Other methods of infection are by di rect inoculation, such as occurs in ex perimental work upon guinea-pigs, and accidentally in those who handle tuber cular meat or infected tissues; in post mortem work, the verruca necrogenica, a local tubercular process, is commonly seen upon the hands of those preparing autopsies and close contact, such as oc curs between man and wife. In the per formance of the rite of circumcision there is a well-known instance of direct inoculability, where a diseased operator communicated tuberculosis to several infants by the practice of suction. Wherever infective material comes in contact with an abraded surface or where tissues are directly exposed to its action, local infection may occur. This is com monly seen in the tubercular nature of the lining membrane of sinuses and tracts leading from foci.
One of the most common predisposing factors to the successful inroads of the tubercle bacillus in the lung is a dis eased condition of the upper air-pas sages. Perhaps the most powerful of these is bad nasal ventilation, such as may be caused by adenomata, nasal spurs, etc., which induce the patient to breathe through the mouth.
A probably frequent focus of tuber cular infection is the ring of lymphoid tissue which surrounds the naso-phar ynx: the cross-roads where the food and air may pass each other. So far as the fancial tonsils arc concerned, infection could, of course, take place either from organisms deposited there (in mouth breathers) by ingested food or by in spired air. In the naso-pharynx it can only be on rare occasions that infected food can contaminate the "third" tonsil. It is concluded that the commonest mode of tubercular infection is by inhalation, and that the inhaled bacillus has in fected the system before the air-current has reached the larynx—most probably through the lymphoid tissue of the naso pharynx. There appears to be no justi fication for the generally accepted idea that the bacillus is inhaled directly into. the pulmonary alveoli. St. Clair Thom son (Practitioner, July, 1901).
Finally, it is to be noted that, while the bacillus of Koch is the specific agent necessary for the development of pulmo nary tuberculosis, one or more of the general and direct predisposing causes before mentioned must have prepared a suitable soil for the bacillus to become inimical to the patient.
Hereditary tuberculosis is seen only in infants or very young children, and its methods of transmission, presumably in vtero, will be considered later. BACTERIOLOGY.—The etiological fac tor of tuberculosis, the bacillus of Koch. is a non-motile, parasitic aerobic and facultative aerobic rod-shaped organism two to four microns in length, or about half that of a red blood-corpuscle, and two-tenths micron in diameter, having —as one of its most characteristic feat ures which distinguishes it from all other bacteria known, with the exception of the bacillus of leprosy which it closely resembles—the ability to retain the stain of the alkaline solutions of the aniline dyes after treatment with acids. This
bacillus is found in all tubercular le sions, and shows a marked tendency to arrange itself in V-shaped pairs. It is frequently bent or slightly curved, and presents, when stained, small, rounded, or oval, clear, bright spaces, — three, four, or five to each bacillus,—which. failing to take the stain, give it the ap pearance of the streptococcus.
It fulfills Koch's circuit: it is found in this and in no other disease. It is capable of cultivation outside the body and of producing, when inoculations are made from these cultures, the original disease, in the lesion of which is found the bacillus. This circuit can be indefi nitely prolonged, and the inoculation carried on through a series of animals always resulting in the production of tuberculosis.
It grows upon a variety of culture media, such as bouillon, potato, and agar to which glycerin has been added, and at a temperature of about 37° C.; best upon blood-serum at a temperature of blood-heat in the presence of moisture and small quantities of oxygen and in the absence of sunlight: conditions highly available in the living tissues of all mammalia, to which the disease usually restricts itself. On artificial media its growth is slow; about the be ginning of the third week may be seen on the surface a thin, grayish-white layer, which fails to penetrate into its depth. It elaborates a toxin which is soluble in glycerin, and which produces, when injected into tubercular animals, a reaction accompanied by temperature; in healthy animals no such effect is ob tained, and in consequence it becomes of diagnostic value.
The tuberculin test, when applied to eases of suspected incipient tuberculosis in which the diagnosis cannot be made by the ordinary methods, has proved generally reliable, and has given positive information which could not be procured in any other way. But it must be re membered that latent tuberculosis of so moderate an extent as not to give any positive symptoms probably exists in about 30 per cent. of individuals, who have no reason to suspect its presence. Susceptibility to tuberculin varies in in dividuals, and no stated dose can be re lied upon to produce reaction in every ease, one reacting readily to I milli gramme, and another requiring 5, 7, or even S before reaction occurs. Again, the various samples of tuberculin differ somewhat in strength. Occasionally ad vanced and recognized eases of the dis ease do not react, either on account of an individual habituation to the toxin, or owing to the profound caehexia of the individual or to the fibrous state of the lesions, which hinders the local reaction by interfering with the circulation around the tuberculous tissue. The test should not be applied to such cases, since the diagnosis can be easily made by the usual methods. E. C. Trudeau (Inter. Med. Mag., Mar., 1900).