Amongst all the articles of human food, milk takes a special position, and this is unanimously recognised, both in daily experience and in the practice of medicine. By everybody milk is considered a light food, and is given in cases of weak digestion as well as in a whole series of severe illnesses; for example, in heart and kidney affections. The extreme importance of this substance, a food prepared by Nature itself, we can now well understand. There are three properties of milk which secure it an exceptional position. As we already know, in comparison with nitrogenous equivalents of other foods, the weakest gastric juice and the smallest quantity of pancreatic fluid are poured out on milk; consequently, the secretory activity requisite for its assimilation is much less than with any other food-stuff. In addition, milk possesses a further important property. Thus, when it is introduced unobserved into the stomach of an animal it causes a secretion both in the stomach and also one from the pancreas; consequently, it appears to be an independent chemical excitant of the digestive canal; and in this action it is remarkable that we perceive no essential difference in the effect when the milk is brought unnoticed into the stomach from that which occurs when it is given to the animal to lap. Although flesh is a better chemical excitant, it is by no means a matter of indifference how it gets into the stomach. It must, therefore, be accepted that milk excites not only a really effective, but at the same time a very economic, secretion, and also that the appetite is unable to stimulate this secretion into a more active or abundant flow. The secret of the relation of milk to the secretion of the digestive juices can, unfortunately, at present be submitted to no further analysis or investigation. We are at liberty, however, to suppose that the fat on the one hand is of importance for the inhibition of the gastric glands, and the alkalinity on the other for the restraint of the pancreas. Thus the gastric glands and the pancreas, notwithstanding the presence of excitants, are maintained by milk at a certain but not too high degree of activity, a matter which is in every way desirable in consideration of the easy digestibility of its constituents. Finally, the third characteristic which is observed to belong to milk, and which is probably only an expression of the first, consists in the following. When one administers to an animal equivalent quantities of nitrogen, in the one case as milk, in the other as bread, and afterwards estimates the hourly output of nitrogen in the urine, it results that the increase during the first seven to ten hours after the milk (compared with the excretion beforehand) amounts only to from 12 per cent to 15 per cent of the nitrogen taken in, while after bread it amounts to 50 per cent. If the hourly rate of absorption and the extent to which milk and bread are respectively used up be taken into consideration, it has to be admitted that these augmentations of urinary nitrogen which appear soon after feeding must be expressions of the functional activity of the digestive canal itself, and that this activity in the case of bread is three or four times greater than in the case of milk (Experiments of Prof. Rjasanzew); consequently, in the case of milk a much larger fraction of its nitrogen is free to be used up by the organism at large (irrespective of the organs of digestion) than in that of any other kind of food. In other words, the price which the organism pays for the nitrogen of milk, in the form of work on the part of its digestive apparatus, is much less than that for other foods. How admirably, therefore, the food prepared by Nature distinguishes itself when compared with all others!
The facts just related bring forward a new aspect from which the relative nutritive values of different foods may be judged. The older criteria must frankly make room for the new or else be displaced by them. Experiments upon the utilisation of food-stuffs, in which what remains undigested is determined as well as what is absorbed into the body fluids, cannot alone be trusted to solve the question in a satisfactory manner. Suppose, for instance, that in the digestion of a given food the alimentary canal has been given a certain work to perform; if it be in health the work will be accomplished in the best possible manner—that is to say, with complete abstraction of everything nutrient. You will thus learn how much nutrient material was contained in the food, but the question of its digestibility remains as obscure as before. In your experiment you do not know how great an effort it has cost the alimentary canal to extract all the nourishment from the food. Nor can artificial digestion experiments settle the question of digestibility, for experiments in which food is normally partaken of are quite different from those in the test-tube, where we have to deal with only one juice, and not with the interaction of different juices and different food constituents. That one must here, as a matter of fact, make a distinction is clear from the observation of Dr. Walther in our laboratory. Fibrin, which is regarded by all as the most easily digested proteid, proved, when compared with a nitrogen equivalent of milk, to be a much stronger excitant of the pancreas, although milk contains, in addition to nitrogenous substances, a good deal of other non-nitrogenous material. The digestibility and nutritive value of foods must obviously be decided by an estimation of the real work which they entail upon the digestive apparatus, both in regard to the quantity and quality of the juices poured out on a given amount of nutrient material. The energy used up in gland metabolism must be deducted from that of food taken in. The remainder will then indicate the value of the food to the organism; that is to say, will give the amount available for use by all the other organs exclusive of the digestive apparatus. From this point of view those materials must be taken as less nourishing and less digestible which are in large part used up to make good the expenditure entailed by their digestion on the part of the alimentary canal; that is to say, those food-stuffs are less useful whose nutritive value little more than covers the cost of their digestion; consequently, it is of great practical importance to compare from this aspect the same foods differently prepared—for example, boiled and roast meat, hard and soft boiled eggs, boiled and unboiled milk, etc.
