In what has been said above it has been implied that the causation of phthisis pulmonalis is connected with the tubercle bacillus. All that has been said in regard to the causation of tuberculosis at pages 303-305 applies here.

There is, in this as in other forms of tuberculosis, not only the action of the specific microbe to be considered, but also the susceptibility of the individual, which may be inherited, but is often acquired. It is .acquired principally in the case of persons so placed as to have the general health reduced, and in whom especially the respiratory functions do not get justice. Persons living in close dwellings, especially when, at their work in factories and otherwise, they are in the habit of breathing vitiated air, in which, it may be, finely divided dust is abundantly suspended, frequently acquire a tendency to phthisis although not originally predisposed.

In relation to the resulting lesions, the Path of entrance of the irritant to the lungs is a matter of importance. In the study of the lesions met with we shall find that they all start at the finer bronchi. A catarrh of the finest bronchial tubes, usually occurring in a number of these simultaneously, is the starting point of a variety of lesions* which, however, for a considerable time remain related to the bronchi in their distribution. This is an indication that the agent finds access to the lungs by the inspired air.

As a general rule the bacilli probably find entrance in small numbers and by accident, but there are cases of a more considerable Insufflation of tubercular matter. Thus in a case observed by the author a tubercular lymphatic gland adherent to a bronchus burst into the bronchus with the result of an acute tuberculosis in a limited district of the lung. There may also be a somewhat rapid extension from insufflation in tuberculosis of the larynx.

The Localization at the apex of the lungs of the earliest lesions is probably related to the fact that the apices of the lungs are the least expansile portions. The first rib even in women is very little raised in inspiration, and in persons with weak respiratory movements the air is apt to stagnate at the apex. This view receives some confirmation from the fact that phthisis so frequently improves when the patients go to reside in high altitudes where the. rarefied air requires more vigorous respiratory efforts. In such persons the size of the chest as a whole generally undergoes an increase. Remembering that the tubercle bacillus is of slow growth, we may presume that it is more likely to obtain a footing when it is left undisturbed in parts where the air is more or less stagnant.