Softened tubercle is nothing more than an intensely granular mass, intermixed with oil-globules; and so peculiar is its dark granular appearance, that it may occasionally be detected in examining microscopically the sputa of consumptive per sons. The conversion of miliary into yellow tubercle, and the process of tubercular softening, essentially consists in the progressive loss even of that low degree of vitality which the morbid deposit originally possessed, the gradual dissolution of everything which had connected it with living tissues; and it is worthy of remark, that the action is brought about precisely in the same way that many other and healthy structures become degraded, viz. by the change of albuminous into oily matters-or, as it is termed, "fatty degeneration". The chemical analysis of tubercle has been, hitherto, signally unsuccessful in helping either to explain its nature, or to advance anything which might contribute to its prevention. The disagreement of chemists respecting its composition, surpasses even that of the microscopists upon its structure. Thénard describes the tubercular deposit as a compound of animal matter with salts of lime and soda. Güterbock has detected within it a peculiar substance, termed phymatin. Scherer's analysis represents it as composed of water, animal matter, fat, and fixed salts. The general conclusion would be, that it is made up of an imperfectly-formed albuminous or other animal substance, fatty matters, and salts―principally of lime. It is probable that tubercle is not always of the same chemical composition; and hence, perhaps, the differences met with by different observers. What I have already expressed as the degree of phthisis, or, in other words, the amount of deviation from healthy function constituting the disease, may determine the kind, as well as the structure, of the morbid deposit; so that this may present every possible shade of departure from healthy substance, -in some cases, perhaps, closely approaching in its constituents the properly organized materials of health, but in others widely differing from them. It may be partly owing to this circumstance, that the pulmonary symptoms of phthisis present such infinite variety in different persons. CHAP. III. THE SITUATION OF TUBERCLE. THE apices of the lungs, at least in chronic phthisis, are well known to be the chosen seat of tubercle; and, except occasionally at an advanced stage of consumption, the morbid deposit rarely extends to the bases of these organs. The limitation of tubercle to the lower lobes is very uncommon: I have met with only two instances of its occurrence. In the acute form of phthisis, the tubercular substance is often scattered throughout the lungs, but even then, the upper lobes are usually more involved than the lower; and the same tendency may often be traced in the few cases of consumption which spring out of pneumonia, where, although the disease may have begun at the bases, it commonly spreads to, and afterwards progresses more rapidly in, the upper parts. An opinion has long existed, that the left lung is far more frequently tubercular than the right; but with a view of testing its accuracy, I have arranged the following table, deduced from my own observations upon a thousand patients, in different stages of the disease. TABLE I. Showing the Position of Tubercle in 1000 Cases of Phthisis. The left lung is here seen to have been oftener diseased than the right; but a marked difference is observable, according to the stage of the malady. During the first stage, the excess is so trifling as to give it the appearance of being what might be termed accidental, were it not that the separate investigations of the most eminent pathologistsamongst whom I might mention the names of Louis, Andral, and Carswell-have led to the same result. I infer, therefore, that the left lung is more liable to become tubercular than the right; but, that the difference in this respect is so small as to render it a subject of curiosity only, and of not the slightest practical value either to the diagnosis or the treatment of consumption. The gradually increasing excess, on the part of the left lung, exhibited by the table as the disease advances, points, however, to the great probability of tubercular softening and vomica being more prone to happen on the left side. Instead of attempting an explanation of these peculiarities in the tubercular secretion-why it prefers the apex to the base of the lung, and appears more frequently upon one side than the other, I shall merely remark, that reasons such as those which have been generally urged, seem insufficient to account for them. Other diseases as well as phthisis have peculiar regions, which they appear to regard as their own. The frequent growth of cancerous matter in the lower lip, and its great rarity in the upper;-the well known selection of certain joints by the gouty deposit ;-the preference shown by certain cutaneous diseases for particular portions of the skin;—the rarity with which idiopathic pneumonia attacks any other part of the lungs than their bases, are familiar illustrations of this principle, whilst all seem likely to remain equally inexplicable. The particular tissue of the pulmonary organs in which tubercle is formed, has been a subject of long, and, as it appears to me, very useless controversy; for, it might be anticipated that anything separated from the blood-which permeates everywhere, would not be limited to any one structure. The tubercular deposit is, accordingly, found both upon free surfaces and within the pulmonary texture. When slowly deposited, it has a manifest tendency to accumulate in the areolar tissue between the air-cells; but even under such circumstances, it is far from being limited to this part, |