III The Digestive System and Senility
Relations between longevity and the structure of the digestive system—The Cæca in birds—The large intestine of mammals—Function of the large intestine—The intestinal microbes and their agency in producing auto-intoxication and auto-infection in the organism—Passage of microbes through the intestinal wall
We have seen that the duration of life in mammals is relatively shorter than that in birds, and in the so-called “cold-blooded” vertebrates. No indication as to the cause of this difference can be found in the structure of the organs of circulation, respiration, or urinary secretion, or in the nervous or sexual apparatus. The key to the problem is to be found in the organs of digestion.
In reviewing the anatomical structure of the digestive apparatus in the vertebrate series, one soon comes to the striking fact that mammals are the only group in which the large intestine is much developed. In fish, the large intestine is the least important part of the digestive tube, being little wider in calibre than the small intestine. Amongst batrachia, where it is a relatively wide sack, it has begun to assume some importance. In several reptiles it is still larger, and may be provided with a lateral out-growth, which is to be regarded as a cæcum. In birds, the large intestine still remains relatively badly developed; it is short and straight. In most birds, at the point where the large intestine passes into the small intestine, there is a pair of cæca, more or less developed. These cæca are absent in climbing birds, such as the wood-pecker, the oriole, and many others. They are reduced to a pair of tiny outgrowths in the eagles, sparrow-hawks, and other diurnal birds of prey, and in pigeons, and perching birds. These organs are larger in the nocturnal birds of prey, in gallinaceous birds, and in ducks, etc.40
In the large running birds, such as ostriches, rheas, and tinamous, the cæca are relatively largest. Thus, for instance, in a rhea (Rhea americana) which I dissected, the cæca were nearly two-thirds as long as the small intestine. The latter was 1·65 m. in length, whereas one of the cæca was 1·01 m., and the other 0·95 m. The weight of the two cæca with their contents was more than 10 per cent. of the total weight of the bird.
Notwithstanding the exceptions, which are relatively rare, the large intestine is badly developed in the case of birds. On the other hand, it reaches its largest size amongst mammals. In these animals, “only the posterior portion of the latter, or rectum, which passes into the pelvic cavity, corresponds to the large intestine of lower Vertebrates; the remaining, and far larger part, must be looked upon as a neomorph, and is called the colon.”41
Gegenbaur,42 another well-known authority on comparative anatomy, writes as follows on this subject:—“The hind-gut is longest in the Mammalia, where it forms the large intestine, and is distinguished as such, from the mid-gut, or small intestine. Owing to its greater length, it is arranged in coils, so that the terminal portion only has the straight course taken by the hind-gut of other Vertebrata.”
The two series of facts are not to be disputed. On the one hand mammals are shorter lived than birds and lower vertebrates, on the other hand the large intestine is much longer in them than in any other vertebrates. Is there here any link of causality, binding the two characters, or is it a mere coincidence?
To answer the question we must turn to the function of the large intestine in vertebrates. In the lower members of the group (fish, batrachia, reptiles, birds, etc.), the large intestine is not more than a mere reservoir for the waste matter in the food. It takes no share in digestion, as that is the function of the stomach and the small intestine. Only the cæcum can be thought to have some digestive property. In reptiles, the lowest vertebrates in which the cæcum is present, it is so little differentiated from the large intestine itself, that it is difficult to assign to it any specialised function. In very many birds, however, the cæca are well separated from the main digestive tube. The food material passes into them in considerable quantities, and is retained there sufficiently long for some digestive process to take place. M. Maumus has found, in the cæca of birds, secretions which can dissolve albumen and invert sugar cane, but he has been unable to make out that the cæcal juice has any action upon fatty matter. Such digestive power, however, is slight, and when M. Maumus removed the cæca in fowls and ducks, no evil consequences followed. As in many birds the cæca are rudimentary and in others absent, it may be inferred that these organs are useless, and are in process of degeneration in the class. The cæca can be regarded as playing an important part in the organism only in the case of large running birds, where they are very highly developed, but we have not precise information as to their digestive function.
