As metamorphoses of tubercle, we have already been made familiar with its decadence or obsolescence, its softening, and its cretefaction. The first is proper to the simply fibrinous, the last two to the fibrino-croupous tubercle. These metamorphoses affect tubercle in common with consolidated blastemata of a certain constitution, whether they occur as exudates (even as extravasate1) external to the vascular system, or as endogenous coagula within the bloodvessels. Their cause is primitively inherent in the tubercle, conformably with our view respecting the primordial properties of blastemata.

1. The obsolescence of tubercle, hitherto disregarded, is cosignificant with its cornification. It implies wasting, extinction of the tubercle.

2. Softening of tubercle, a metamorphosis which it enters upon without distinction of volume - resolves itself into that elementary phenomenon, the breaking down of solidified protein substances, and especially of solidified fibrin, - a phenomenon pertaining to this substance only in its determinate croupous constitution. It is proper to fibrino-croupous tubercle alone, and is determined by a conversion of the chemical components arising out of an interchange of the elements.

Genuine gray tubercle-granulation never softens. A combination of its blastema with that of fibrino-croupous tubercle alone capacitates it for softening. It was, indeed, formerly taught, that gray tubercle-granulation lost its gloss, its transparency, became opaque, of a yellowish-white or yellow, and ultimately softened and deliquesced. The error probably arose from a readiness, in the frequent cases where the two forms of tubercle are concurrent and even now and then mingle together into a kind of transition link from the one to the other, to take for granted that they represented in reality two different stages of development.

We have, however, a second error to rectify besides. Long ago the softening of tubercle was described as a development, - a progressive metamorphosis, - but in general and not very lucid terms. Present pathology, whilst adopting the older views concerning the softening of tubercle, is influenced by the microscopic discovery of an incomparably greater number of nuclei, and especially of cells, in softened than in gray tubercle. They are looked upon as new formations out of the liquefied tubercle blastema.

1 The term "extravasate" is used by German pathologists in a restricted sense only, namely, to signify the effusion of substantive blood, with blood-globules, into surrounding textures, - in other words, internal hemorrhage from ruptured or wounded bloodvessels.

We cannot participate in this view. Those elements are not recently generated out of the liquefied blastema, but proper to the tubercle from the commencement, and isolated by the softening process. That they are more numerous in softened tubercle than in the gray, is explained by the fact, that only that tubercle softens which originally holds them in abundance, namely, the yellow (croupous) form.

In point of fact, no fluid is less adapted to furnish the blastema for new growth than the so-called pus of tubercle. The softening of tubercle takes place sometimes early, sometimes late, - rapidly, or by slow degrees. All this depends upon certain peculiarities in the character of the (croupous) tubercle. In this process it is worthy of note, that in tubercle masses deposited all at once, the softening proceeds from the central part; whereas, in aggregate masses thrown out at different epochs, and perhaps embracing different forms of tubercle, the softening may commence at any part, - even at the periphery. This fact is fraught with interest, as corresponding with kindred processes in certain other morbid products, - for example, the central softening in globular endocardial vegetations, - in intra-arterial coagula-layers, etc. Moreover, it is important as offering - if at this time of day it be wanting - a conclusive argument against the assumption of the softening of tubercle being a process evoked from without through the agency of surrounding textures. The utter absence, in tubercle, of bloodvessels of its own, the compression and closure affecting such as penetrate the larger tubercle masses from without, - the fact that in textures surrounding tubercle engaged in incipient softening no trace of inflammation is generally discoverable, - that both the latter and suppuration supervene only upon completed softening of the tubercle, - lastly, the ocular proof that the softening commences at the point most remote from surrounding textures, are so many arguments against the assumption referred to, and especially against that of a mechanical melting down of the tubercle, through pus thrown out from the inflamed encircling textures.

The sum of these negations is, that the softening is a spontaneous metamorphosis essentially proper to the nature of tubercle.

The softening is that which constitutes (yellow) tubercle a malignant growth, inasmuch as it commonly leads to that ulcerous destruction of the textures which represents tuberculous phthisis.

The complete solution of a tubercle determines in the implicated parenchyma, a gap, generally corresponding to the tubercle in size, replete with so-called tubercle-pus. The parenchyma has suffered a loss of substance to the extent only of the texture particles which happen to have been involved in the tubercle, and have now perished in the tubercle-pus. This gap represents the primitive tuberculous cavity within a parenchyma. The contact of the tubercle-pus with the surrounding.textures, occasions a corrosion of the latter. The moderate enlargement of the primitive cavity thus engendered, is substituted, upon membranous expansions, the mucous coats for instance, by a deepening destruction of the tissues; that is, of the inner stratum of the mucous membrane. This manifests itself as a millet- or hemp-seed-sized ulcer, which, to distinguish it from the different form arising from consecutive enlargement, has been designated as the primitive tubercle-ulcer.