This mechanical irritation contracts the leucocyte into a ball, in which shape it remains until a slowing or partial stoppage of the blood current permits their dormant elasticity to reassert itself. The cell division, which follows closely on this cellular migration, must be regarded as a further outcome of this individual activity. During the division, amoeboid movement sets in, we perceive a pellucid band running transversely across the cell, giving the optical impression of an annular constriction of the surface, which often vanishes and reappears, until, at length, the separation is suddenly accomplished and the bisected halves, recoiling from each other, pursue independent paths.

Corpuscular exudation occurs in very varying degrees of intensity. Unimpeded migration and division lead to the formation of pus. We apply the term pus to a liquid which owes its yellowish color and thickish consistency to the suspension of numberless cells in an otherwise clear and albuminous menstruum. The cells of perfectly fresh pus are equal in size, globular in shape, have sharp outlines and a whitish protoplasm. The nuclei are not visible, but become so by the addition of acetic acid.

Pus appears (1) as a diffused infiltrate; (2) as a superficial secretion; (3) as an abscess.

The diffused purulent infiltrate shows us the colorless blood corpuscles and their derivatives in the first stages of their journey through the connective tissue which envelops the blood vessels, and accompanies them in all their ramifications. This connective tissue is succulent, swollen, and yellowishwhite in color, the latter completely covering and changing the normal coloring of the inflamed part. The pus corpuscles are situated partly in pre-existing connective-tissue spaces, in those crevices and juice canals in which are also found the fixed connective tissue cells, and partly in the fibrils and lamellae of the basement membrane. The space occupied by them here is only obtainable by a corresponding melting away of the fibrous texture, and thus it happens that, in proportion as the infiltrate becomes thicker and richer in cells, the inflammatory product will be composed solely of cells, which, finally, upon the addition of serum, constitute a pus focus or abscess.

If the inflamed part be a membrane, for instance, a mucous,

serous, or synovial membrane, the wandering pus corpuscles, following the direction of the least resistance, soon reach the surface, where they appear as a purulent secretion. This secretion rarely furnishes pure and unmixed pus, such as is occasionally met with in purulent inflammations of serous membranes ; * on the surface of synovial membranes, the exuded pus corpuscles are brought into contact with a large amount of fluid called synovia, by means of which the interstitial pus becomes synovial in character; while in the mucous membranes the pus corpuscles mingle with the increased mucous secretion and transform it into a muco-pus. The presence of pus, even in very trifling quantities, may always be detected from the streaked and yellowish-white coloring which it imparts to colorless fluids.

An abscess or apostema is a large accumulation of pus, which interferes with the normal continuity of the bodily parenchyma, as, for instance, a collection of pus between the muscles in the skin, brain, glands, etc. Pathologically considered, abscesses are also those collections of pus in preexisting cavities, such as joints, mucous and serous sacs, etc. Abscess-pus consists of cells, which, although already discarded by the organism, are still temporarily enclosed by it. This circumstance may suffice, for the present, to explain the behavior of the surrounding tissues toward the abscess, and especially that immediate tendency to reject the pus and discharge it externally or somewhere along the course of the mucous membrane. To accomplish this discharge, deep seated collections of pus often follow unusual and roundabout courses, guided in part by the law of gravity and in part following the direction of the least resistance. This latter is found in the strata composed of loose, areolar connective tissue. Generally speaking, the pus pursues a downward course through such strata, retaining, however, invariably, the tendency to pass from within outward, and finally reach the skin, which it sooner or later perforates.

(d) TERMINATIONS.-(a) Secondary Arterial Hyperœmia.— With the establishment of the inflammatory exudate we

*A moment's reflection will convince us that a moderate infiltration of the membranes in question must co-exist with the free purulent secretion, because the blood vessels are everywhere separated from the surface by more or less dense sheaths of connective tissue, and these sheaths must be traversed, i. e., infiltrated, before the blood cor puscles can appear on the surface.

reach the point up to which the inflammatory cause is directly and plainly operative. The exudate, both in quantity and quality, is determined first of all by the nature of the inflammatory cause. We may, as physicians, have done our utmost to limit the quantity of the discharge; we may have tried to contract the supplying vessels by persistent cold applications; we may, perhaps, by the use of quinine and other spanæmics," have attempted to check the migration of the colorless blood corpuscles; but all this with very indifferent results. We still have before us an inflammatory exudate, whose composition and extent can only be determined approximately, and we must console ourselves with the reflection that in most cases the cause of the disease has exhausted itself in the production of the exudate, and trust to nature and medical skill to remove the exudate and restore the normal condition.

