The term phthisis pulmonalis was originally used to designate a wasting of the body associated with disease of the lung. In its modern use it is applied to cases in which the lungs are affected by a progressive lesion, the ordinary and regular result of which is destruction of the lung tissue and the formation of cavities. The idea of wasting is thus transferred to the lungs, and associated with the anatomical character of the lesion. It has always been recognized that tuberculosis plays a considerable part in the pathology of phthisis pulmonalis, but it is only of late years that a more complete demonstration has been furnished of the fact that virtually all cases conforming to the above definition are really cases of tuberculosis of the lung. There may be a few cases of actinomycosis, and possibly some of syphilis, in which lesions somewhat similar in character are produced, but they are so few that phthisis pulmonalis may now be regarded as synonymous with local tuberculosis of the lung.

It may be well here to refer briefly to the various phases through which the views as to the pathology of phthisis have gone since the time of Laennec, especially as many of the terms in common use in connection with the disease are related to some of these views.

Laennec believed that there was a particular tubercular matter which was liable to be deposited in the lungs or elsewhere. It might be deposited in isolated places, forming miliary tubercles, or infiltrated into a considerable portion of lung, forming infiltrated tubercle. In both cases the deposit usually began as a Grey transparent structure, which, however, was prone to change into a yellow or whitish material that was drier and harder. This yellow material was called Yellow or Crude tubercle, whether occurring in the isolated or in the infiltrated form. All cases were regarded as tubercular in which there were either isolated nodules or extensive infiltrations, whether these were grey or yellow.

By and by it came to be seen, however, that many of the conditions in phthisis are simply inflammatory. The minute histological characters of what we now call the tubercle were discriminated, and the essentially inflammatory processes were sought to be separated from the tubercular. It was shown that the existence of caseous material is no evidence of tuberculosis, since the ordinary products of inflammation and other new-formations, such as tumours, may undergo this change, which, in its essence, is really a necrosis with degeneration of the structures concerned. In phthisis, then, the process is largely an inflammatory one, with the special tendency in the products of inflammation to undergo a caseous metamorphosis. In this way arose Virchow's designation Caseous pneumonia - an inflammation with a caseous tendency in its products, just as scrofulous disease of the glands is an adenitis with a similar tendency.

When the lungs in phthisis were more particularly examined, however, it was found that the condition is not such a purely inflammatory one as Virchow's position would indicate. In all stages of the disease Tubercles are to be found alongside the inflammatory products. The tubercles undergo changes similar to the latter, and it is often difficult to discriminate between the two, especially when caseous metamorphosis has occurred. But, in nearly all cases where the disease is advancing, proper tubercles are to be found along with the inflammatory conditions.

The more modern position brings us back more nearly to that of Laennec. Again, we regard phthisis as a tubercular disease, but not merely in the general sense of Laennec. We are to observe carefully the inflammatory processes and distinguish their effects in the lung tissue. Our position differs also from Laennec's in respect that he regarded a particular state of the constitution as the essential cause of the tuberculosis. It is not to be denied that the lungs must be in a state of susceptibility before they can be affected by the tubercular Virus, but the same may be said concerning any form of tuberculosis, and indeed concerning ordinary inflammatory processes. We know,that different persons, or the same person at different times, are very variously susceptible to catarrhs, and to inflammations of all sorts.

We are to regard phthisis pulmonalis as a Local tuberculosis in which inflammatory processes and the actual formation of tubercles manifest themselves, and both lead on to necrosis and ulceration.