of the alimentary canal. As we thus have proofs the one is a secretory process, why not admit that of the other to be so also, since the circumstances of each so perfectly agree.

One hundred years later the obscure importance of the absorbing alimentary tract must still be emphasized. In the words of a popular textbook: the energy that controls absorption resides in the wall of the intestine, presumably in the epithelial cells and constitutes a special form of imbibition which is not yet understood. Thus the dignity of the living structures still remains unchallenged.

The uncertainty regarding the acidity of the gastric juice which still prevailed twenty years after Young's paper was. published has already been mentioned. Even as late as 1812 Montegre insisted that what was supposed to be gastric juice is nothing but swallowed saliva. An American, Professor Smith, suggested that digestion is performed "by the veins of the stomach, and by the liver." Vague ideas like these, in contrast with modest experimental inquiries illustrated by the monograph which we have reviewed in some detail, led Dr. Beaumont to remark:

It is unfortunate for the interests of physiological science that it generally falls to the lot of men of vivid imaginations, and great powers of mind, to become restive under the restraints of a tedious and routine mode of thinking, and to strike out into bold and original hypotheses to elucidate the operations of nature, or to account for the phenomena that are constantly submitting to their inspection. The process of developing truth, by patient and persevering investigation, experiment and research, is incompatible with unrestrained genius. The drudgery of science is left to humbler and more unpretending laborers. The flight of genius is, however, frequently erratic.
  1. I am indebted to Dr. C. F. Langworthy, of Washington, for directing my attention to this paper. It is reprinted in the Medical Theses, edited by Charles Caldwell, M.D., Philadelphia, Thomas and William Bradford, 1805, which was obtained for the Yale University Library through the courtesy of the Library of the Surgeon General's Office in Washington.
  2. I have assumed that the writer must refer to William Cullen (1712-90), of Edinburgh, under whose influence the abler young men from the English colonies in America came.
  3. One is reminded of J. Loeb's demonstration nearly a hundred years later that certain fishes can be put into distilled water without the least injury.