The most important and frequent of the infectious diseases common to animals and man is _tuberculosis_. This appears usually as a subacute or chronic affection, although in a small proportion of cases it assumes an acuteness of type which may make it fatal within as short a time as fourteen or fifteen days, or even less, from the first recognizable symptom. Tuberculosis is more prevalent than any other form of disease, and is the cause of death of a proportion variously estimated at from 20 to 30 per cent. of mankind. It is a disease which perhaps concerns the surgeon more than the physician, inasmuch as it is also the most common of the so-called surgical diseases. Its frequency varies in different parts of the country. In the average surgical clinic of the United States probably 20 to 25 per cent. of cases are manifestations of this affection. Surgical tuberculosis covers the entire range of diseases formerly described as _scrofula_. The term scrofula is now expurgated from medical terminology. All of the active manifestations formerly regarded as scrofulous are known to be due to tuberculosis. To the presence of tubercle bacilli in the tissues is due that distinctive aggregation of cells which constitutes the so-called _miliary tubercle_. Its presence and arrangement are apparently the direct outcome of the irritation produced by these minute foreign bodies, and its method of grouping is so characteristic that it may be everywhere and usually easily recognized. Its centre is composed of one, possibly several, _giant cells_, whose nuclei are generally arranged around its margin, with perhaps degenerative changes going on in the interior of the cell itself. In this giant cell, as well as outside of it, may be seen one or several _tubercle bacilli_. Around this centre are clustered a number of large cells known as _epithelioid_, which may also contain bacilli. These cells are probably derived from epithelium when at hand, or from the endothelium of the vessel walls, or from the fixed tissue cells. Outside of these are other, usually spindle-shaped, cells, contained in a connective-tissue network and regarded mostly as _lymphoid cells_. When tubercle is experimentally produced the bacilli seem more numerous than they do in instances of spontaneous disease. This little aggregation of cells constitutes a mass which may be recognized by the naked eye--a minute, usually white point or nodule, which is known as a _miliary tubercle_. It is subject to any one of several changes to be presently considered, and it is usually found in large numbers. The punctate appearance of miliary tuberculosis is perhaps best seen upon the cerebral membranes or the peritoneum in cases of acute miliary tuberculosis. By coalescence of a number of these nodules larger tubercles are formed, and by combination of coalescence and caseous degeneration are produced the large cheesy masses which were formerly called _yellow tubercle_. (See Plate VI.)