The tubercle bacillus—we speak of the human type in particu lar—is very widely distributed, although not so nearly ubiquitous as formerly considered. Wherever the tuberculous individual goes, pro vided his excretions and secretions contain bacilli, there exists the possibility of dissemination. The vehicle of infection exists not alone in the sputum, but also in the finest particles of moisture expelled in the act of speaking, sneezing, etc. These become dry, and are later raised with the dust. In the open air this is of little consequence, as the sun and even diffused daylight serve as most excellent disin fectants. It is, however, in the rooms and in the thickly populated dwellings of the poor, where sunlight and air do not enter or find their way but meagerly, that opportunity is offered for the increase of the masses of virulent bacilli. The bacillus of the bovine type is found more especially in the cow stalls, in all the excretions and secretions of tuberculous animals, as well as in their flesh and blood. It is found particularly in the milk of tuberculous cows. even in those not suffering from tuberculosis of the udder, and in the products of raw milk, such as cream, butter, cheese, curds, etc. Thus we see that all of us are brought into close relation with the bacillus and are in this manner exposed to infection.
Pathogenesis.—We must mention the paths, especially in child hood, by which the tubercle bacillus is able to enter the human body and produce tuberculosis. This question is by no means settled, but we shall attempt to discuss the contradictory views impartially, and to give the pros and cons of all sides. But let it be understood from the
outset that no one claims that there is but one mode of infection by tuberculosis.
Innumerable opportunities for entrance are offered the tubercle bacillus, and at one time or another it may gain admittance any where in the body. Indeed the bacilli may even pass through the intact skin, if they are briskly rubbed over it; far easier, however, does infec tion follow if there is an injury to the cutis, whether it is caused by a tuberculous object, or whether the infection is added later. Marked examples of this class of cases are the local lesions that follow the hand ling of cadavers. I observed a case where a tuberculous mother, fearing tuberculosis following vaccination of her child, sucked the wound, and thus produced a localized tuberculosis of the skin. In short, the tu bercle bacillus may enter the human body at any point. For us, however, the important questions are: Where does it enter in the majority of the cases? Where are the loci minoris resistentire? Where must we in general expect tuberculous infection? There are three different views upon this subject, each of which explains the roost frequent mode of infection in a different way:—(1) by heredity of tuberculosis (prenatal infection): (2) infection through the air-passages (aerog enic infection) ; (3) infection through the digestive tract (enterogenic, or better, alimentary infection).