The variations in the structure in the large intestine are greater in mammals than in birds. In some mammals, the large intestine is a simple prolongation of the small intestine, similar in calibre and in structure. In these conditions it may fulfil a definite digestive function. Th. Eimer43 has determined that in insectivorous bats the large intestine digests insects like the small intestine. Such cases, however, are rare. In most mammals the large intestine is sharply separated from the small intestine by a valve, and opens directly into the cæcum which may be very large. In the horse, the cæcum is an enormous bag, cylindrical and tapering, generally well filled, and holding on an average 35 litres. It is equally large in many other herbivorous animals, such as the tapir, the elephant, and most rodents. In such cases, the food remains for a considerable time in the organ and without doubt undergoes some digestive changes. In many other mammals, particularly carnivorous forms, the cæcum may be quite absent, whilst in some, as for instance, the cat and dog, it is very small; in the latter cases its digestive function must be non-existent or insignificant.44
As for the large intestine itself, apart from the special cases, such as bats, it cannot fulfil any notable digestive function. Th. Eimer was unable to find a proof of any such action in rats and mice, and the very many investigations that have been made in the case of man seem to have established the absence of digestive power in the colon.
Dr. Stragesco,45 in a recent investigation carried out under the direction of the famous Russian physiologist Pawloff, established that, in normal conditions, digestion and assimilation of food are confined almost exclusively to the small intestine in mammals, and that the large intestine plays only the smallest part. It is only in certain diseases of the digestive tract, in which, on account of increased peristaltic action, the contents of the intestine with the digestive juices are passed quickly from the small intestine to the large intestine, that some digestive work is done in the latter organ.
The large intestine (excluding the cæcum), then, cannot be regarded as an organ of digestion, although absorption of the liquids which have been formed in the small intestine, may take place within its walls. It is known that in the large intestine the contents of the gut give up their water and assume the solid form of fæcal matter. However, whilst the mucous membrane of the large intestine rapidly absorbs water, it has not a similar action on other substances.
The question of the extent to which the large intestine can absorb has been closely investigated, because of its practical importance. It sometimes happens that invalids cannot take food by the mouth, so that their life would be in danger if it were not possible to supply them with food otherwise. Attempts have been made to inject nutritive substances through the skin, or, and this is a more usual procedure, by the rectum. By such means the organism can be kept alive for a certain time, but the absorbing power of the large intestine is extremely small. According to Czerny and Lautschenberger46 the entire colon of the human being can absorb no more than 6 grammes of albumen in 24 hours, an amount which, from the point of view of nutrition, is very small. It was thought that the large intestine might more rapidly absorb albuminous material which had been previously digested and transformed to peptones, but the experiments of Ewald47 showed that even in that case the absorption was very small. According to more recent experiments of Heile,48 carried out upon dogs which had cæcal fistulas, and in the case of a man who had an artificial aperture in the colon, the large intestine does not absorb undigested white of egg, and absorbs water, cane sugar, and glucose only very imperfectly. The only substances which are rapidly absorbed through the wall of the colon are the alkaline fluids from fæcal matter. It is possible, however, to nourish invalids by rectal injections of certain nutritious substances, the most important of which is milk.49
The large intestine, which has really very slight digestive properties and cannot absorb any considerable bulk of nutriment, is an organ which secretes mucus. The latter serves to moisten the solid fæcal material, so aiding in its expulsion.
We must conclude, therefore, that the large intestine, the organ so highly developed in mammals, is an apparatus the general function of which is the preparation and elimination of the waste products of digestion. Why should such an organ be so much more developed in mammals than in the other vertebrates?
In answer to the question, I have formed the theory that the large intestine has been increased in mammals to make it possible for these animals to run long distances without having to stand still for defæcation. The organ, then, would simply have the function of a reservoir of waste matter.
Batrachia and reptiles lead a very idle life, and can move slowly, sometimes because they are protected by poison (toads, salamanders, serpents), sometimes because they have a very hard shell (turtles), sometimes because they are extremely powerful (crocodiles). Mammals, on the other hand, have to move very actively to catch their prey, or to escape from their enemies. Such activity has become possible because of the high development of the limbs, and because the capacity of the large intestine makes possible the accumulation of waste matter for a considerable time.
In order to void the contents of the intestines, mammals have to stand still and assume some particular position. Each act of this kind is a definite risk in the struggle for existence. A carnivorous mammal which, in the process of hunting its prey, had to stop from time to time, would be inferior to one which could pursue its course without pausing. So, also, a herbivorous mammal, escaping from an enemy by flight, would have the better chance of surviving the less it was necessary for it to stand still.
According to such a view, the extreme development of the large intestine would supply a real want in the struggle for existence. M. Yves Delage,50 the well-known biologist, is unable to accept this hypothesis. He thinks that the rectal enlargement would fulfil the purpose, and adds that everyone has seen herbivorous animals pass their excretions whilst running. The rectum of mammals, however, cannot serve as a reservoir for waste matter, because as soon as such matter reaches the rectum it excites the need of excretion. The waste matter accumulates in the large intestine, from which it passes into the rectum at intervals. When it has reached that region, a sensation is caused which leads to defæcation.