A discussion of some further medical questions may here be taken up. The first concerns the therapeutic use of the neutral and alkaline salts of sodium. In clinical, pharmacological, and physiological text-books it is stated now, as ever, that these salts promote a flow of gastric juice. We may look in vain, however, for any experimental foundation to support this doctrine. The experiments brought forward cannot be regarded as conclusive. When Blondlot sprinkled sodium bicarbonate upon flesh, or Braun and Grützner introduced sodium chloride solutions directly into the blood, they began with methods either false in themselves or far removed from normal conditions. In this case, however, the gaps in the experiment were happily made good by the clinician, for the experiment appeared to be confirmatory of clinical experience. That sodium salts (the chloride and bicarbonate) are useful in disorders of the digestive apparatus there can be no doubt. How do they act, however? It appears to me that here, as in some other cases, medical science has fallen into error. When we know that an effect takes place it does not by any means imply that we know the mechanism by which it occurs; and although medicine is broad enough and comprehensive enough to make free use of empiricism in practice, yet it often thinks in narrow grooves when it turns to the explanation of facts. It frequently tries to explain complicated healing processes in the simplest way, on supposed physiological data. And this is true in the present case, which affords an example of prevalent medical reasoning; the alkalies work favourably in digestive disturbances—therefore they are succagogues. Naturally the stomach, under the influence of alkalies, sometimes begins to secrete a greater quantity of juice. This means, however, that it has recovered from a disordered state and has returned to normal conditions. Consequently, the effect is due to the fact of recovery, and not to a direct influence of the alkalies. This latter, however, must be specially proved. The assistance afforded by the alkalies to the organism might be capable of another explanation; for example, that which is ordinarily given. In this case, however, I venture to offer a reason for the effects of sodium chloride, and of the alkaline salts of sodium, which is exactly the opposite of that generally accepted. We were unable to convince ourselves of any succagogue influence on the part of these salts. Indeed, both on the stomach and pancreas they proved in our hands to have an inhibitory effect. In addition to the experiments which I previously brought forward concerning the relation of alkalies to gastric and pancreatic juice, I may relate the following observation. A dog which fortunately had survived the performance, one after the other, of a gastric fistula, a pancreatic fistula, and an œsophagotomy, received daily during the course of several weeks an addition of soda to its food. The animal enjoyed good health and had an excellent appetite. When the first sham feeding experiment was carried out, the relatively small effect of this otherwise very active juice-exciting procedure at once struck us. At the same time we observed that the pieces of flesh which fell from the upper end of the œsophagus, contrary to the ordinary rule, were hardly at all insalivated. In this dog, therefore, a greatly lowered activity of several digestive glands—viz., of the gastric, pancreatic, and salivary glands—simultaneously existed. With regard to the salivary glands the circumstance was naturally submitted to closer investigation. I believe that the inhibitory influence of the alkalies on the digestive glands, which was here proved experimentally, may furnish a basis for the following representation of their mode of action in producing healing effects. Catarrhal affections of the stomach are characterised by an incessant or very protracted secretion of slimy, weakly acid gastric juice. Further, in many cases the affection begins with a hypersecretion, that is an abnormal excitability, of the secretory apparatus which makes itself evident in a superfluous and useless flow. The same must be conceived to happen in disorders of the pancreatic gland; at least such a condition sets in after operations performed for physiological purposes. It is, further, justifiable to suppose that, when an affection is once set up by this or that cause, it may later maintain itself independently; for continuous activity has undoubtedly a harmful influence on the glands. The due nourishment, and the restoration of organs after activity, proceeds best during rest. In the normal course of events, after a period of active work follows a pause, during which the latent work of restoration is accomplished. When, therefore, a remedy effectively restrains the excessive work of a diseased organ, it may in this way contribute to the removal of the pathological condition, and thus to a restoration of the normal state. In this consists, in my opinion, the healing effects of the alkalies. One might draw a parallel between the action of these substances in digestive disturbances and that of digitalis in compensatory disturbances of the heart. An uncompensated heart beats rapidly, and thereby only aggravates its condition. Its time of rest, that is of recovery, of restitution of the organ, is shortened. A vicious cycle is set up. The weak action of the heart lowers blood pressure; the lowering of this leads (from known physiological causes) to an increase in the number of beats; the quickening leads to weakening of the organ. Without doubt the digitalis aids by breaking through this vicious cycle, in that it greatly slows the pulse, and thereby gives new power to the heart. With our explanation of the action of the alkalies harmonises the further circumstance that, with the use of the salts in question, a strict diet is generally prescribed, which means that a certain amount of rest is secured for the digestive glands. It is interesting that in clinical investigations with the stomach-tube, after a period when the alkalies were looked upon as succagogues, a new phase has also set in, mention being now more frequently made of a restraining effect.
