This has been found by Metschnikoff to occur both in the blood and the tissues. In the daphne, or water-flea, where the tissues are transparent, he has been able to observe the spores of a kind of yeast passing from the intestinal canal into the body-cavity (Figs. 18, 19). As they pass through they are attacked by leucocytes - sometimes by one, sometimes by many. These leucocytes occasionally coalesce so as to form a plasmodium. When they are sufficiently powerful they digest and destroy the spores (Figs. 19, 20, and 21). Sometimes the spores may be left sufficiently long intact to germinate and give off buds, which become free in the body-cavity, and may also, like the parent spores, be attacked and digested by leucocytes.

When there are many spores they destroy the leucocytes instead of being destroyed by them (Fig. 25).

The connective-tissue cells also take up and destroy the microbes, and, from the property the cells possess of eating up the microbes, Metschnikoff names them phagocytes.1 He finds that bacillus anthracis is eaten up in a similar way by white blood-corpuscles;2 and Fodor3 has observed that various kinds of bacteria, viz. bacterium termo, bacillus subtilis, and bacterium megatherium, as well as the spores of the latter, disappear in four hours after they are injected into the blood of living rabbits;

1Virchow's Archiv, vol. xcvi., p. 177. 2 Idem, vol. xcvii., p. 502.

3 Arch, fur Hygiene, Bd. 34, p. 129.

Fig. 18.   A piece of the anterior part of the body of a Daphne, with a number of spores, some of which are still in the intestinal canal, others are penetrating the intestinal wall, and others are free in the abdominal cavity, where they are attacked by leucocytes.

Fig. 18. - A piece of the anterior part of the body of a Daphne, with a number of spores, some of which are still in the intestinal canal, others are penetrating the intestinal wall, and others are free in the abdominal cavity, where they are attacked by leucocytes.

The Struggle For Existence Between The Organism An 46The Struggle For Existence Between The Organism An 47

Fig. 19.

1. A spore which has penetrated the intestinal wall and entered the abdominal cavity, where four leucocytes have surrounded its end. m, the muscular layer of the intestine; e, epithelial layer; s, the serous layer.

2. A spore surrounded by leucocytes from the abdominal cavity of a Daphne.

3. Confluent leucocytes enveloping a spore.

4. A spore, of which one end is being digested by a leucocyte.

Fig. 20.   Different stages of the changes undergone by spores through the action of phagocytes

Fig. 20. - Different stages of the changes undergone by spores through the action of phagocytes.

Fig. 21.   A germinating spore with leucocyte adherent to it

Fig. 21. - A germinating spore with leucocyte adherent to it.

the abdominal cavity.

Fig. 22.   A spore germinating and forming conidia, which drop off and become free in

Fig. 22. - A spore germinating and forming conidia, which drop off and become free in

Tig. 23

Tig. 23. - a and b, two stages in the process of a leucocyte eating up two conidia.

Fig. 24.   A leucocyte enclosing conidia.

Fig. 24. - A leucocyte enclosing conidia.

leaving only an empty vesicle and fine detritus.

Fig. 25.   A group of conidia which have caused the leucocytes surrounding a spore to dissolve,

Fig. 25. - A group of conidia which have caused the leucocytes surrounding a spore to dissolve,

Fig. 26.   A connective tissue phagocyte, containing three fungi cells.

Fig. 26. - A connective-tissue phagocyte, containing three fungi-cells.

Fig. 27.   Leucocyte of a frog from the neighbourhood of a piece of the lung of a mouse infected with anthrax about forty two hours after the piece of lung had been placed under the skin of the frog's back.

Fig. 27. - Leucocyte of a frog from the neighbourhood of a piece of the lung of a mouse infected with anthrax about forty-two hours after the piece of lung had been placed under the skin of the frog's back. The leucocyte is in the act of eating up an anthrax bacillus.

but if the animals are weak, or depressed by hunger or cold, they have much less power of destroying the foreign organisms, and so a longer time elapses before the bacteria disappear.

Fig. 28.   The same leucocyte, a few minutes later, after it has completely enveloped the bacillus.

Fig. 28. - The same leucocyte, a few minutes later, after it has completely enveloped the bacillus.

When only a small number of pathogenic bacteria, such as the bacillus anthracis, is injected into the blood at once, they are destroyed in the organism; but when they are in larger numbers, they have the best of the struggle, and the organism itself is destroyed. It is probable that bacteria are constantly entering the organisms of men and animals from the lungs and digestive canal, but unless they are excessive in number, and virulent in their nature, they are quickly destroyed.1

The septic poisoning which occurs from wounds is not due merely to bacteria entering the blood from them, but is due chiefly to the absorption of the poisons which the bacteria have formed in the wound. The dead or enfeebled tissues which occur in the wound afford a soil favourable to the growth of the bacteria, and for the formation of their deadly products. When these are absorbed they not only poison the tissues generally, but, by doing so, convert the whole body into a soil suitable for the growth and development of bacteria, as is shown by the fact that the tissues of animals killed by the injection of sepsin decompose very quickly, and swarm with bacteria shortly after death.