before death occurs. Such a manifestation is termed acute miliary tuberculosis. The intra-cellular or extra-cellular toxins produced by the tubercle bacillus in the course of its develop- ment are the cause of many of the pathological features associ- ated with the disease the bacillus produces.
The tubercle bacillus is a minute rod-like, often slightly curved, organism, i-SM 3'SM in length and 0-3/1 in breadth. It may stain uniformly or present a beaded appearance, the unstained beads being regarded by Koch as spores. Metchnikoff ad- vanced the view that the bacillus as ordinarily met with is but a stage in the developmental cycle of a filamentous fungus. The
- organism is regarded by many as a member of the streptothrix
group belonging to the hyphomycetes or mould fungi. It stains with difficulty but retains its stain, once received, with remarkable tenacity, resisting decolorization by strong acids and hence called acid-fast.
Exposed to direct sunlight or ultra-violet radiation it is rapidly slain but is of retentive vitality under certain conditions. Dr. Stenhouse Williams has shown that it remains viable and virulent in cow-dung on pasture land for at least five months, and in dung stored in the dark for twelve months, a discovery of great importance and significance in veterinary practice. It retains its virulence and capacity for development for six weeks or longer in decomposing sputum and for six months in dried sputum. The thermal death point varies between 65C. and ooC. and Swithinbank has shown that it will survive a tempera- ture of i86C. for 42 days.
Certain antiseptics are fatal to the tubercle bacillus, 5% carbolic acid will slay it in less than a minute, and endeavours have been made to destroy the organism in living tissues by the administration of drugs, but hitherto without demonstrable success. Methylene-blue and certain copper salts injected into guinea-pigs infected with tuberculosis, can be demonstrated in their tubercle, bacilli, and a fascinating, but hitherto unfruit- ful line of chemotherapeutic research has thus been opened up, aiming at the destruction of the tubercle bacilli while parasitic in their host.
The tubercle bacillus is widely parasitic through the animal world, but different animals show widely varying degrees of susceptibility. Generally, domesticated animals are more liable to infection than wild, and captive wild animals than those in their natural state. Domestic cattle are particularly prone. Much controversy has arisen as to the communicability of tuberculosis from animals to man. After Koch had thrown doubt on its probability at the British Congress on Tubercu- losis in 1901, the British Government appointed a Royal Com- mission to enquire into the relationship of human and animal tuberculosis. In the second interim report of the Commission, issued in 1907, the conclusions arrived at were: " That there seems to be no valid reason for doubting the opinion, never seriously doubted before 1901, that human and bovine bacilli belong to the same family. On this view, the answer to the question Can the bovine bacillus affect man? is obviously in the affirmative. The same answer must also be given to
- those who hold the theory that human and bovine tubercle
- bacilli are different in kind, since the ' bovine kind ' are readily
to be found as the causal agents of many fatal cases of human 1 tuberculosis." In later years the investigation has been further pursued. Pulmonary tuberculosis is rarely bovine in origin,
- md non-pulmonary tuberculosis does not appear to be so
! commonly bovine as previously supposed, the latest researches ! (up to 1921) in England going to show that probably approxi- mately some 30% of cases of non-pulmonary tuberculosis are af bovine and 70% of human origin. While cows are the
- ommonest domesticated animals infected, tuberculosis occurs
pigs, less commonly in dogs, cats, and but rarely in horses ind sheep. Tuberculosis in rats has been demonstrated. In 'domestic fowls another variety of the tubercle bacillus, the ivian, is found. Even reptiles, fishes and invertebrate creatures such as worms may be infected. While bovine bacilli are
- apable of infecting the human subject, especially in childhood,
ivian bacilli are of little human pathogenic significance.
In the evolution of pulmonary tuberculosis, human or, very rarely, bovine bacilli may be detected in the sputum, and usually are demonstrable in tuberculous pus derived from any focus. The bacilli may also sometimes be found in the blood or stools of infected subjects.