M. Delage is not quite definite when he speaks of mammals voiding their excretions whilst they are in motion. A horse, harnessed to a vehicle, may defæcate whilst it is walking or even running slowly. But these animals cannot defæcate when in rapid motion, and competent observers state that horses never do so whilst racing. In zoological gardens, where animals have room to run about, they stand still before emptying the rectum. M. Ch. Debreuil, who keeps antelopes in a very large park at Melun, has noticed that the excreta are always to be found in masses and not scattered about as if they had been discharged by animals in motion. Antelopes, which are animals that run and leap extremely actively, have to come to a standstill before discharging their small pellets of deer-like excreta.
In the struggle for existence, when a mammal is pursuing its prey or escaping its enemy, there is no question of the leisurely movement of a horse harnessed to an omnibus or cab, but the greatest possible activity is necessary. In such circumstances the possession of an organ within which the excreta could accumulate would be of real importance. My theory of the origin of the mammalian large intestine is intrinsically probable.
Although the capacity of the large intestine may preserve a mammal in emergencies, it is attended with disadvantages that may shorten the actual duration of life.
The accumulation of waste matter, retained in the large intestine for considerable periods, becomes a nidus for microbes which produce fermentations and putrefaction harmful to the organism. Although our knowledge of the subject is far from complete, it is certain that the intestinal flora contains some microbes which damage health, either by multiplying in the organism, or by poisoning it with their secretions. Most of our knowledge on this matter has come from the study of human patients.
Persons have been known who do not defæcate except at intervals of several days, and who, none the less, do not seem to suffer in health. But the opposite result is more common. The retention of fæcal matter for several days very often brings harmful consequences. Organisms which are in a feeble state from some other cause are specially susceptible to damage of the kind referred to. Infants are frequently seriously ill as the result of constipation. Dr. du Pasquier51 describes such cases in the following words:—“The infant is leaden in hue, with sunken eyes, dilated pupils, and pinched nostrils. The temperature may reach nearly 104° Fahr.; the pulse is rapid, feeble, and often irregular. Restlessness, insomnia, sometimes convulsions, stiffness of the neck and strabism show that the nervous system is being poisoned by toxins, and even collapse may be reached. The foul and dry tongue, the vomiting and fetid discharges show the disturbance of the digestive tract. Very often an eruption appears, as described by Hutinel, chiefly on the back and buttocks, the front of the thighs and fore-arms.” The illness may lead to death but is generally cured by simple purging.
Women in pregnancy and child-birth frequently suffer much as the result of retention of fæcal matter, and physicians are familiar with the symptoms, which have been described as follows by M. Bouchet52:—“After normal parturition, in the course of which the usual antiseptic precautions have been fully pursued, and where delivery has been complete and natural, occasionally the patient is seized with chill and headache. The breath is fetid and the tongue foul. The temperature, taken in the axilla, is nearly 101° Fahr. The abdomen is inflated and painful in the umbilical region. Palpation in the iliac fossæ reveals lumps or consolidations along the colon. Thirst is intense, and there is complete anorexy. On questioning, it is found that there has not been defæcation for several days. The treatment consists of purgatives, enemas, and milk diet. In the next few days the bowels are emptied freely, the abdominal pain ceases, the temperature becomes lower, appetite is restored, and the patient recovers.”
Those who suffer from affections of the heart, liver, or kidneys are specially susceptible to the evil results of retained fæcal matter. In such patients an error of diet or constipation may bring about most serious consequences.
Such facts are well known to physicians, and it has been established that complete emptying of the lower bowels leads at once to favourable symptoms. From the other side, it has been shown by experiment that artificial retention of the fæces by ligature of the rectum puts the body in a grave condition.
If we collect our knowledge of all the facts, we cannot doubt but that the cause of the evil is multiplication of microbes in the contents of the large intestine. When the fæcal matter is free from microbes, as is the case with the meconium of the fœtus or new-born infant, it is not a source of danger to the organism. The waste of cells and the secretions which are added to the undigested food cannot do any harm. Amongst the microbes of the gut, there are some that are inoffensive, but others are known to have pernicious properties.
