and in this form is inhaled by those in attendance upon the patient. When infection or transmission occurs, it is probably to be explained in this manner.

The bacillus of tuberculosis is a slender, rod-shaped organism usually presenting the appearance of being somewhat beaded. It is believed to form spores, it is motionless, it cannot be readily cultivated on the ordinary nutrient media, though it can be induced to lead a saprophytic form of existence on sterilized blood-serum, on nutrient agar-agar, and in bouillon to which glycerin has been added, and sometimes on potato. It develops under these conditions only at the temperature of the body. It is destroyed by the temperature of boiling water in a few minutes, and by lower temperatures also, but after a longer time-viz., in four hours by 55° C., in fifteen minutes by 65° C., in five minutes by 80° C. It is not destroyed by either drying or freezing. In the dry state it is much more resistant to heat than in the moist. It is recognized by its microchemical reaction with particular dyes.

This reaction is characterized by the tenacity with which the tubercle bacillus retains the staining, even when subjected to the action of comparatively strong decolorizing solutions. The same reaction is common to several other organisms

viz., the bacillus of leprosy, the so-called bacillus of syphilis, and the smegma bacillus.' (For details of the method of staining and the differential tests for these organisms, the reader is referred to works on Bacteriology.)

Geographical Distribution, Race, Sex, etc.-The disease occurs in all countries, among all peoples, and at all ages. It is more frequent in overcrowded than in sparselysettled localities, and somewhat more common in low than in high altitudes.

The American Indians and the negro races exhibit a marked susceptibility to the disease, as do also the offspring from the commingling of the white and colored races. It is seen in many of the domestic animals, notably bovines. It is tolerably common in pigs, rare in sheep, dogs, and cats. Many of the wild animals in which tuberculosis does not occur while they are in the native state develop the disease after captivity and confinement. It is of the greatest sanitary importance to note the fact that, among the domestic animals. in which the disease occurs, bovines, especially milch cows, are most often affected.

Mode of Dissemination.-As has been already stated, the part played by heredity in disseminating the disease is most conspicuously exhibited in the perpetuation of a particular condition of vitality that renders the individual endowed with it peculiarly susceptible to this form of infection.

The frequency of tuberculosis in the young of tuberculous parents cannot always be referred to the congenital existence of the disease; in fact, authorities are pretty well at one in the opinion that this is rarely the case. It is more probably due to direct infection from parent to child through their very intimate association. It cannot be denied, however, that intra-uterine tuberculosis of the fetus is occasionally encountered.

Every tuberculous individual is a source from which the disease may be further disseminated. This is conspicuously true with regard to those afflicted with the pulmonary mani

1 See article on this subject by Cowie: Journal of Experimental Medicine, 1900, vol. v., p. 205.

festations. As said above, the sputum of consumptives contains the specific causative micro-organism-bacillus tuberculosis-in enormous numbers. When such sputum is allowed to dry and become ground into dust, and is inhaled as such by those in the vicinity, we see, most probably, the commonest mode of dissemination of consumption. The greater frequency of the pulmonary over other forms of the disease speaks in support of this view.

The secretions from those manifestations of the disease usually classed as "surgical tuberculosis" also contain the bacilli or their spores, and are capable of causing tuberculosis. With tuberculosis of the genito-urinary tract the bacilli are found in the urine, while from intestinal tuberculosis they are thrown off with the discharges from the bowels. From skin-tuberculosis-lupus-they escape with the secretions and with the particles of exfoliated epidermis.

An important factor in the dissemination of tuberculosis is its occurrence in certain domestic animals, especially those used as food. Though there is still a controversy with regard to the danger of infection from the use of flesh and milk of tuberculous animals, it should be borne in mind that bacillus tuberculosis has been found in both, and so long as this is the case these substances must be considered as possible disseminating factors, if not rendered harmless by thorough cooking. Rapidly accumulating evidence points to the probability of infection by way of the alimentary tract being much more frequent than is ordinarily supposed.

In addition, there are a multitude of ways in which dissemination may occur when consumptives are closely and continuously associated with healthy individuals, as in the case of man and wife, of parent and child, of patient and nurse, etc. Without detailing every possible mode of dissemination under such circumstances, it will suffice to say that the most frequent are perhaps infection through the introduction into the mouth of the specific causative organism, by the hands that have been soiled with sputum or secretions from the patient, by kissing and caressing, by using the same eating utensils and drinking from the vessels

used by the patient, and by sleeping in the same bed with the diseased person.

Portals of Infection.-The commonest portals of infection in man are the lungs, the alimentary tract, and wounds of the skin. The disease may be produced experimentally in susceptible animals by subcutaneous inoculation, by interperitoneal inoculation, by direct injection into the vascular circulation, by the feeding of tuberculous materials, by the introduction of the bacilli into the air-passages, and by inoculation into the anterior chamber of the eye.

Contrary to the general rule for pathogenic bacteria, bacillus tuberculosis apparently has the property of forming spores, or of passing into a resistant condition analogous to that of the spore-stage, within the living body. When thrown off from the diseased body these resistant forms of the bacillus survive the unfavorable conditions that they encounter, such as drying, low temperature, scarcity or lack of nutritive material, etc., and retain the power of producing tuberculosis, when conditions favorable to the exhibition of this function again present. The investigations of Cornet demonstrate that bacillus tuberculosis in full possession of its pathogenic powers may often be found in the dust of rooms occupied by tuberculous individuals. Some of the details of these investigations are most instructive: In 147 samples of dust collected from general hospital wards, from asylums, from prisons, from private apartments of consumptives, from surgical wards of hospitals, and from other localities occasionally visited by consumptives, bacillus tuberculosis was demonstrated (by inoculation into guinea-pigs) to be present 59 times-viz. :

In 38 dust samples from seven hospitals

it was found 20 times

From this it is clear that the inhalation of dust contaminated

66

о

with this organism, by individuals naturally susceptible or vitally predisposed to this variety of infection, must be a comparatively frequent channel through which the disease is contracted..

It is not my intention to open here the familiar controversy as to whether flesh and milk of bovines affected with the lymphatic or pulmonary form of the disease contain the bacillus of tuberculosis or not, but rather to call attention to the point that through the careless handling of such flesh and milk by butchers who slaughter tuberculous cattle, and by dairymen who obtain the milk from diseased cattle, both meat and milk may become accidentally infected, and if used in the uncooked state may serve as sources of infection.

Tuberculosis as a result of direct inoculation-i. e. through wounds of the skin-is less important to man than are the preceding modes of infection. When it does occur, the process is usually localized to the site of infection. There is reason to believe, however, that general tuberculosis may follow this mode of infection, though such a result is

rare.

The localized tubercular nodule resulting from infection of wounds of the skin, familiarly known as "post-mortem " tubercle, is encountered most frequently in those whose duties bring them in intimate contact with the morbid tissues of deceased tuberculous individuals and animals, as pathologists, butchers, workers in raw hides, etc.

Prophylaxis. According to the eleventh United States Census, 14.8 per cent. of all deaths occurring in New York City for the six years ending May 31, 1890, were from one manifestation or another of tuberculosis; 13.5 per cent. in Brooklyn; 14.1 per cent. in Philadelphia; 18.5 per cent. in Boston; 14.4 per cent. in Baltimore; and 16.5 per cent. in the District of Columbia. According to the same document, the number of deaths from this disease in the abovenamed cities for the same six years was greater, and in some cases much greater, than the deaths from scarlet fever, diphtheria, typhoid fever, and diarrheal diseases (excluding cholera infantum) put together (see Table XIII.).