The practical importance of the facts already ascertained respecting the communicability of phthisis is obvious. They constitute the foundation for a reasonable supposition that the disease may be communicated to man by means of the meat of tuberculous animals, by milk, and by breathing an atmosphere charged with particles of tubercle. That the instances in which the disease is communicated, however, are rare seems to be a rational inference from the difficulty of obtaining clinical proof of communicability. That susceptibility is an essential factor is made evident by the well-known predisposition pertaining to certain periods of life. It is to be considered that while the communicability of the disease to certain animals is abundantly shown by the experiments to which reference has been made, the existence of a special virus or a contagium is not as certainly established by these experiments. They leave to be settled, by further investigation, the question whether or not the communicability of the disease involves only the agency of a septic matter devoid of the special character expressed by the terms virus and contagium. Without waiting for data sufficient to settle this important question, prudence would dictate the propriety of all practicable precautionary measures.
Still more recently, and since the foregoing remarks on the communicability of phthisis were written, have appeared the remarkable experimental researches of Koch of Berlin. Koch claims to have demonstrated the constant presence in tuberculous products of a specific organism which he calls the bacillus tuberculosis, and that it is not found in non-tuberculous products. This parasite he has isolated, and by cultivation carried through several successive generations. By its introduction, after, as well as before, cultivation, into the pleural cavity, the peritoneal cavity, the anterior chamber of the eye, and in other situations, he produced tuberculous disease, not only in rabbits and guinea-pigs, but in dogs and rats, the latter animals being less susceptible than the former to tuberculous infection. In his experimental observations, animals not inoculated, placed under the same external conditions as those inoculated, did not become tuberculous. The same parasite, alike capable of infecting healthy animals, he found in miliary tubercles, in the cheesy tuberculous deposit, in scrofulous glands, and in the sputa from tuberculous patients. The parasite was found not to have lost its vitality in dried sputa.7
7 For the details of Koch's researches vide his report in the Berliner klinische Wochenschrift, April 10, 1882; vide, also, Verhandlungen des Congresses für Innere Medicin, Erster Congress gehalten zu Weisbaden, 20–22 April, 1882.
The researches of Koch had been continued for two years before the publication of the results in March, 1882. Moreover, his ability as a skilled experimental observer in the study of micro-organisms, and his sincerity as a truth-seeker, are universally admitted. Naturally, the publication of the results of his researches excited at once great interest in all countries. At the present moment (April, 1885) questions connected with the bacillus tuberculosis are more considered than any others relating to medical pathology and etiology. Thus far, the observations of competent medical mycologists are confirmatory of the results of the researches by Koch. It seems to be established that the so-called bacillus tuberculosis is uniformly present in tuberculous products, and as uniformly absent in other morbid products; that it is generally present in the sputa of phthisical patients, and never present in the sputa of non-phthisical patients; and that tuberculous disease in animals may be produced by inoculation with this organism after cultivation has been sufficiently continued to eliminate all else pertaining to the tuberculous product. On these data are based the conclusions that phthisis is an infectious disease—in other words, that it involves in its causation a specific agent capable of self-multiplication; that it is a communicable disease, and that the agent of the communication is the bacillus tuberculosis—that is, this agent is the contagium. The supposition that the presence of the bacillus is secondary to the tuberculous affection is not tenable in view of the fact that the affection is produced by the introduction of this organism after it has passed through several generations by culture out of the body.
As has been already seen, clinical experience fails to furnish positive proof of the communicability of phthisis. There are many striking instances which, taken by themselves, render it probable that the disease was communicated; but, on the other hand, there are so many cases of its development under circumstances not pointing to contagion, and of the number of persons in close proximity to tuberculous patients the proportion of those who become affected is so small, that it has seemed impossible to establish the doctrine of contagion by clinical evidence.
The insufficiency of clinical proof, however, cannot invalidate the demonstration by inoculation. Assuming it to be demonstrated that the disease involves a specific agent, and that this agent is proven to be a contagium by its capability of producing the disease when introduced into a healthy body, the conclusion as to communicability is not to be shaken by the lack of corroborative clinical evidence or by inability to explain certain facts which seem to be inconsistent with that conclusion. Having accepted a demonstrated truth, the endeavor should be to reconcile therewith facts which do not sustain it and which may appear to be opposed to it. It remains to inquire in what way the communicability of phthisis by means of a contagium vivum is to be reconciled with facts furnished by clinical experience.
If we accept the conclusion that a particular parasitical organism is the primary and efficient causative agent in the production of phthisis, the development and multiplication of this organism must require certain local conditions. Without these the parasite is innocuous. The conditions are to its development and multiplication what the peculiarities of soil are to the production of different vegetables. Of the nature of these conditions we are at present ignorant. When they exist the bacillus develops and multiplies; when they are wanting the parasite is incapable of development and multiplication. This dependence of specific morbific agents upon particular conditions is exemplified in other infectious diseases. For example, the contagium of the eruptive fevers, received into the system ever so abundantly, is inoperative in some persons, and, as a rule with rare exceptions, it is never operative after the disease which it occasions has been once experienced. In these instances it is not the contagium itself which has lost the capability of producing the disease, but the conditions for its activity are wanting. Of the nature of these conditions we know as little as of those which are essential to the development and multiplication of the bacillus tuberculosis. The inoculation of animals with tuberculous matter shows that the disease is produced in some species of animals much more readily than in other species, and some animals of the same species much more than others are susceptible to this contagium. These facts are to be explained by variations in different species of animals, and in different animals of the same species, as regards the conditions required for the efficiency of the morbific agent.
The facts in the clinical history of phthisis which denote a constitutional predisposition thereto or a tuberculous cachexia are explicable by reference to the conditions requisite for the development and multiplication of the parasite. A predisposition which may be innate, inherited, or acquired involves the existence of these conditions. The latter may be greater or less in degree. The causative agencies of confinement within doors, humidity of soil, pregnancy, etc. operate by either giving rise to or increasing these conditions. If this view be correct, it is evident that the curative influence of climatic changes, alteration of the habits of life, and other hygienic agencies must be by means of an effect exerted upon these conditions; and probably it is in this way chiefly that remedies are useful. Of the essential nature of these conditions we know neither more nor less than of what consists the tuberculous cachexia. We are, of course, as ignorant of the one as of the other if it be assumed that they are identical—that, in other words, the different expressions have the same meaning. The only difference is this: If phthisis be an infectious and a communicable disease, a contagium enters into its etiology; whereas if the existence of a contagium be denied, it follows that the cachexia is itself sufficient for the causation of the disease.
In connection with the etiology of phthisis a theory which of late years has found favor with many should be referred to. It is, that this disease may be a result of the absorption of caseated non-tuberculous morbid products in different parts of the body. This theory of autochthonous infection derives but little support from clinical observation. In much the larger proportion of the cases of phthisis it is impossible to discover anywhere caseated morbid products which may be supposed to have a causative connection with the disease. To assume that, when not discovered, foci of infection nevertheless are concealed somewhere within the organism is evidently begging the question. On the other hand, how often do suppurations, necroses, and degenerated morbid products occur in different situations without being followed by phthisis!
SYMPTOMATOLOGY AND COMPLICATIONS.—Giving under this head a fuller account of the symptomatology and complications than has been already given in sketching the history of the disease, it will be a convenient arrangement to consider these topics in their relations to the different anatomical systems of the body—namely, the respiratory, circulatory (including temperature), hæmatopoietic, digestive, nervous, and genito-urinary systems.