Makers of Modern Medicine

by James J. Walsh

His first investigation concerned stomach digestion. It was important mainly because it directed his mind to digestive questions. In these he was to make his great discoveries. [{277}] His first independent investigation concerned the differences to be found in the digestive apparatuses and functions of the carnivora and herbivora--that is, of the meat and plant-eating animals. The differences in the natural habits of these two classes of animals had long been noted. While the meat-eaters invariably bolt their food, the plant-eaters chew theirs very carefully. Many of these latter, like the cow, are ruminants--that is, they bring up their food to chew it over again at their leisure. The instinct that makes them do this is most precious. Their food is mainly composed of starch, in the digestion of which the saliva takes a large part. The thorough mixture of the food with saliva, then, is an extremely important matter. Human beings, who are both herbivorous and carnivorous, must learn to masticate thoroughly at least the starch-containing portions of the food. Bernard's first researches concerned the nerves that supplied the salivary glands, and which consequently influence the flow of saliva. Curiously enough, the conclusions of his first experiments were erroneous. The topic led him, however, into the general subject of the influence of nerves upon glandular secretion, a problem that he was destined to illustrate in many ways.

After the salivary glands the most important structure for the digestion of starches in the animal economy is the pancreas. It was early evident, however, that the pancreatic secretion effected more than the conversion merely of starch into sugar. Its most important rôle, that of influencing the digestion and absorption of fats, was only recognized as the result of a classical observation of Bernard's upon the rabbit. He noticed that fat introduced into the digestive tract of a rabbit undergoes no change until it has advanced a considerable distance beyond the stomach. When fat is introduced into the dog's digestive apparatus a marked change [{278}] begins in it almost as soon as it leaves the stomach. At first this seemed very mysterious. Observations were made over and over again, always with the same result. There was evidently some important distinction between the intestines of the two animals. Careful investigation showed that the difference between the behavior of the fat in the rabbit and the dog was due to the presence or absence of the pancreatic fluid from the intestinal contents. In the dog the pancreatic duct which carries the secretion of the gland to the intestine empties into the intestine just beyond the stomach. In the rabbit the duct and its secretion empty into the intestine only some eight to ten inches below the intestinal orifice of the stomach. It is just beyond where the pancreatic duct reaches the intestine in both animals that the digestion of fat begins. This observation solved the seeming mystery of fat digestion, and at the same time made clear the importance of the pancreatic secretion in the general work of digestion.

Bernard's attention was directed by this first observation to the other properties of the pancreatic fluid. He soon demonstrated by experiment, not only that it split up fats into fatty acids and glycerin, and so made their absorption possible, but that it had a powerful action upon proteids--that is, upon the albuminous portions of the food, and also upon the starches and sugars. Up to this time the principal role in digestion had been assigned to the stomach and the gastric juice. After Bernard's observations it was evident that the action of the stomach was mainly preliminary to intestinal digestion, and that the chief work in the preparation of food for absorption into the system was really accomplished by the secretion of the pancreas. It took some years to make all this clear. Much of the advance in our knowledge of the effect of pancreatic juice upon proteids--that is, upon meat and other albuminous materials--is due to Kühne, a pupil [{279}] of Bernard; but not only did the inspiration for the pupil's work come from the master, but the important fundamental principle of pancreatic proteolysis--i.e., the solution of proteids by pancreatic secretion--was clearly laid down in Bernard's original publications on the subject. Only in our own day has come the greatest confirmation of the notion then first introduced into physiology, of the surpassing importance of intestinal digestion. The removal of the whole stomach for malignant disease is now undertaken without any fears as to the ultimate result on the patient's general nutrition. The operation has been done many times, and the surgeon's confidence that the intestines would compensate, as far as digestion of food was concerned, for the absent stomach has been amply justified. Patients who survived the operation have all gained in weight, and some of them have enjoyed better health than for years before the removal of their stomachs.

From his studies of the pancreas, Bernard, whose mind was always of a very practical bent, was very naturally led to the study of that puzzling disease, diabetes. The question of how sugar was absorbed into the system was an interesting one even at that time. It was not realized, as it is now, that saccharine material was a most valuable food-stuff. Its use in the world's great armies of recent years has brought sugar very prominently before the medical profession of to-day. The bone and sinew for hard fighting and exhausting marches would not seem to be derivable from the favorite dainty of the child, which has besides fallen into such disrepute as a health disturber; yet tons upon tons of sweets are now shipped to fighting armies, and are distributed in their rations when especially hard work is required of them. Bernard did not quite realize that he was attacking, in the question of the digestion and consumption of sugar in the system, one of the [{280}] most important problems of nutrition, especially as far as regards the production of heat.

