Fibrino-Croupous Tubercle appears in the shape of roundish nodules, as also, and that very frequently, of irregular, gibbous, branched masses of considerable diameter, or, upon free surfaces as gibbo-stellate layers of various thickness. The nodules in size often equal the gray tubercle granulations, still oftener do they equal hemp-seed or peas. Usually, every variety of size coexists. The substance of this tubercle is, as we may here once for all remark, - opaque from the very first, now resplendent, in various degrees, yellow, of fibrous or of granular fracture, firmly elastic, or friable, of a lardaceous, curd-like aspect. We distinguish it from the gray tubercle by the designation of yellow tubercle. It most probably constitues the pyin-holding tubercle.

The microscopic examination of this tubercle shows, as in the case of the foregoing one, a fixed base, and the aforesaid form-elements. The former is a fibro-glebous, or else an amorphous, opaque blastema. With respect to the latter much variety obtains. The number of cells, of nuclei, especially of the dull, granulated nuclei, of the elementary granules, and especially the quantity of the finest point-molecule predominate.

The metamorphosis proper to this tubercle is softening, and again cretefaction.

1. The first, namely softening, also termed suppuration, consists in this: after the tubercle has tarried for a certain time in the above-described condition of crudity, it loosens up, - for the most part with considerable increase of volume, readily breaks asunder through compression, moistens. Hereupon it changes into a yellowish, glutinous, fatty, tenacious substance, like melted cheese, and eventually liquefies to a thin, whey-like fluid of acid reaction, wherein flocculent and fragmentary particles, the remnants of tubercle imperfectly broken up, float as tubercle-pus.

In the larger tubercle masses there is often observable, during the said process, a cleft formation on a large scale; or, where the tubercle is spread out in a layer, a Assuring of this latter.

With regard to the elementary character of the tubercle at this stage, we would observe:

The softening consists in a liquefaction and breaking up of the solidified base of the tubercle to a fluid loaded with point-molecule. This transformation results in a separation or isolation of the form-elements of the tubercle, which at the same time undergo within the fluid a more or less marked change. Thus, the cells become turgescent, corroded, dissolved; the nuclei shrivelled and misshapen, irregularly angular, pouched, etc. At length free fat becomes developed in the softened tubercle.

Hence the liquefied tubercle consists:

(a.) Of a fluid with point-molecule.

(b.) Of the isolated nuclei and cells changed in the manner just now specified.

(c.) Of free fat in the shape of elementary granules and larger scattered globules.

The softening determines the malignancy of tubercle, leading as we shall presently see to ulcerous destruction of the textures, - tuberculous 'phthisis.

2. The other metamorphosis of this tubercle is cretefaction. It never affects the tubercle blastema in its primitive condition, but only in its liquefying or liquefied state.

During the softening process, or after its completion, the tubercle takes up lime-salts and fats, in the shape of free, discrete, or aggregated elementary molecule, or else in granule-cells in the form of big drops and of cholesterine crystals. In this act the softened tubercle is progressively thickened into a moist, unctuous chalk-pap, and eventually converted into a concrete mortar.

Let us now attempt to institute an inquiry respecting the nature of tubercle, in its two cardinal forms, as just delineated; whereupon we will proceed to discuss its varieties, its metamorphosis, its local process of deposition, its seat, and, lastly, its relation to the blood-crasis.

In the first place, the ground-work of rapidly solidifying tubercle blastema is, without the least doubt, fibrin. Again, in the two cardinal forms of tubercle, it is easy to recognize the two principal forms of fibrin, the simple and the croupous (see Fibrin). Why the former, which we have elsewhere denominated plastic, enters into no textural conversion, why the latter fails to undergo that prompt liquefaction proper to the croupous exudates, are questions which we shall endeavor to reply to in a more appropriate place.

With reference to the varieties of these cardinal forms, we would observe -

(a.) Of croupous tubercle there occur several varieties, together reminding us of croupous fibrin and its resulting exudates. They are determined by opacity, coloration, consistency, tendency to liquefaction, by the corrosive property of their ichor, the proportion of their form-elements, of their point-molecule, and by the character and import of their nucleus and cell-formations.

(6.) Like blastema in general, tubercle blastema is especially unwont to exude pure. The combination of the two cardinal tubercle-blastemata in different proportions, and their manifold grades of co-ordination and of blending; again, the union of varieties of croupous tubercle with each other, and with organizable blastema (fibrin), break up tubercle into countless varieties.

In like manner, the gray tubercle granulation presents many variations in respect to transparency, coloration, etc, the greenish shade, for example.

A peculiar variety is the pigmental tubercle, for the most part hemorrhagic, as to its origin.