The cause of the erroneous belief that alkalies promote a flow of juice obviously lies in this, that people omitted to compare the effects of the saline solutions with those of like quantities of water (Dr. Chigin).
The second point which we may consider is the following. The chief difficulty of the physician who wishes to regulate the diet of patients when they suffer from digestive disturbances consists in the fact that idiosyncrasy plays a very important rôle. In one and the same illness, different patients react to the same diet in wholly different ways. That which is agreeable to one, and is well borne and useful, may be rank poison to another. Consequently, the golden rule in dietetics is to give no directions with regard to food till one has made inquiries concerning the inclinations and habits of the patient. What does all this indicate? Till now physiology had no experimental answer to the question. But our facts, it appears to me, contribute to a clearing up of the situation. Every food determines a certain amount of digestive work, and when a given dietary is long continued, definite and fixed types of glands are set up which can only slowly and with difficulty be altered. In consequence, digestive disturbances are often instituted if a change be suddenly made from one dietetic régime to another, especially from a sparse to a rich diet; such, for instance, as happens after the long Russian fasts. These disturbances are expressions of the temporary insufficiency of the digestive glands to meet the new demands made upon them.
Finally, it may be of some use to relate the following here. There are often cases of sudden and unaccountable digestive disturbances. From the standpoint of modern physiology they might be explained by an activity of the secreto-inhibitory nervous system, which from some cause or other has been excessively and abnormally stimulated. In any case this system is now a factor of which the physician has to take due account.
SWALLOWING AND MOVEMENTS OF THE STOMACH AND INTESTINES
By W. B. Cannon, M.D.
Of the Physiological Laboratory of the Harvard Medical School Boston, Mass., U. S. A.
[Note.—In the beginning of 1896 Dr. Professor Henry Pickering Bowditch, one of our Board of Scientific Assessors in the Nutrition Case suggested the use of the Röntgen ray as a means of learning more than was then known about the mechanism of swallowing. There was much difference of opinion among research physiologists about this important function, and the question was far from settled. Magendie published a theory of deglutition, in Paris, in 1836, which was practically accepted until 1876, when Dr. Professor Angelo Mosso, of the University of Turin, Turin, Italy, established the theory of sole peristaltic assistance in swallowing. Again in 1880 Dr. Professor Kronecker, of Berne, Switzerland, in connection with Dr. Falk, and later in connection with Dr. Meltzer, of New York, produced evidence to prove a more complicated process in deglutition than that of peristalsis alone. But even Kronecker and Meltzer found, as they went on, evidence to modify their earlier beliefs, and hence the subject was not cleared up to a point of general agreement.
The suggestion made by Dr. Bowditch was taken up in the Harvard Physiological Laboratory and formed the beginning of a series of studies of the mechanical factors in digestion. The reports of these studies, presented by Dr. W. B. Cannon and collaborators, in the American Journal of Physiology, in the volumes of 1898 and 1903, are so understandable, even to the layman ignorant of physiological nomenclature, that we are prompted to give them, almost entire, leaving out only the technical description of the methods employed, which are only interesting to research students who have access to the Journal.
It will be noted that three of the professors of physiology mentioned in connection with this preliminary study of the nutrition problem—Bowditch, Mosso, and Kronecker—are members of our presently organised Board.—Horace Fletcher.]