=Caseation.=--Caseation comprises a series of changes in the chemical constitution of the cells by which an albuminoid mass much resembling casein in composition and appearance is produced. The English equivalent _cheesy_ well describes many of these masses, which both cut and appear very much like domestic cheese. They have a yellowish color, and are met with in masses in size from a pin’s head up to a robin’s egg. These are the yellow tubercles of the older writers, and such a cheesy tumor has been called _tyroma_. =Calcification.=--Calcification refers to a peculiar deposition of calcium salts within the interior of these nodules, the first precipitation occurring usually in the centre of the giant cell, which is itself the topographical centre of the miliary tubercle. It may spread from this until a mass easily recognizable by the naked eye and detectable by the finger is produced. Such calcareous particles are frequently found in sputa, and are always an index of the tuberculous character of the case. They differ markedly from the yellow calcareous nodules found in the pus of actinomycosis, the circumstances under which they are likely to be confused being met in pulmonary disease. COLD ABSCESSES. =Suppuration=, as indicated, is the result of a mixed or secondary infection with pyogenic organisms. In the previous chapter tubercle bacilli were grouped as among the facultative pyogenic bacteria, yet pus is not formed in this disease except in consequence of coincident activity of other bacterial organisms. Suppuration of tuberculous foci is of importance to the surgeon, because thereby is produced a distinct class of so-called abscesses--namely, the _cold_ or _congestion abscesses_. These are of the chronic type, and are generally free from the ordinary signs of abscess formation. They are invariably the result of local infection, sometimes perhaps by the tubercle bacilli alone, but frequently by the combined action of these with pyogenic forms. For their formation a previous tuberculous lesion is essential. Wherever old tuberculous lesions are encountered cold abscesses also may form. No tissue or organ is exempt: they are found in the brain, in the bones, viscera, joints, skin--in fact, in all parts of the body.