The collective term "tubercle" is made to embrace sundry formations, which have nothing in common beyond their outward form.

Still, after having well sifted this side of the question, we shall ourselves feel bound to comprise under "tubercle," formations in external appearance quite dissimilar to what is commonly called tubercle, nevertheless essentially identical with it; for instance, the primitively yellow, fibrino-croupous tubercle.

If we except the rare instances in which it represents an endogenous deposition within the circulating system, tubercle is in the broadest sense an exudate - an exudate of solidified protein substances (fibrin, albumen), which as blastema persists at the lowest grade of development; that is to say, in the primitive crude condition determined by its consolidation. It thus occupies the point of transition to the non-organized new growths.

This last attribute is essential and indispensable, imparting to solid blastema the impress of tubercle. It is so important, that every blastema, however much its characters may assimilate to tubercle in other respects, loses the distinctive mark the moment it enters upon a transformation of texture.

This exudate (in its broadest sense) is for the most part distinguished by the tubercle-form; that is, by its appearance as scattered or collected nodules, or where more copiously produced, by its deposition in granulations and stellate masses. It is hereby cognizable at the first glance. Still this is open to exceptions.

Gelatinous and fibrous cancer appear now and then in a tubercle-like form; that is, in the form of little discrete nodules or stellate bodies; and, upon serous membranes, the peritoneum, for example, there occur granular exudates of fibroid and areolar tissues. These are distinguishable from tubercle by their texture.

But, again, even tubercle itself occurs in extensive, irregular masses. There are inflammatory products endowed with an indwelling tuberculous character, although manifesting a total absence of the external habitudes of tubercle.

Tubercle has therefore sometimes a local, but far more frequently a general import and significance. It is invariably so closely linked with dyscrasial processes, that, for a profitable consideration of tubercle, an incessant retrospect to the dyscrasial relations is imperatively demanded.

Nevertheless, the basis and starting-point for an anatomical inquiry concerning tubercle itself, must in our opinion still be the aforesaid fixed blastema abiding at its primitive stage of crudity.

In this sense tubercle offers sundry distinctions, some obvious and essential, others less marked. They relate to its color and lustre, its transparence, its consistence, its elementary fabric, chemical composition, etc. These are characters referable to more or less manifest special crasial relations - modifications of a fundamental tubercle-crasis. They determine several, and some of them essential, forms of tubercle, which we shall proceed at once to portray, selecting for our basis the purest possible forms.

(A.) Simple Fibrinous Tubercle

Simple Fibrinous Tubercle appears as scattered or stellate conglomerations of granules of about the size of millet-seeds. It presents, moreover, as the product of inflammation upon serous membranes, smooth pseudo-membranous exudates, as we often find exemplified upon the pleura of lungs involved in florid phthisis.

In the first-known form this tubercle represents the gray semi-transparent granulations of Laennec.

The investigations and theories hitherto instituted relate almost exclusively to this tubercle, from which all other tubercle-formations have been derived as from a stereotype basis.

The question of old - What is tubercle? must at this day be changed into - What is this particular tubercle?

In its early stage, at which acutely generated tubercle is often enough to be obtained in the human subject, it appears in the form of the aforesaid granulation, - to the naked eye a roundish, resistant, solid nodule, of about the size of a millet-seed. Not unfrequently, however, we encounter amongst them tubercles somewhat smaller, and representing a less firm, a softer, at the same time more transparent, almost vesicle-like granule.

Nevertheless, however much tubercle may at a first glance wear a vesicular appearance, it invariably originates as a solid corpuscle; and the results of a careful analysis of this substance, as well as its very nature and import, serve to corroborate this fact.

Minutely examined, it only seemingly represents a spherical body. Under a moderate magnifying power, - nay even on a narrow inspection with the naked eye, it is seen at its circumference to branch out more or less. With the textures it is only in so far connected as to lodge betwixt their elementary parts, to take up some of these into its substance, and - what is especially discoverable in tubercle upon serous membranes, - to adhere to them by dint of an indwelling tenacious property. It represents a tolerably homogeneous - now toughish, gritty, fibro-granular, fragile, now softish, uniformly compressible - substance, in various shades and modifications, of a pearly gray color.

Under the microscope it reveals the following elementary composition:

It consists mainly of a more or less pellucid base (blastema), which affords a sort of binding medium for certain form elements. Its components therefore are -

1. The said basement-mass, - for the most part a fibro-glebous, gray, fixed blastema, rendered turgescent and transparent by acetic acid.

2. Certain embryonic form-elements, namely: (a.) Elementary granules of various magnitude.

(B.) Nucleus Formations

Nucleus Formations, both black-contoured, lustrous, spherical, even oblong nuclei, - and more delicate, dull, granulated nuclei, under various phases.

(C.) Nucleated Cells

Nucleated Cells; commonly in such small numbers as to tempt one to doubt their occurrence altogether. Nuclei and cells are often to a great extent misshapen, disorderly, jagged, angular, bulging, dumbbell-shaped, rudimental, stunted.

Along with this, the tubercle is wont to include various elements appertaining to the textures in which it nestles. The tubercle purest in this respect is that upon serous membranes, which, therefore, like many other new growths upon serous membranes, is the best adapted for examination. Nay, tubercle will even take up and incorporate compound textural constituents, and in particular bloodvessels. The question here suggested as to tubercle-containing vessels of its own will be discussed hereafter.

The metamorphosis which this tubercle undergoes, is limited to decadence. After abiding in the primitive, crude condition before described it becomes transformed, with the loss of its moisture, - with condensation - to a hard nodule, and shrivels, into a tough, amorphous or indistinctly fibrous, horn-like mass, - in a word, cornifies. This determines a complete wasting and death of the tubercle, subversive of all further change. Occasionally this process is associated with bony deposition, the tubercle becoming a partly ossified nodule.

This tubercle does not undergo any other metamorphosis independently. Every other change suffered by it is based upon a combination of its blastema with another, and its softening in particular, upon a combination with the ensuing tubercle, namely, the fibrino-croupous. This softening process plays so momentous a part in the doctrine of tubercle, that we deem it right to declare emphatically our dissent from the opinion that gray tubercle, the gray tuberculous granulation of Laennec, softens.