As a consequence of the absorption of a pseudoplastic process into the living blood, or, more rarely, as a consequence of the spontaneous disease of that fluid, there is a process developed which is fully discussed in its general bearings under the head of "the diseases of the blood:" it, however, affects the lungs more frequently than any other organ, and usually occurs simultaneously at several circumscribed spots. It consists in the deposition of a fibrinous product in the lung-substance, or of a coagulation in its capillaries (phlebitis capillaris), either of which undergoes matamor-phoses corresponding to the principle taken up into the blood.

As, on the one hand, the veins seem to be the seat in which deleterious substances are produced, or in which they are collected from without, and as, on the other, the whole of the venous blood passes through the lungs, - the principal organ in the process of haematosis, - it is easy to understand why it is that in general these deposits are most frequent and most abundant in the lungs.

As is generally the case in all parenchymatous organs, these deposits almost always occur in the superficial layers of the lungs. We find deposits of various dimensions, from the size of a millet-seed to that of a lentil, a pea, a bean, or even a nut, scattered through the tissue of the lung, and separated from one another by large patches of healthy tissue; the smaller they are, the more they resemble, in form, a roundish granulation, while on the other hand, the larger they are, the more they lose the round form, and appear as irregular, angular, ramifying masses. Large deposits, when lying near the surface, and pressing upon the pulmonary pleura, like those occurring in the spleen, have a wedge-like shape, being thick externally, and growing small towards the interior. They are at first of a blackish-red or brownish-red color, and firm although fragile, and can be distinguished by their sharply defined outline, and by their apparently homogeneous structure, from the surrounding tissue, which at the commencement is normal, or at most the seat of hyperemia and oedematous infiltration; but subsequently, when the deposit begins its progressive metamorphoses, a reactive inflammation, in the form of croupous pneumonia and hepatization, is set up in the lungs, and its extent is usually proportioned to the size of the deposit.

The deposit subsequently becomes of a lighter color, and undergoes one of the following metamorphoses:

In one case (and this is what commonly occurs), the deposited mass becomes more or less decolorized, and dissolves into a cream-like, purulent, or ichorous fluid, which destroys the tissues. This process commences in the centre of the deposit, much as we observe to take place in secondary phlebitis of one of the larger veins, and we then find the above-named fluid enclosed within the outer remains of the deposit, around which a reactive inflammation is established. In the course of time these, and the adjacent tissue also, undergo a similar process of fusion, and the extent of this change is proportional to the destructive tendency of the product of the reactive inflammation. Moreover, this process is very often essentially of a septic nature, and is based on the absorption of gangrenous ichor, or, on the other hand, it often undergoes degeneration, and gives rise to gangrene of the surrounding tissues. These deposits are very frequently combined, from the first, with a secondary pleurisy of a croupous nature; sometimes, however, the latter occurs as a consecutive affection, arising from the inflammatory reaction that is set up around the superficial deposits, or as a purulent or ichorous abscess in the immediate vicinity of the pleura. In the latter case, we observe the abscess as roundish, nodular, furuncular, yellow prominences, or if gangrenous destruction has occurred, as dirty greenish or brownish collapsed spots, shining through the pulmonary pleura, which itself undergoes destruction from suppuration or gangrene, with or without perforation, and gives rise to general pleurisy.

In the other metamorphosis, which, however, is extremely rare, the deposit, without dissolving or undergoing any intermediate change, passes directly from its crude state into that of obsolescence, that is to say, it shrinks into a callous, grayish nodule, which is seated in a capsule of cellulo-fibrous tissue, and in the course of time becomes converted into an osseous concretion. Many of these peripheral deposits, after their conversion into concretions, have doubtless been mistaken for chalky tubercles. The more complicated retrograde process which is sometimes manifested in deposits in other parenchymatous organs, as for instance the spleen and the kidneys, namely, the cheesy disintegration of the product, and its subsequent conversion into chalky matter, may, as we should presume from analogy, also occur in the lungs; but in the whole course of our observations, we cannot recollect a single case in which it has occurred.

We have spoken in the first volume of the rarity of obsolete deposits in the lungs, and have also accounted for it.

We have already explained how these deposits become combined with pleurisy; they are also associated with similar deposits in other parenchymatous structures - as the spleen, kidneys, liver, brain, and thyroid gland, in the tissue of mucous membranes, especially that of the intestines, in the skin, the subcutaneous cellular (areolar) substance, and all interstitial cellular layers, and in the muscles; also in the exudative processes on mucous, serous, and synovial membranes (as, for instance, metastases in the joints).

They must be carefully distinguished from lobular pneumonia, for which they have sometimes been mistaken.

h. Gangrene of the Lungs is an affection of not unfrequent occurrence, and one which, as Laennec very correctly remarks, must not be regarded as the result of an excessively acute inflammation. We do not, however, intend to assert, that it cannot by any possibility occur in an inflamed lung, for under certain conditions hepatization of a portion of the lung is unquestionably the most common complication.

We will first consider it in an anatomical point of view, and then proceed to notice the conditions under which it is developed.

There are two perfectly distinct forms of gangrene of the lungs, namely, diffuse gangrene and circumscribed gangrene or gangrenous eschar.

In diffuse gangrene, we find a portion of the lung presenting an abnormal greenish or brownish tint, filled with a similarly colored, somewhat frothy, turbid serosity, soft, rotten, and readily breaking down into a pulpy, shaggy tissue. The whole evolves the characteristic odor of sphacelus. Towards the outer portion the discoloration, infiltration, and alteration of consistence are less marked, and finally become imperceptible; and there is no line of demarcation between the gangrenous and the adjacent tissue, which only differs from the normal state in being cedematous and anaemic. It corresponds to diffuse gangrene of the bronchial mucous membrane, with which it is almost always associated. Upon the whole it is a rare affection; but when it does occur, it always attains a considerable extent, as it commonly attacks the whole of a lobe, or, at all events, its greater part. It especially attacks the upper lobes, when, in consequence of excessive activity, they have become the seat of emphysema and anaemia, the lower lobe being at the same time in a state of passive stasis. It is perhaps scarcely entitled to rank as an essentially independent affection, inasmuch as it is almost always associated with gangrenous eschar of the lungs; and hence it is the more readily developed from the contact of the ichorous, gaseous, and fluid products of the gangrenous eschar coming in contact with the bronchial and pulmonary mucous membrane, inasmuch as in all probability the disease extends from the bronchi to the lung-tissue. The above description of gangrene, as it occurs in the upper lobes, is sufficient to render this form intelligible, as well as to explain why there is no inflammatory reaction, and consequently no line of demarcation around the affected tissue.

As we have already remarked, it must be carefully distinguished from softening of the lungs.