Tuberculosis occurs more frequently in some situations than in others; it is common, for example, in lymph glands, in bones and joints, in the peritoneum, the intestine, the kidney, prostate and testis, and in the skin and subcutaneous cellular tissue; it is seldom met with in the breast or in muscles, and it rarely affects the ovary, the pancreas, the parotid, or the thyreoid.

Tubercle bacilli vary widely in their virulence, and they are more tenacious of life than the common pyogenic bacteria.  In a dry state, for example, they can retain their vitality for months; and they can also survive immersion in water for prolonged periods.  They resist the action of the products of putrefaction for a considerable time, and are not destroyed by digestive processes in the stomach and intestine.  They may be killed in a few minutes by boiling, or by exposure to steam under pressure, or by immersion for less than a minute in 1 in 20 carbolic lotion.

#Methods of Infection.#—­In marked contrast to what obtains in the infective diseases that have already been described, tuberculosis rarely results from the infection of a wound.  In exceptional instances, however, this does occur, and in illustration of the fact may be cited the case of a servant who cut her finger with a broken spittoon containing the sputum of her consumptive master; the wound subsequently showed evidence of tuberculous infection, which ultimately spread up along the lymph vessels of the arm.  Pathologists, too, whose hands, before the days of rubber gloves, were frequently exposed to the contact of tuberculous tissues and pus, were liable to suffer from a form of tuberculosis of the skin of the finger, known as anatomical tubercle.  Slight wounds of the feet in children who go about barefoot in towns sometimes become infected with tubercle.  Operation wounds made with instruments contaminated with tuberculous material have also been known to become infected.  It is highly probable that the common form of tuberculosis of the skin known as “lupus” arises by direct infection from without.

[Illustration:  FIG. 33.—­Tubercle Bacilli in caseous material x 1000 diam.  Z. Neilsen stain.]

In the vast majority of cases the tubercle bacillus gains entrance to the body by way of the mucous surfaces, the organisms being either inhaled or swallowed; those inhaled are mostly derived from the human subject, those swallowed, from cattle.  Bacilli, whether inhaled or swallowed, are especially apt to lodge about the pharynx and pass to the pharyngeal lymphoid tissue and tonsils, and by way of the lymph vessels to the glands.  The glands most frequently infected in this way are the cervical glands, and those within the cavity of the chest—­particularly the bronchial glands at the root of the lung.  From these, infection extends at any later period in life to the bones, joints, and internal organs.