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IV

MICROBES AS THE CAUSE OF SENILITY

Relations between longevity and the intestinal flora-Rumi-
nants-The Horse-Intestinal flora of birds-Intestinal
flora of cursorial birds-Duration of life in cursorial birds
-Flying mammals-Intestinal flora and longevity of bats-
Some exceptions to the rule-Resistance of the lower verte-
brates to certain intestinal microbes

IN the actual state of our knowledge it is impossible to make a final examination of my hypothesis, as there are many factors about which we are incompletely informed. Nevertheless, it is possible to confront the hypothesis with a large number of accurately established facts.

Although the life of most mammals is relatively short, there are to be found in the group some which live relatively long, as well as others whose life is short. The elephant is an example of the long-lived mammals, whilst ruminants are short-lived forms. In the last chapter, I stated that sheep and cattle became senile at an early age, and did not live long. They are striking exceptions to the rule according to which the duration of life is in direct relation with the size and length of the period of growth. The cow, which is much larger than a woman, and the time of gestation of which is about the same, or a little longer, acquires its teeth at four years old, and becomes senile at an early age; it is quite old at between sixteen and seventeen,

an age when a woman is hardly adult; at the age of thirty, practically the extreme limit for bovine animals, a woman is in full vigour.

The precocious old age of ruminants, the constitution of which is well understood, and which are carefully tended, coincides with an extraordinary richness of the intestinal flora. Food remains for a long time in the complicated stomach of these animals, and afterwards the digested masses remain still longer in the large intestine. According to Stohmann and Weiske,1 in the case of sheep it is a week until the remains of a particular meal have finally left the body of the animal. The excreta of sheep, normally solid, do not betray any special putrefaction in the intestine, but if the body is opened there is abundant evidence of the process. The intestinal contents are richly charged with microbes and give off a strong odour of putrefaction. It is not surprising that under these conditions, the life of sheep should be short.

Another large herbivorous animal, the horse, also dies young, after a premature old age. Although it does not ruminate and possesses a simple stomach, the process of digestion is slow, and enormous masses of nutritive material accumulate in the huge large intestine. Ellenberger and Hofmeister2 have shown that food remains in the alimentary canal for nearly four days. It remains in the stomach and the small intestine only 24 hours, but about three times as long in the large intestine.

This is remarkcase of birds, in

ably different from what happens in the which there is no stagnation during the passage of food through the digestive canal.

1 Quoted by Frédericq et Nuel, Eléments de physiologie humaine, 4th edition, 1899, p. 256.

2 Quoted by Fredericq et Nuel, op. cit.

The structure of birds is adapted for flight, the body being as light as possible, many of the bones and the cavities of the body containing air-sacs. The absence of a bladder and of a true large intestine prevents the accumulation of excreta, these being ejected almost as rapidly as they are formed. The process of ejection, which takes place often in birds, is not so inconvenient as in mammals. The hind limbs are not used in flight, so that they offer no obstacle to evacuation. Thus birds may discharge their droppings while flying.

Such structure and habits make it not surprising that the alimentary canal of many birds contains only a scanty intestinal flora. Parrots, for instance, which are remarkably long-lived birds, harbour very few microbes in the intestine. The small intestine contains almost none, the rectum so few that the fæcal matter appears to be formed of mucus, the waste of the food, and only a very few microbes. M. Michel Cohendy, who has examined the intestinal flora at the Pasteur Institute, was unable to isolate more than five different species of microbes living in the alimentary canal of parrots.

Even in birds of prey which feed upon putrid flesh, the number of microbes in the intestine is remarkably limited. I have investigated the case of ravens which I fed on flesh which was putrid and swarming with microbes. The droppings contained very few bacteria, and it was specially remarkable that the intestines had not the slightest smell of putrefaction. Although the opened body of a herbivorous mammal, such as a rabbit, gives off a strong smell of putrefaction, the body of a raven with the digestive tube exposed has no unpleasant smell. This absence of putrefaction in the intestine is probably the reason of the great longevity of such birds as parrots, ravens, and their allies.

It might be said, however, that the long duration of life in birds is due to the organisation of these animals, rather than to the scantiness of their intestinal flora. To meet this objection, it is necessary to turn to the case of cursorial birds.

There are some birds incapable of flight, the wings of which are badly developed, but which have strong limbs, and can run with great rapidity. Ostriches, cassowaries, rheas, and tinamous, are well known examples of cursorial birds. They live on the surface of the ground, and their habits resemble those of mammals. When they are attacked by enemies, they escape by running so quickly that some of them (ostriches and rheas) outstrip even a horse. However, like mammals, they cannot discharge their secretions when they are running quickly. Tinamous (Rhynchotus rufescens), which I have observed in captivity, however quickly they may be running, stop abruptly to discharge their excretions. M. Debreuil, at my request, made observations on this matter, and assured me that the tinamous and rheas (Rhea americana) in his park always stood still for this purpose. He has noticed that the droppings, however abundant, were always deposited in heaps. With regard to ostriches, M. Rivière, director of the experimental Gardens at Hamma, Algeria, has been kind enough to give me the following information. "The discharge of excreta," he said in a letter in January, 1901, "is less frequent than in other birds, but the comparatively small size of the enclosures here makes it impossible for me to assert that the animal could discharge its droppings if it were running for a length of time; a priori I should think that this did not happen. Normally the bird stands still for defæcation, the tuft of feathers on the tail is lifted up, and there is a violent contraction of the abdominal muscles

before the sphincters of the cloaca are suddenly opened to discharge the excrement with violence."

I believe that the remarkable development of the large intestine in these running birds has been acquired to obviate the danger which is caused by the animal having to stop for defæcation. Although the huge cæca of these birds have a digestive function, particularly on plants rich in cellulose, I cannot think that the cæca of cursorial birds have been developed for digestion. As a matter of fact, some birds which are not cursorial live on the same kind of food (herbage, seeds, and insects) and have much smaller

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FIG. 14.-Intestinal microbes from the cæca of a Rhea.

cæca, the cæca indeed, in some, for instance, the pigeons, being quite rudimentary.

It is not surprising that the accumulation of food material in the large intestine of running birds is associated with the presence of an extremely rich intestinal flora. Microscopic examination of the excrement of such birds shows this at once. Although the intestinal contents and excrement of many other birds show the presence of very few microbes, belonging to a small number of species, the same materials taken from running birds show enormous quantities of microbes, belonging to a large number of species. In the cæcum of the rhea (Fig. 14) there are bacterial

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