Channels of Infection. Congenital origin, though rare, has been proved to exist. By Cobbett it has been regarded as commoner than usually supposed. Tubercle may be introduced by direct inocu- lation. Much more commonly the infection is produced by inhala- tion of dried sputum as dust into the lungs, or by the ingestion of tuberculous material into the alimentary canal. Cornet has esti- mated that as many as 7,200,000,000 bacilli may be expectorated by a consumptive patient in the course of a day, and it requires little consideration to show what a ready means is thus presented of infection. Recognition of this fact is of importance in prophylaxis. The vehicle of infection by ingestion is commonly tuberculous milk or butter. The portal of entry, carious teeth, tonsils or some por- tion of the intestine. Fatal bovine infection, though rare in adults, is not uncommon in children. Dr. Cobbett has calculated that about one-third of all fatal cases of tuberculosis in children under five is attributable to a bovine source, a matter of great importance to farming interests and preventive medicine. There is reason to believe that the relative incidence of bovine or human infection may_vary according to locality. Thus in Scotland, bovine infec- tion is probably relatively commoner than in England.
Contagiousness of Tuberculosis. Tuberculosis is not contagious in the sense that such diseases as measles and other of the exanthe- mata are. The danger of infection of healthy subjects where reason- able precautions are taken, as in institutions for the tuberculous, is extremely _remote. But under conditions due to overcrowding, bad hygiene, imperfect nutrition, when lowered resistance is encount- ered, where massive doses of bacilli are absorbed and contact is intimate and prolonged, the danger of infection is very real. The vexed question of marital or conjugal tuberculosis has been much discussed of late and should not be neglected.
Predisposing Causes of Infection. In discussing infection, atten- tion should be drawn to the " soil " of the patient as well as to the " seed " sown. There is variation in both individual and racial susceptibility. Thus the Irish are said to be relatively susceptible, the Jews immune. " It has been learned of late years that the number of individuals who can be shown by radiography, tuberculin tests, and autopsy findings to have some focus of tuberculosis is enormously in excess of those who are usually classed as tubercu- lous " (Bushnell). Tuberculous infection amongst civilized com- munities is well-nigh universal. The tubercle bacillus may remain latent in the individual for indefinite periods, may rapidly and in varying degrees give rise to local lesions or generalized dissemina- tion. _ Its innocuousness depends both on the good health of the individual attacked, his immunity inherited or acquired, and the absence of conditions calculated to break down that immunity. Amongst the more important factors likely to facilitate morbid infection are heredity, a constitutional liability to the disease, gen- eral debility due to various circumstances ; poverty and its associated conditions; overcrowding, insanitary surroundings, bad hygiene, insufficient or unsuitable food, exposure, trauma, alcoholic excess, insanity, syphilis and other diseases; unfavourable climatic influ- ences, occupations, etc. Dr. Browning has shown that the common age-period of phthisis may vary in different localities. Thus, it is earlier in the Shetland Is. than in London. The age of maximum mortality appears to be increasing. In the middle of the igth cen- tury it was 25 to 35 years; in 1921 it was 45 to 55 in males, 35 to 45 in females.
Control and Eradication of Tuberculosis. With the discovery of the causal factor of this disease and an ever-extending acquaint- ance with the conditions favouring its pathogenicity, it became possible to initiate, investigate and undertake scientific methods for its control and eradication. The problem, however, is of infinite complexity. Up to 1921, no certain method of im- munization against infection had actually been discovered, and no specific treatment comparable to that employed in dealing with syphilis had been devised. Till the uninfected popula- tion can be safely immunized against tuberculosis and the infected population cured by specific measures, the laborious and often complicated methods now employed, constantly im- proved and elaborated, will constitute both our means of prophylaxis and of attempted cure. Tuberculosis is protean in its manifestations and the methods directed against the dis- ease are of corresponding variety. Certain general principles in treatment may, however, be laid down. These comprise the adoption of all those measures calculated to increase the patient's power of resistance and preserve and restore the part or parts attacked. Life in the open air under suitable climatic and
