On the other hand, leukocytes may and do englobe virulent microbes. In man the mononuclear forms do not take up either the streptococcus of erysipelas or the gonococcus; whereas these two organisms are readily attracted by the polynuclear neutrophile cells. The bacillus of leprosy, on the other hand, is never attacked by the polynuclear forms, but is speedily devoured by the mononuclear cells. This shows that the various leukocytes may exercise a marked selective ability. This inclusion of minute bodies within ameboid cells seems to be an evidence of a peculiar tactile sensibility upon the part of the latter. In fact, this is clearly established, and seems to be inseparable from the peculiar attraction between leukocyte and bacterium, to which the name _chemotaxis_ has been given, and which is described in an ensuing chapter. If the included organism is, as is usually the case, killed, it is disposed of by a true process of intracellular digestion in a neutral or alkaline protoplasmic medium, and its inert portions are again extruded. On the other hand, if the leukocyte is poisoned or die in this phagocytic attempt, it presents usually as a so-called _pus cell_ or _corpuscle_, and the solid part of pus is made up in large measure of cells which have perished in this way. (See Inflammation and Suppuration.) _To regard phagocytosis as an affair mostly of certain tissue cells and invading bacteria_ would be altogether too narrow a view to take of it. It is really a process of the greatest importance and of constant performance in our systems. By virtue of it disintegrated muscle fibers and other tissue cells are disposed of, sloughs are separated, certain absorbable foreign bodies (catgut, etc.) taken away--_i. e._, absorbed--cellular tissue reduced in numerical strength (progressive atrophy), and a great variety of changes, either normal, as those pertaining to health and advancing years, or abnormal, like those incident to many diseases, are actually the product of this kind of phagocytic activity. The protective power, then, which the phagocytes exert as against bacteria is only one part of their normal functions, by virtue of which they become, in effect, perhaps the most important cells within our bodies. Their powers are limited, however, as will be seen when describing pus, for the so-called pus corpuscle is nothing but a phagocyte which has perished in its self-assumed task. It is known also that in certain instances phagocytes, which are incapable of defence as against the mature bacterial organism, are nevertheless capable of englobing its spores and preventing their development. This is true, for instance, in case of anthrax in animals ordinarily immune, as, for instance, the frog and fowl. If, however, in these very animals the vitality of the phagocytes be affected--as by cooling in fowls or heating in frogs--phagocytosis is so far interfered with that the spores germinate within the enfeebled leukocytes and the entire