Phagocytosis in anthrax pustule. (Gaylord.)
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 organism is infected.
CHAPTER III.
INFLAMMATION.
Inflammation is an expression of the effort made by a given organism to rid itself of or render inert noxious irritants, arising from within or introduced from without (Sutton, modified).
After having duly considered hyperemia as a phenomenon having an identity and termination of its own, we are prepared to study the more complex processes included under the term inflammation, the first of which is the hyperemia already considered. The characteristic of the truly inflammatory process is that it does not stop with mere congestion nor with any of its previously mentioned terminations, but goes on to something more complex. It must be understood, therefore, in this consideration that hyperemia is the first act of the vessels, resulting from peculiar stimuli which will shortly be considered. Even the hyperemia seems to be now more distinct than under other circumstances, and, along with the dilatation of vessels and the stagnation of blood current, the capillary vessels seem crowded with blood corpuscles to an abnormal degree, the rapidity of their motion is checked, and there occurs accumulation of blood cells along the walls of the small veins, to which they seem to adhere as if by some new cohesive property. The result is that before long the vessel wall appears to have received a new coating of white corpuscles, this being more marked in the veins than in the arterioles, while in the latter the red are more numerously mingled with the white than in the veins, in which the distinction between the two classes of cells is better maintained.
Next comes the phenomenon whose clear recognition and description is inseparably connected with Cohnheim’s name. This is known under different names as migration or diapedesis of the leukocytes. The program is about as follows: A little protrusion of the vascular wall, a marked alteration in the shape of a leukocyte, which yet adheres to this point of its lumen, and then the curious fact so often seen under the microscope—the gradual passage of this cell through the vascular wall, from its inner to its outer side, by what is generally known as its ameboid movement. This migration of the leukocyte is not confined to its mere escape from the restriction of the vessel lumen, but goes on to an indeterminate extent after it has detached itself from the outer surface of the vessel. This seems to occur by virtue of the same ameboid characteristic which it exhibited in passing through between the cells of the vessel itself. If this occurs at one point, it occurs at innumerable points, in consequence of which a large number of leukocytes escape into the tissues of the part involved. This diapedesis occurs most markedly from the smaller veins, to a less extent from the capillaries. The cells which escape from the latter are usually accompanied by red cells, the consequence being that the exudate which necessarily occurs at the same time is more or less tinged with the coloring matter of the blood, and is known as a hemorrhagic exudate.
The above phenomenon, described in so few words, is in its minutiæ a really complex one, depending on a variety of causes not easily appreciated; but it is at least positive and well known, because it can be observed at will in the mesentery or web or tongue of certain animals which can be confined upon the stage of the microscope. The phenomena of inflammation, therefore, comprise, first, hyperemia, and then escape from the bloodvessels of the corpuscular and fluid elements of the blood. The former may be due, as already seen, to various irritations of a non-specific character; while, as we shall learn, the latter practically never take place save when the irritation has been, as pathologists say, specific or infectious.
The phenomena of true inflammation comprise practically the roles played by the three elements which conspire to produce those changes—namely, the tissues, the blood, and the specific irritants which are the primary cause of the entire lesion. Each of these should be considered separately.
All observers agree that in actively inflamed tissues the number of cells is very greatly increased. A certain increase may be accounted for by that which has already been described—namely, the escape into the tissues of the wandering cells from the bloodvessels. But neither this alone nor the products of their rapid proliferation are sufficient to account for all the cells found in the truly inflammatory condition. It is now well established that in connective tissue there are two varieties of cells—the fixed and the wandering—the former concealed in the trabeculæ of the intercellular substance, while the latter are small, ordinarily round in shape, much resembling the white corpuscles, possessed of ameboid characteristics, and having the power of changing position. These are known as the wandering cells, which meander through the lymph spaces of the tissues or back and forth into and out of the blood-vascular system, their migration being regulated by causes not yet known. Under natural conditions their number is relatively small. Once given a true inflammatory disturbance they are reproduced with amazing rapidity; and their numbers, added to those produced by diapedesis of leukocytes, with the combined proliferative activity of both forms, serve to account for the new cells whose presence characterizes phlegmonous and other similar disturbances. That these wandering connective-tissue cells have much to do with these changes is shown by the unmistakable evidences of excessive activity known as karyokinesis (i. e., nuclear activity).