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Medical opinion has undergone a great change with regard to the influence of heredity. The frequent occurrence of consumption among members of the same family used to be explained by assuming the existence of a tuberculous “diathesis” or inherent liability to consumption which “ran in families” and was handed down from one generation to another. As the real nature of the disease was not understood, the inherited diathesis was regarded as a sort of latent or potential consumption which might develop at any time and could hardly be avoided. The children of consumptive parents had the “seeds” of the disease in them, and were thought to be doomed with more or less certainty. Great importance was therefore attached to heredity as a factor in the incidence of tuberculosis. The discovery that it is caused by a specific parasitic infection placed the question in a different light, and led to a more careful examination of the facts, which has resulted in a general and increasing tendency to minimize or deny the influence of heredity. At the Berlin Congress on Tuberculosis in 1899 Virchow pronounced his disbelief in the theory on pathological grounds. “I dispute this heredity absolutely,” he said. “For a course of years I have been pointing out that if we examine the bodies of infants newly born, who have had no life apart from the mother, we find no tuberculosis in them. I am convinced that what looked like tuberculosis in the newly born was none of it tuberculosis. In my opinion there is no authenticated case of tubercle having been found in a dissected newly-born infant.” Observations on animals similarly tend to disprove the existence of congenital tuberculosis (Nocard). The theory that the germs may remain latent in the offspring of tuberculous parents (Baumgarten) is unsupported by evidence. The occurrence of disease in such offspring is ascribed to infection by the parents, and this view is confirmed by the fact that the incidence in consumptive families is greater on female children, who are more constantly exposed to home infection, than on the male (Squire). The statistical evidence, so far as it goes, points in the same direction. It is even denied that the children of consumptives are specially predisposed.

Recognition of the communicability of tuberculosis has directed attention to the influence of conditions in which people live massed together in close proximity. The prevalence of the disease in large centres of population has already been noted, and the influence of aggregation is no doubt considerable; but it does not always hold good. The distribution in England and Wales does not correspond with density of population, and some purely rural districts have a very high mortality. Broadly, however, the rural counties have a low mortality, and those containing large urban populations a high one. In France in the department of the Oise, in purely industrial villages, the mortality from pulmonary phthisis is from 56 to 61 per 10,000; in a village in which part of the population worked in the fields and part in factories the mortality was 46 per 10,000; and in purely agricultural villages it ranged from 0 to 10 per 10,000.

Other Conditions.—Poverty, insufficient food and insanitary dwellings are always more or less associated with overcrowding, and it is difficult to distinguish the relative influence of these factors. An analysis of 553 deaths in Edinburgh according to rentals in 1899 gave these results: under £1O, 230; from £10 to £20, 190; above £20, 106 (Littlejohn); but the corresponding population is not stated. An investigation of selected houses in Manchester gave some interesting results (Coates). The houses were divided into three classes: (1) infected and dirty; (2) infected but clean; (3) dirty but not infected; infected meaning occupied by a tuberculous person. Dust was taken from all parts of the rooms and submitted to bacteriological tests. The conclusions may be summarized thus: The effects of overcrowding were not apparent; a large cubic space was found to be of little avail if the ventilation was bad; the beneficial effects of light and fresh air were markedly shown even in the dirtiest houses; ordinary cleanliness was found not sufficient to prevent accumulation of infectious material in rooms occupied by a consumptive; no tuberculous dust was found in dirty houses in which there was no consumption. The upshot is to emphasize the importance of light and air, and to minimize that of mere dirt. This is quite in keeping with earlier investigations, and particularly those of Dr Tatham on back-to-back houses. Darkness and stuffiness are the friends of the tubercle bacillus.

It is now generally accepted that tubercle bacilli may enter the body by various paths. At the International Congress on Tuberculosis held in Vienna in 1907 Weichselbaum summarized the channels of infection in pulmonary tuberculosis as follows:

(1) By inhalation directly into the bronchioles and pulmonary alveoli, or by way of the bronchial glands through the blood and lymph channels into the lung. (2) Through the mucous