PATHOLOGY AND BACTERIOLOGY. Tuberculosis is invariably due to the presence in the body and to the action upon the body tissues of a specific microorganism known as the tubercle bacillus.
The tubercle bacillus is a thin bacillus from three to four mieromillimeters long, i.e. about one half the diameter of a red blood cell. It is often slightly curved and there is a marked tendency of the bacilli to hang together, two often lying in the shape of a letter V. The bacillus grows on ordinary culture media at body temperature, but the growth is extremely slow. Thus, cultures on blood serum or on nutrient agar rarely show any signs of growth earlier than two weeks, when thin grayish dry scales begin to appear on the surface of the medium. Cultures grow best in the (lark; in fact, the bacillus is killed by exposure for a few hours to the direct rays of the sun. From a pure culture successive cultures may be made in definitely. Such cultures show a gradual loss of virulence, but no changes in growth char acteristic. Inoculation of susceptible animals, such as the guinea-pig, with these cultures re produces the disease. See Plate of DISEASE GERMS.
The bacillus stains with difficulty with the aniline dyes, but when once stained retains the dye with great tenacity. It is upon this staining characteristic that our most important method for morphologically differentiating the tubercle bacillus depends. This consists in staining a sus pected specimen with a strong solution of a red dye known as fuehsin, to which carbolic acid has been added to increase its penetrating powers. After staining, the specimen is washed in dilute acid, which extracts the stain from all other bac teria. The tubercle bacilli retain the red stain in spite of the action of the acid and may thus be differentiated. But three other bacilli re semble the tubercle bacillus in this staining re action. They are the bacillus of leprosy, the smegma bacillus, and the so-called 'hay bacillus.' The first and last of these can usually be excluded on the ground of the impossibility of their being present. The smegma bacillus as a rule enters into consideration only in examining urine for the tubercle The changes which the tubercle bacillus causes in the body tissues are various. Thus it may stimulate the passage of serum and white blood cells from the blood vessels into the tissues, caus ing an exudative inflammation; or it may stimu: late the production of new connective tissue, caus ing what is known as a productive inflammation; o• its effects may be more severe and lead to death of tissue or necrotic inflammation. Any one of these effects may be produced separately, or, as is commonly the ease, they are emnbined.
The most distinctive feature of the action of the tubercle bacillus in the body is the production of what are known as tubercles or tubercle tissue. This tubercle tissue occur either as the nodular tubercle or as diffuse Whereto tissue. in either ease, according to Oster, "The `tubercle' presents in its early formation nothing,' distinct ive or peculiar either in its components or in their arrangement. Identical structures are pro duced by other parasites, such as the actionomy cosis and the ctrongylus in the lungs of sheep." In the production of the 'tubercle' the bacilli. AC cording to the extensive researches of Baum garten. first cause a proliferation of the connec tive tissue elements with the production of epi theliod and giant cells. Following this is an in flammatory reaction with the exudation of serum and white blood cells. The 'tubercle,' which is now fully developed, may next undergo degenera tive changes consisting of either caseation or sclerosis. Caseation begins in the centre of the growth and consists in a degeneration of the cells and their reduction to a homogeneous. strue tureless mass. Coincident with the caseation or following it is a sclerosis by which the tubercle is converted into a firm, hard substance composed of connective tissue rich in fibres. Osler describes very graphically these two processes as follows: "In all tubercles two processes go on: one—easea tion—destructive and dangerous; and the other —sclerosis—eonservative and healing. The ulti mate result in a given ease depends upon the capabilities of the body to restrict and limit the growth of the bacilli. There are tissue soils in which the bacilli are, in all probability, killed at onee—the seed has fallen by the wayside. There are others in which a lodgment is gained and more or less damage done, but finally the day is with the conservative protecting forees—the seed has fallen upon stony ground. Thirdly, there are tissue-soils in which the bacilli grow luxuriantly, caseation and softening, not limita tion and sclerosis, prevail and the day is with the invaders—the seed has fallen upon good ground." Small tubercular nodules are called miliary tubercles. Seen with the naked eye the nodular tubercle is a small white, gray, o• yellowish body. the smallest not visible without a miscroscope. Microscopically these tubercles show around their edges proliferated connective tissue and endo thelial cells, with often some giant cells. while the centre is composed of tissue undergoing caseation or coagulation necrosis. Tubercle ba cilli may be few or numerons.