Pathologists are in the habit of recognizing only one form of pneumonia. It is true that this is by far the most frequent form; but even in regard to this there are several points in which we cannot agree with the accepted view. We may provisionally and very briefly remark that the evidence of its croupous nature will be the more manifest in proportion to the epidemic constitution and the special cause of the disease, the rapidity of its course, the degree of its intensity, etc. We shall treat of this, the most common form of pneumonia, under the designation of:

1. Croupous Pneumonia

The course of this disease is divided, as is well known, into three stages, which have received the names of inflammatory engorgement, hepatization, and purulent infiltration. We shall first consider the case in which a whole lung, or at least a whole lobe is affected.

The first stage, inflammatory engorgement, is always preceded by the above described condition of simple stasis and splenization of the parenchyma; but, conversely, this condition is not always developed into inflammatory stasis or engorgement. This affords the explanation of the contested question regarding the inflammatory nature of simple stasis and its significance as a stage of the inflammatory process. It is only by a careful examination that we can distinguish inflammatory engorgement from simple stasis. The lung is generally of a dark red color, heavy and tough; it pits on pressure, and we perceive that it contains a fluid and little or no air. On cutting it we find its substance denser than in the normal state, in consequence of the swollen condition of its tissues and of its being filled with a sero-sanguineous fluid; and according to the degree of this state the lung may either crepitate and swim in water, in consequence of its still containing a little air, or it may sink and not crepitate; it is easily torn, very moist, and pours forth a sero-sanguineous fluid, which is sometimes rather frothy and sometimes not at all so.

This condition has, as we have already remarked, the greatest similarity and affinity to simple stasis, especially when the latter is combined with oedema.

We will now direct attention to the characteristic symptoms by which inflammatory engorgement may be distinguished from the above-named similar condition. Amongst them we may mention the color tending to. a brownish-red, and the moisture of the parenchyma, which in itself is sufficient to distinguish the inflammatory from the simple stasis, and also from that combined with oedema, by the special circumstance of its depending on the tissue being filled with blood that has already undergone the inflammatory metamorphosis, or, in other words, with a brownish or brick-red, thin but viscid fluid mixed with black, crumbling flocculi. As soon as the transition to the second stage commences, there is a secretion of a very viscid, tough, reddish-brown fluid, - the characteristic sputum, as may be proved by an examination after death; and, finally, there is the true exudation with which appears -

The second stage, or that of hepatization, in which the lung appears both externally and internally of a dark brownish-red color, is solid but friable, does not crepitate, and sinks when placed in water. On examining the cut surface we either observe the above color uniformly distributed, or it is deposited in the form of irregular spots amongst the black pulmonary tissue; while the pale red interlobular tissue presents ramifications, and the whitish bronchial tubes and the bloodvessels form stripes or islands which destroy the uniformity of the coloring, and give the cut section a marbled appearance. Further, the cut or torn surface presents a change of texture which is perfectly characteristic; when the light falls obliquely on this surface we perceive that it has a granular appearance, which is the special reason why it resembles the tissue of the liver, although the similarity is aided by the firmness, fragility and color of a hepatized lung. Hence the origin of the term hepatization, which is now generally adopted and understood. The character of the granulation is uniform, and the individual granules are roundish. - Scarcely anything exudes from the cut surface, and it is only by a certain amount of pressure, or by passing the scalpel over it, that a brownish-red, turbid, sanguineo-serous fluid escapes, mixed with blackish-brown and a few reddish-gray flocculi.

The volume of the hepatized lung does not in general exceed that of the healthy lung in a state of full inspiration: hence its surface is smooth, and never indented by the ribs, and there is no dilatation of the thorax. Sometimes, however, we find single lobules projecting higher above the surface than others, in consequence of a want of uniformity in the progress and the degree of the exudation; and the granulation of these tissues is coarser in consequence of the products of inflammation being here deposited in greater quantity than in other parts.

This form of hepatization is named the red, to distinguish it from those varieties in which this color is no longer present, although the granular texture remains.

On what does the granular texture of the hepatized lung depend? This is a most important question; the ordinary answer to which is, that it results from so great a swelling of the walls of the air-cells that their cavities become obliterated, each granulation being thus represented by an air-cell. We can by no means give our assent to this generally received opinion, for we are convinced that the granulations are produced by the inflammatory product deposited in the cavities of the air-cells; we shall, however, postpone bringing forward our evidence on this point, since the perfect solution of the question is intimately connected with the determination of the seat and nature of the pneumonic process.

Each granulation is a hardish, fragile, dark-red, roundish plug, which adheres so closely to the dark-red, swollen wall of the air-cell, that it is difficult to separate and extract it.