In the matter of natural healing it is well known that nature employs no immediate means of relief. But should assimilation be impaired in any part of the organ, should there be an accumulation of substances which impede the functions and render normal nutrition impossible, there occurs, as in all physiological crises, by the intervention of the centripetal nerves, an arterial hyperæmia. The phenomena of this arterial hyperemia are, of course, associated with those of inflammatory hyperæmia, but in character and effect this secondary active hyperæmia cannot be too sharply sundered from the genuine inflammatory hyperæmia.

The same local centre is common to both, but the boundaries of the arterial hyperemia are extended in proportion as the surrounding arterial vascular territory is sympathetically affected. The characteristic phenomena of arterial hyperæmia, viz. dilatation of the blood vessels, and acceleration of the blood current, are everywhere apparent. In inflanmatory hyperæmia a slowing of the blood current takes place, almost amounting to stasis, and under these conditions the colorless blood corpuscles congregate on the walls and wander out. In this secondary hyperemia, the blood flows so rapidly through the blood vessels, as not only to prevent any further adhesion of colorless blood corpuscles to the walls, but also to detach any cells already there, and, so to speak, sweep the walls clean again. When this has taken place, the most important source of exudation is cut off, and

normal circulation is again restored in the inflamed part. The process of resolution can now proceed. This consists, (1) in relieving the parenchyma from the exudate, (2) in restoring the part to its former condition.

The two processes are entirely distinct. The perfect and complete restoration of the former condition is only possible when the structure of the inflamed part has suffered no injury from the exudate. But as soon as the structure of the organ is, in the slightest degree, destroyed by the accumulation of pus, the restoration can only be indirectly and imperfectly accomplished. The injurious effects of the cause of disease must also be taken into account. These consist very frequently in direct injuries, and even total death of the tissues, and we must decide, in such a case, whether, and how far, these tissues may be preserved, or whether they must be thrown off and lost.

(b) Granulation and Cicatrization.-The chief measure employed by the organism to effect a definite, even though incomplete re-establishment of function is the formation of granulation and scar tissue. This formation depends upon the arterial hyperemia in proportion as the latter furnishes improved nutrition to the cells which form the blood vessel wall and those which come in immediate contact with it. The dilated capillaries are, accordingly, enveloped with young cells, which are only loosely united together, and present the histological characteristics of young, embryonic, connective tissue. If they continue to increase in such numbers that the nourishment from the mother blood vessels becomes insufficient, new vascular loops form in a very simple manner, pushing their way, first in one direction, then in another, through the densest accumulations of cells, until they finally empty into a neighboring blood vessel. To favor their development, the cells of the germinal tissue recede from each other, and, simultaneously, the blood vessel wall gives way at those points where the future blood vessels are to arise, opening the way for the arterial blood to rush in and enlarge them. This phenomenon is especially well marked at those points where the smallest terminal arteries merge into the parenchyma which they are to nourish. Hence it is, that when it occurs on a free surface, like that of an exposed wound, small, soft, bright-red warts spring up, which have been known from time immemorial as proud flesh, or granulations.

This connection with the arterial system produces in the granulation tissue a decided constructive tendency. It fills up interstices, smooths over inequalities, and often replaces, with astonishing rapidity, any loss of substance. At the same time, the granulations are, through their large and numerous blood vessels, in intimate relationship with the bodily organism, and may even be regarded as exceptionally well nourished parts of the body. But, with the abatement of the intense inflammatory process, a change occurs. The arterial hyperæmia yields, and the recently formed tissue becomes metamorphosed into fibrous connective tissue. The closely aggregated cells produce out of their fused protoplasm a fibrous substance, which differs from ordinary connective tissue by the incomplete terminations of individual fibrils, and a constant increase in thickness. This we call scar tissue, and speak of a cicatricial retraction, or shrinkage of the original granulation tissue. The blood vessels of the granulation tissue suffer in consequence. Many of them are obliterated, but enough remain to supply the steadily decreasing parenchyma with sufficient nourishment.

SPECIAL VARIETIES OF INFLAMMATION.

The foregoing description of the process of inflammation applies, to a certain degree, to all inflammations, but it suffices fully only for local inflammations of the interstitial connective tissue, called interstitial inflammations or phlegmon. Beyond this the course of inflammation undergoes, according to cause and locality, so many modifications, that, in order to be explicit, it would be necessary to insert at this point the greater portion of general pathological anatomy. It is not my intention to describe all known inflammations. I shall, therefore, limit myself to some of the most important varieties, which include a large number of minor types.

(a) Parenchymatous Inflammation.-Organs which consist mainly of large parenchyma cells, like the liver, kidneys, and muscles, often present peculiar forms of inflammation, which may be called parenchymatous inflammations. The chief symptoms are a moderate enlargement of the whole organ; a whitish, opaque discoloration, and a perceptible change into an inelastic, doughy consistency; but no hyperæmia, or interstitial exudation. The microscope shows that these changes result chiefly from a granular opacity and swelling of the