Page:EB1911 - Volume 27.djvu/371

Rh TUBE (Lat. tuba), a pipe or hollow cylinder. Tubes play an important part in engineering and other works for the conveyance of liquids or gases, and are made of diverse materials and dimensions according to the purpose for which they are intended, metal pipes being of the greatest consequence. According to the process of manufacture metal tubes may be divided into seamed and seamless. One of the earliest uses of seamed wrought-iron tubes was for gun-barrels, and formerly these were made by taking a strip of wrought iron, bending it so that the edges overlapped and then welding by hammering, with or without the aid of grooved swages. The development of gas lighting increased the demand for tubes, and in 1824 James Russell introduced the butt-welded tube, in which the edges of the skelp are not made to overlap, but are brought into closest possible contact and the welding is effected in a double swage, having corresponding grooves of the diameter of the tube required; this method required no mandrel as did those previously in use. The following year saw another improvement in making these pipes, when Cornelius Whitehouse effected a butt weld by drawing the bent skelp through a die. Stronger tubes are obtained by using grooved rollers instead of a die, the skelp being mounted on a mandrel. This is the method commonly adopted at the present day for making this class of tube. Seamed tubes, especially of copper and brass, are made by brazing or soldering the edges of the skelp. Another method is to bend the edges so that they interlock, the contact being perfected by rolling. Seamless tubes, which are stronger than those just described, are made by drawing a bloom of the metal perforated by an axial hole or provided with a core of some refractory material, or, in certain cases, by forcing the plastic metal by hydraulic pressure through an appropriate die. The seamless steel tube industry is now of great dimensions owing to the development of steam engineering. Another type of seamless tube is the cast-iron tube, usually of large diameter and employed for gas and water mains; these pipes are made by casting.

TUBERCULOSIS. The word "tuberculosis," as now used, signifies invasion of the body by the tubercle bacillus, and is applied generally to all morbid conditions set up by the presence of the active parasite. The name is derived from the "tubercles" or "little lumps" which are formed in tissues invaded by the bacillus; these were observed and described long before their real nature or causation was known. (For an account of the organism, which was discovered by Koch in 1882, see 1911 Encyclopædia Britannica/Parasitic Diseases.) The bacillus attacks every organ and tissue of the body, but some much more frequently than others. The commonest seats of tuberculous disease are the lungs, lymphatic glands, bones, serous membranes, mucous membranes, intestines and liver. Before the discovery of the bacillus its effects in different parts of the body received separate names and were classified as distinct diseases. For instance, tuberculosis of the lung was called "consumption" or "phthisis," of the bones and lymphatic glands "struma" or "scrofula," of the skin "lupus," or the intestinal glands "tabes mesenterica." Some of these names are still retained for convenience, but the diseases indicated by them are known to be really forms of tuberculosis. On the other hand, there are "tubercles" which are not caused by the tubercle bacillus, but by some other source of irritation, including various parasitic organisms, some of which closely resemble the tubercle bacillus. To these forms of disease, which are not as yet well understood, the term pseudo-tuberculosis has been given. Lastly, the word "tubercular" is still sometimes applied to mere lumpy eruptions of the skin, which have no connexion with tuberculosis or pseudo-tuberculosis.

Pathology.—The effects of tuberculosis on the structures attacked vary greatly, but the characteristic feature of the disease is a breaking-down and destruction of tissue. Hence the word "phthisis," which means "wasting away" or "decay," and was used by Hippocrates, accurately describes the morbid process in tuberculosis generally, as well as the constitutional effect on the patient in consumption. According to the most recent views, the presence and multiplication of the bacilli excite by irritation the growth of epithelioid cells from the normal fixed cells of the tissue affected, and so form the tubercle, which at first consists of a collection of these morbidly grown cells. In a typical tubercle there is usually a very large or "giant" cell in the centre, surrounded by smaller epithelioid cells, and outside these again a zone of leukocytes. The bacilli are scattered among the cells. In the earliest stages the tubercle is microscopic, but as several of them are formed close together they become visible to the naked eye and constitute the condition known as miliary tubercle, from their supposed resemblance to millet seeds. In the next stage the cells forming the tubercle undergo the degenerative change known as "caseation," which merely means that they assume in the mass an appearance something like cheese. In point of fact, they die. This degeneration is believed to be directly caused by a toxin produced by the bacilli. The further progress of the disease varies greatly, probably in accordance with the resisting power of the individual. In proportion as resistance is small and progress rapid the cheesy tubercles tend to soften and break down forming abscesses that burst when superficial and leave ulcers, which in turn coalesce, causing extensive destruction of tissue. In proportion as progress is slow the breaking-down and destructive process is replaced by one in which the formation of fibrous tissue is the chief feature. It may be regarded as Nature's method of defence and repair. In tuberculosis of the lungs, for instance, we have at one end of the scale acute phthisis or "galloping consumption," in which a large part or even the whole of a lung is a mass of caseous tubercle, or is honeycombed with large ragged cavities formed by the rapid destruction of lung tissue. At the other end we have patches or knots of fibrous tissue wholly replacing the original tubercles or enclosing what remains of them. Such old encapsulated tubercles may undergo calcareous degeneration. Between these extremes come conditions which partake of the nature of both in all degrees, and exhibit a mixture of the destructive and the healing processes in the shape of cavities surrounded by fibrous tissue. Such intermediate conditions are far more common than either extreme; they occur in ordinary chronic phthisis. The term "fibroid phthisis" is applied to cases in which the process is very chronic but extensive, so that considerable cavities are formed with much fibrous tissue, the contraction of which draws in and flattens the chest-wall. Tuberculosis commonly attacks one organ or part more than another, but it may take the form of an acute general fever, resembling typhoid in its clinical features. "Acute miliary tuberculosis" is a term generally used to indicate disseminated infection of some particular organ—usually the lungs or one of the serous membranes—in which the disease is so severe and rapid that the tubercles have not time to get beyond the miliary state before death occurs. Tuberculosis is exceedingly apt to spread from its original seat and to invade other organs. The confusing multiplicity of terms used in connexion with this disease is due to its innumerable variations, and to attempts to classify diseases according to their symptoms or anatomical appearances. Now that the cause is known, and it has become clear that different forms of disease are caused by variations in extent, acuteness and seat of attack, the whole subject has become greatly simplified, and many old terms might be dropped with advantage.

Tuberculosis in the Lower Animals.—Most creatures, including worms and fishes, are experimentally susceptible to tuberculosis, and some contract it spontaneously. It may be called a disease of civilization. Domesticated animals are more susceptible than wild ones, and the latter are more liable in captivity than in the natural state. Captive monkeys, for instance, commonly die of it, and of birds the most susceptible are farmyard fowls, but it is practically unknown in animals in the wild state. In cattle coming chiefly from the plains (United States Bureau of Animal Industry Reports, 1900–1905) the number found diseased was only 0.134% in 28,000,000. Of the domesticated animals, horses and sheep are least, and cattle most, affected; pigs, dogs and cats occupy an intermediate position.