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special attraction for the black carbonaceous matter deposited in the lungs, as the two are very frequently associated; the latter either surrounding the other, or being scattered through it.

Tubercles may remain in this condition for almost any length of time, even during a long life; but they more generally, within a limited period, increase in number, and are converted into a yellow or yellowish-white, homogeneous, dull, cheesy-looking material, known by the name of crude tubercle. This is ultimately transformed into calcareous matter, or it breaks up into a soft purulent, or sometimes flaky substance, intermixed with fluid.

The calcareous matter owes its origin either to the simple absorption of the animal matter which had existed in the tubercle, and the continuance of its earthy constituents (in which case the resulting chalky mass bears a very small proportion to the original tuberculous one); or, it depends upon a process identical with ordinary petrifaction, in which each organic molecule, as it is removed, is exactly replaced by a calcareous one, so that the chalky substance ultimately acquires the precise form of the original tubercle. The shape and size of such bodies are consequently found to differ materially in different cases; occasionally they are minutely granular, but much oftener rounded, and of the size of a pin's head or a small pea, whilst, very frequently, their form is large and irregular. I have now a specimen, in which there are several

ramifying canals, evidently arising from the tubercular mass whence it derived its shape, having surrounded some of the smaller bronchial tubes. Sometimes they are perfectly white and opaque; at others, they are grey, brown, or amber-coloured, and semitransparent. They also vary in consistence, from the hardness of stone to the softness of putty, according to the relative amount of their earthy and organic constituents. Carbonate, with a small proportion of phosphate of lime, a salt of soda, and animal matter, have constituted all the specimens I have examined; but it is very probable that their chemical characters are not always the

same.

In the case of tubercular matter originating in an inflammatory attack, its course is somewhat different. The stage of grey granulation is then remarkably short, or perhaps altogether wanting; the product of the inflammation is, from the first, of a low kind, allied to, if not identical with, the crude tubercle, or it rapidly becomes so; and the calcareous transformation seldom, if ever, occurs.

The microscopical examination of tubercle has led to very different statements as to its minute structure, scarcely two observers agreeing in all respects upon its composition; all, however, have represented it as being entirely free from bloodvessels, and every other form of highly organized tissue. M. Lebert has described peculiar cells, which he considers characteristic of tubercular

formations, serving to distinguish them from every other morbid product. Gerber has divided tubercle into different grades, according to its amount of vitality; thus, he describes the unorganized or almost granular tubercle, the cytoblast tubercle, the cell tubercle, and the filimentous tubercle, one being just a step higher than the other in the scale of organization. Vogel describes tubercle as consisting of three elements, viz. amorphous stroma, granules, and cells, all of which differ in their respective proportions in individual cases. More recently, Mr. Simon has denied the existence of cells in the tubercular deposit, or, at least, thinks them unessential parts of its composition. Gruby differs from every other observer, in describing peculiar and very large cells-many times larger than pus-corpuscles—as its characteristic constituent. And several other authorities might be quoted, equally differing from each other upon its composition.

There is, I think, a ready explanation of all this disparity, in the circumstance that tubercle is not always of the same structure: it differs in different cases, according to the degree of phthisis under which it is generated; and it may differ in the same case, according to the particular organs in which it is formed.

Miliary tubercle from the lungs consists of a dense mass of imperfect cell-nuclei and ruptured cells, thickly studded with granules, through which

are distributed a few oil-globules, elongated fibres, and black matter, identical with the ordinary carbonaceous substance so common in the lungs. I have never observed pulmonary tubercle wanting in these materials, though I have examined it from a vast number of very different cases; but they are found associated in variable proportions, according to the amount of vitality in the tubercular mass, in other words, to the degree of phthisis which has produced them; the closer the tubercle approaches to living structures, the more numerous and perfect will be its cells; and it is probable that, to such differences, the subsequent varieties, both in the course and duration of the phthisical disease, are in a great measure attributable. The cells themselves are generally so broken and irregular, that they can scarcely be said to have any definite form; but when sufficiently perfect to be examined, they are seen to be of various sizes-from that of a bloodglobule to that of a large pus-corpuscle, of irregular shape, and devoid of nuclei. The fibres do not interlace, but are irregularly scattered, and appear to be derived from the surrounding tissues, rather than as an essential constituent of the tubercular

mass.

Tubercle from other internal organs presents the same general characters, the only difference being in the absence of the carbonaceous matter, and in the cellular arrangement. Sometimes, however, no difference is discoverable in the latter respect,

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the cells being precisely of the same form, from whatever structure the tubercle is taken. But, very frequently, both the size and arrangement of the cells will be found to vary in particular organs : thus, I have seen them more than usually distinct, and of considerable size, in tubercle found in the spleen and absorbent glands; and have noticed that their shape has been less irregular, and their form larger, when taken from the liver, than when they had their origin in the lungs. I do not believe, however, that they assume definite shapes in certain organs, but only that they are liable to vary according to their situation, in consequence, probably, of sometimes approaching the cellular character of the particular organ in which they may be formed.

The proper definition of tubercle would be, that it is a mass of dead and dying cells, and abortive cytoblasts—an attempt at reproduction, ending in the formation of a cell-mass, of different degrees of vitality in different cases, and sometimes also of different structure in different organs, but incapable of long resisting the chemical and other changes to which unorganized bodies are liable.

As tubercle advances in its stages of transition, the minute structure becomes modified. The yellow or crude tubercle is composed of granular matter, and an increased number of oil-globules, with which may be interspersed a few remains of cellgrowth, in the form of broken cell-walls and cytoblasts.

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