Tuberculosis is an infective disease in which the tissue-changes are due to the action of a specific virus or infective agent. As in the case of syphilis there is always a local or primary lesion, but the infective agent does not usually extend to the blood and infect distant parts of the body. On the other hand, the local lesion is, for the most part, a constantly extending one, infecting neighbouring parts till the death of the person, which is usually brought about, directly or indirectly, by the tubercular process.

Causation

While tuberculosis is due to a specific virus, there are undoubtedly conditions of the body which predispose certain persons to its action. Considering that this virus is very largely present in nearly all inhabited places, it must be apparent that most people are ■ exposed to its action, and yet only a limited number of persons become affected. The predisposition is frequently the result of inheritance. It has been abundantly proved to be so in the case of the commonest form of tuberculosis, namely, that of the lungs, but not so obviously in other forms. Inheritance determines a local predisposition usually in one organ or system (see ante), but when a tuberculosis is once established in the body, it is not uncommon to find it extend by one path or other to other organs, even without there being evidence of a general extension by the blood.

The Bacillus of tubercle is the essential agent of infection. This is a microbe, consisting of minute threads, each measuring in length about two-thirds of the diameter of a red blood-corpuscle. The thread contains what, appear as small rounded bodies or spores. (See Fig, 127.) The bacillus is present in somewhat varying degrees of abundance in different tubercular lesions, but it has been met with in all forms of tuberculosis.

The connection of the bacillus with tuberculosis and the essential identity of the various forms of the disease have been proved by the cultivation of the microbe outside the body, and by the results obtained by inoculating the cultivated products. (See section on Bacteriology.) The tubercle-bacillus grows slowly and only at a temperature approaching to that of the living body. We may therefore infer that it does not to any appreciable extent multiply except in the body, as the conditions will seldom be favourable, as regards soil and temperature, unless these are carefully arranged for experimental purposes. On the other hand, the bacilli, and especially the spores, are very resistant to external influences, so that ■ they retain their vitality under adverse circumstances, and are ready to begin growth when they obtain a fitting locale. The bacilli and spores survive even when dried, and they may be suspended in the air and carried as dust, ready to deposit on surfaces or to be inhaled during respiration. Tuberculosis is not commonly produced by direct communication by contact between person and person, in this respect contrasting with syphilis.

The bacilli in themselves, consisting of minute threads, cannot be supposed to have much effect upon the tissues. They produce their results by means of irritant products which they evolve. These products are present in cultures of the bacillus, and are separable from the microbe itself. The so-called tuberculin, which Koch introduced for the treatment of tuberculosis, is a glycerine extract of such cultures. It is an exceedingly active agent, an extraordinarily minute dose introduced hypodermically inducing serious general symptoms.

The minimum dose of Koch's fluid is one gramme of a fluid which has been thrice diluted to the extent of 10 times. The gramme thus contains 1/1000 of a gramme of the original fluid, or about 1/60 of a minim. Koch calculates that the original fluid contains about 1 per cent, of the essential principle, and the minimum dose will therefore contain 1/100000 of essential principle or 1/6000 of a grain. Such a small quantity as this, in a person who is already tuberculous, often sends the temperature up rapidly and produces marked local disturbance.

Fig. 127. - Tubercle bacillus from sputum in phthisis pulmonalis. x 1500.

These products, evolved by the bacilli in the living body, have somewhat varying effects. The resulting lesions are in part specific new-formations, which are called tubercles, and in part the more usual results of ordinary inflammation. These two, the more specific and the more ordinary lesions, are generally both present, but the one or the other may exist alone.

The infective nature of tuberculosis does not rest alone on the facts connected with the bacillus; it was inferred long before the discovery of this micro-organism. The most direct proof consisted in the production of tuberculosis by the inoculation of tubercular material in animals. This was first demonstrated by Villemin in I860, and was repeatedly confirmed by many observers, among whom may be mentioned Lebert, Waldenburg, Klebs, Cohnheim, Tappeiner, Bollinger, before the discovery of the bacillus in 1882. Tuberculosis may be readily produced in animals, and the material used may be tubercular products obtained from post-mortem examinations or otherwise, the sputum of patients suffering from phthisis pulmon-alis, or artificial cultures of the bacillus. The disease has been induced by inoculation (Cohnheim, Koch, Baumgarten), by inhalation (Tappeiner), and by feeding (Wesener, Bollinger).

As an example of the mode of inoculation and its results we quote the following from Koch's work on the Etiology of Tuberculosis (Sydenham Society's translation, 1886). It recounts the effects of inoculating fragments of tissue from various organs in cases of human tuberculosis, such as from the lungs in phthisis, from strumous joints, scrofulous glands, lupus, and from tuberculosis in various animals.