The ill-health which follows retention of fæcal matter is certainly due to the action of some of the microbes of the gut. There are difficulties, however, in determining the precise mode of action of these microbes. It is generally believed that they form poisonous substances which are absorbed by the walls of the intestine and so pass into the system. The phrase auto-intoxication as applied to infants, women in labour, and patients affected with diseases of the heart, liver, or kidneys, is based on this interpretation of the morbid processes involved. Attempts have been made to isolate and study the poisons in question, but there are many difficulties in the way. To distinguish between the actions of the poisons and of the microbes themselves, the latter have been destroyed by heat or by antiseptics, or been removed by filtration. Such methods, however, may alter the poisons and so are inconclusive. MM. Charron and Le Play53 have tried to obtain exact results by heating the intestinal microbes to a temperature of about 136° Fahr., a process which probably does not seriously deteriorate the microbial poisons. Such material, injected into the veins of rabbits in large quantities, rapidly produced death, or in smaller quantities, proportionate ill-health.
Kukula54 has tried to produce this toxic action in animals, employing microbial secretions obtained from cases of intestinal obstruction. He succeeded in producing serious symptoms, such as vomiting and curvature of the neck and back, in fact, precisely the sequence of events familiar in cases of obstruction of the bowels or other retentions of fæcal matter.
Some of the products of the intestinal flora are undoubtedly toxic, such as the benzol derivatives (phenol, etc.) ammonium and other salts. Many of these toxins have been insufficiently studied, but it is well known that certain of them can be absorbed by the wall of the gut and act as poisons. A well known case is the toxin of botulism which was isolated and studied by M. van Ermenghem.55 The poison, the product of a microbe which causes serious intestinal disturbance, is so fatal that a single drop given to a rabbit produces death after symptoms similar to those observed in cases of human beings poisoned by stale food. Butyric acid and the products of albuminous putrefaction are amongst the most pernicious of the microbial poisons produced in the large intestine. It is familiar that digestive disturbance is frequently associated with discharges of sulphuretted hydrogen and putrid excreta, and there is no doubt but that the microbes of putrefaction are the cause of these symptoms.
It has been assumed for long that the retention of fæcal matter tends to putrefactive changes in the intestines, and that the evil consequences of constipation are due to this. Recently, however, bacteriologists have criticised this accepted view, on account of the small number of microbes found in the excreta of constipated persons. Strasburger was the first to establish the fact, and his associate, Schmidt, showed that putrefaction did not follow when readily putrescible substances were infected with material taken from cases of constipation. However, notwithstanding the exactness of these facts, I cannot accept the inference which has been drawn from them. The excreta discharged naturally in cases of constipation do not give a correct indication of the conditions inside the gut; whilst such matter contains few microbes, the substance removed after injection by an enema is extremely rich in bacteria. Moreover, analysis of the urine, in cases of constipation, shows an excess of the sulpho-conjugate ethers which are known to be products of intestinal putrefaction.
Not only is there auto-intoxication from the microbial poisons absorbed in cases of constipation, but microbes themselves may pass through the walls of the intestine and enter the blood. In the maladies that are the result of constipation some of the symptoms recall those of direct infection, and it is highly probable that, if special investigations were made, microbes of intestinal origin would be found in the blood of the sick children and the pregnant or parturient women whose symptoms I have described above.
The question as to the passage of microbes through the intestinal walls is one of the most controversial of bacteriological problems, and there is little agreement in the numerous publications regarding it. None the less, it is far from impossible to get a general idea of what goes on in an intestinal tract richly charged with microbes.
Although the intestinal wall in an intact state offers a substantial obstacle to the passage of bacteria, it is incontestable that some of these pass through it into the organs and the blood. Numerous experiments performed on different kinds of animals (horses, dogs, rabbits, etc.) show that some of the microbes taken with food traverse the wall of the alimentary canal and come to occupy the adjacent lymphatic glands, the lungs, the spleen and the liver, whilst they are occasionally found in the blood and lymph. Discussion has taken place as to whether the passage takes place when the wall of the gut is absolutely intact or only when it is injured to however small an extent. It would be extremely difficult to settle the question definitely, but it is easy to see that it has little practical bearing. It is known that the wall of the gut is damaged extremely easily, so that the bluntest sound can hardly be passed into the stomach without making a wound through which microbes can pass into the tissues and blood. In the ordinary course of life, the delicate wall of the gut must often undergo slight wounding, and the frequent presence of microbes in the mesenteric ganglia of healthy animals shows clearly what takes place.56
It is indubitable, therefore, that the intestinal microbes or their poisons may reach the system generally and bring harm to it. I infer from the facts that the more a digestive tract is charged with microbes, the more it is a source of harm capable of shortening life.
As the large intestine not only is the part of the digestive tube most richly charged with microbes, but is relatively more capacious in mammals than in any other vertebrates, it is a just inference that the duration of life of mammals has been notably shortened as the result of chronic poisoning from an abundant intestinal flora.