Sugar is a substance that dissolves easily and in considerable quantity in water. When in solution it easily passes through an animal membrane by osmosis, and so the question of its absorption seemed simple enough. The disease diabetes showed, however, that sugar might exist very plentifully in the blood and yet the nutrition of an individual suffer very much for the lack of it. Something else beside its mere presence in the system was necessary to secure its consumption by the tissues. Bernard thought that the liver was active in the consumption of sugar, and that disease of this organ caused diabetes. He therefore secured some of the blood going to the liver of a living animal and some of the blood that was just leaving it. To his surprise the blood leaving the liver contained more sugar than that entering it. After assuring himself that his observations were correct, he tried his experiments in different ways. He found that even in the blood leaving the liver of an animal that had been fed only on substances containing no sugar, sugar could be demonstrated. Even in a fasting animal the liver itself and the blood leaving it showed the presence of a form of sugar. The only possible conclusion from this was that the liver was capable of manufacturing this form of sugar out of non-sugar-containing material, or even from the blood of a fasting animal.

This was the first time in physiology that the idea of an internal secretion was advanced. Glands within the body that gave off a secretion always possessed a duct by which this secretion was conducted to where it was to produce its effect. The idea that glands exist which pour their secretion directly into the blood-stream had not occurred.

This branch of physiology has developed wonderfully since [{281}] Bernard's discovery. The chapter of the functions of the ductless glands is one of the most interesting and most practical in modern medicine. The spleen, the thyroid, the suprarenal glands have taken on a new significance. Mysteries of disease have been solved, and, most wonderful of all, we have learned that many of the substances derived from these glands, when not present in the human body, may be effectually supplied by corresponding substances from animals, with results upon suffering human beings that are little short of marvellous. To mention but one example: the stunted, idiotic child that, because of congenital absence of the thyroid gland, formerly grew up to be a repellent, weak-minded man or woman, can now in a few short months be made the peer of most of its kind. All the modern tissue-therapy, with its hopeful outlook, is due to Bernard's far-reaching conclusions from his experiments upon sugar digestion and absorption.

His studies on sugar logically led Bernard to the investigation of heat production and heat regulation in the human body. Glycogen, the sugary substance produced by the liver, occurs abundantly in all the muscles of the body, and it was evident that muscular movement leads to its consumption and the consequent production of heat. Sugar is a carbon-containing substance, and its combustion always produces energy. The question of heat regulation was a much more complicated problem. Heat is always being produced in the human body and always being given off. Very different amounts of heat are required to keep up the temperature of the human body in the winter and summer seasons. Near the pole or at the equator man's temperature in health is always the same. To secure this identity of temperature some very delicately balanced mechanism is required. Without the most nicely adjusted equilibrium of heat production and dissemination human tissues would soon freeze up at a [{282}] temperature of 70° below zero, or the albumin of the body fluids and muscular tissue coagulate at a temperature above 110° F.

While engaged in the investigation of this interesting problem Claude Bernard found that the cutting of the sympathetic nerves in the neck of a rabbit was followed by increased heat on the side of the head supplied by the nerve, and that this increased heat coincided with heightened sensibility and greater blood-supply in the parts affected. Here was an important factor in heat regulation laid bare. It was evident that the sympathetic nerve trunk supplied filaments to the small arteries, and that when these nerves no longer acted, as after the cutting of the nerve trunk, these arteries were no longer controlled by the nervous system and became dilated. The presence of more blood than usual in the tissues and its slower flow gave occasion to more chemical changes in the part than before, and consequently to the production of more heat.

These vasomotor nerves, as they have been called, because they preside over the dilatation and contraction of the walls of the bloodvessels (vasa) of the body, are now known to play an important rôle in every function. When food enters the stomach, it is dilatation of the gastric arteries, brought on by the reflex irritation of the presence of food, that causes the secretion of the gastric juices necessary for digestion. It is the disturbance of this delicate nervous mechanism that gives rise to the many forms of nervous dyspepsia so common in our day. It is its disturbance also that makes digestion so imperfect at moments of intense emotion, or that makes severe mental or bodily exertion after the taking of food extremely inadvisable. The vasomotor nerves, however, control much more than heat processes and digestion. The familiar blushing is an example of it, and blushes may occur [{283}] in any organ. Excitement paralyzes the efforts of some individuals, but renders others especially acute. It is probable that the regulation of the blood-supply to the brain has much to do with this. While one student always does well in an oral examination, another, as well gifted, may always do poorly. Just as there are those who cannot control the vasomotor nerves of the face, and blush furiously with almost no provocation, so there are brain-blushers in whom the rush of blood interferes with proper intellection. On the other hand, there are those, and they are not always unaware of it, in whom the slight disturbance of the facial vasomotor mechanism only gives rise to a pleasing heightened color, and in the same way the increased blood-supply to the brain only gives them more intellectual acumen.