It has already been stated that the milk of a cow, affected in any part of the body by tuber cular disease, may be infective. Similarly the milk of a nursing mother or wet nurse may be infective, and the infant at the breast may thus become tubercular. But infants are not sus ceptible to tubercle of the lungs. This is paralleled by a similar fact of another infection, namely scarlet fever, to which infants of less than one year of age are little liable. But an infant infected by the milk of a tubercular mother may afford a resting-place for the organ ism in the glands, those of the neck, for instance, or bowel, and there the disease may lie quiescent for years. Then when the child has grown up, the organism hitherto quiescent may become active owing to some other circumstance, may multiply and pass from glands into blood vessels, so reach the lungs, take root there, and produce consumption. This case, by the old view, would be a proof of the inherited tendency, whereas it is only an illustration of a well-known fact of the tubercular infection, that it may long lie dormant. Incidentally, therefore, we have emphasized the need of another precaution, namely that a consumptive mother should not suckle her children.
It must, however, be noted that there are certain types of constitution more liable to attack than others. This is, of course, true of every disease. Everyone could probably give illustrations from their own knowledge of families that seemed to fall a prey to every infection going, and other families that seemed to be proof against even the commonest infec tion. So there is a type of constitution more liable to tubercle than another, a type of light build, clear complexion, light-coloured hair, bright eyes, quick and active but not muscular. Inheritance also affects the power of resistance, as is shown in the offspring of too early mar riage, or of marriage between persons too nearly related, or between persons of considerable difference of age.
In short, a person may be born with a delicate constitution and with a strong tendency to the disease ; an ordinarily healthy individual may acquire a weakened constitution owing to his occupation, surroundings, as a result of various diseases, or owing to the pressure of poverty, anxiety, overwork, &c.; and a person may ac quire a weakened constitution by his habits, by excesses, undue and prolonged excitements, &c. In all these persons the natural resistance of the body is lessened, and, amid the numerous oppor tunities of infection, they sooner or later fall victims to the attack of the organism.
The Site of' disease begins most commonly in the upper part of the left lung, about the level of the middle of the collar-bone, spreading backwards into the apex from this place. Sometimes the first part attacked is the apex of the middle lobe ; this corresponds, on the outside of the chest-wall behind, to an area between the middle of the posterior edge of the shoulder-blade and the spinal column. Another place where the disease manifests itself early corresponds to the side wall of the chest, about the middle when the arm is raised.
Changes in the tubercle ba cillus then, introduced in one way or other into the lungs, settles down in the lung tissue, and, if it flourishes at all, goes on to produce certain changes. What the precise nature of these changes is, and whether they occur quickly or slowly, will depend on the number and virulence of the organisms and on the amount of resist ance the person in general and the lung tissue in particular offer to their operations. The
result is that while the changes produced in different individuals have all certain characters in common, the manifestations of the disease in each individual may vary considerably, being rapid in one and slow in another, one symptom being prominent in one person, and another in another person. Moreover, in one person the reaction of the lung to the operation of the organism may vary in kind or degree from that of another person, and thus the extent and character of the changes will present different features in different individuals. In consequence of these differences different types of disease have been described, at different times and under different names, either according to the prominent symptoms during life, or the appearances of the diseased lungs after death, which are now recognized as merely due to varieties of operation of the same cause, and all capable of being included under the general term of consumption of the lungs, or tubercular disease of the lungs, or wasting of the lungs, or pulmonary plithisis. Thus while tubercular disease of the lungs is a term applied to the disease when one thinks of its cause, decline or consumption is a more popular term, applied because of the prominent symptom, in numerous cases, of loss of strength and flesh, or because of the apparent breaking down of the lung and the matter that is expelled in the spit. The nature of the changes we shall indicate as briefly as possible.
The tubercular organism causes around it a growth of cells, some large ones and numerous small ones, so that the organisms come to lie in and among a mass of round cells. This mass pushes aside and usurps the place of the healthy tissue. It is about the size of a millet seed and has a clear gray appearance, the gray tubercle. But there are no blood-vessels in this mass; it is therefore of low vitality, breaks down in the centre, and degenerates into a yellowish cheese like mass. This is called the yellow tubercle. Here the process may stop, and the little caseous mass may further become walled off from the healthy tissue around by the conversion of a zone of round cells into fibrous tissue. The cheesy mass may shrink and be absorbed, or it may be infiltrated with lime salts and become chalky. This is to be regarded as a process of cure, the resistance of the tissue having triumphed, and there is only left a patch of fibrous tissue—a scar—or a nodule of chalk to mark the spot where the battle was fought. On the other hand, the mass may continue to grow, one mass may coalesce with another. They may soften and liquefy, burst and be discharged, the matter being expelled in the spit, and a cavity may be thus left in the lung substance. The process going on in the walls of a cavity, it enlarges, several cavities com municate with one another, and the lung may thus become riddled with small or large or irregular cavities. Such a mass may break down and discharge through a bronchial tube, or form and break down on the wall of a tube, leaving an ulcer. But even though a cavity has been formed, or several cavities, the active process may cease, lung tissue become con densed round the wall of the cavity, which shrinks, and, the fibrous tissue round it grad ually contracting, it may be more or less obli terated, the site being again marked by an irregular fibrous patch.