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says: “Among the latest and most important additions to modern science

must be enumerated the discovery of minute organisms in almost all diseases of the zymotic class, and in tuberculous and other pathological products. The importance of the investigations which have led to our knowledge of this subject cannot be overrated, and the facts so accumulated must be accepted as invaluable additions to our conceptions of the origin and spread of such affections.”

“It will be well, however, not to build up too hasty theories of disease on even such facts. There

palpable danger of results being mistaken for causes.

The invariability of certain morbid products being found is no doubt a proof of the unchangeableness of the diseased process, but it would be surpassing the limits of the Harveian method of exploration by experiment, if we were to assume that in all cases these, and these alone, were the causes.

The decay of almost every form of organic life ends in a parasite, or lower grade, and it may yet be found that our assumed causes were only results.”

It is not now that bacteriologists are for the first time warned, in solemn words, of the danger of mistaking for its cause what may be a mere result of disease. Again and again has this warning been urged upon them, until one unacquainted with bacteriology might well believe that the warning was based

upon observed facts, and that the mistaking of the result of a disease for its cause had become a pitfall into which bacteriologists almost habitually stumbled. It is not surely unreasonable to ask, that

grave doubts

when next this familiar danger signal is unfurled it shall be accompanied by a statement of the facts which have prompted its use.

Dr. Pollock's reference to tubercular disease is plainly stated in his words of warning ; the application of them to Koch's work on tuberculosis is clearly intended.

It is not, then, going too far to ask, on what grounds that work is challenged? What error is there in it to justify the casting of

upon

its trustworthiness? If any man knows of such an error, it is his duty to lose no time in placing that knowledge at the service of the medical profession in every country. This matter is of the utmost consequence, because very grave issues hinge upon it, and amongst them is the fact that one of the strongest arguments in favour of the view which regards consumption as a communicable disease, rests wholly upon the truth of the belief that all tubercular disease is caused by a living organism. That being so, it follows that any attempt to show just cause for classing consumption amongst communicable diseases, should be preceded by a clear statement of what was Koch's work concerning tuberculosis, and of the facts and arguments upon which it rests.

CHAPTER II.

DR. KOCH'S RESEARCHES CONCERNING

TUBERCULOSIS.*

On March 24, 1882, in a paper read before the Physiological Society of Berlin, Dr. Robert Koch claimed to have established, by experiment and by observation, the existence of a micro-organism which is associated with tubercle, and not only associated with tubercle, but also the cause of all tubercle. This organism is a bacterium, of the kind known as a bacillus, and it is, of course, rod-shaped. In length it varies from about sooo to zoomin., and its breadth is about one-fifth of its length. In looking at a specimen of this bacillus it will be seen that it sometimes contains spores, two to four in number, ranged along the length of the organism.

Since the above date, no observations have been published which disprove Koch's work. On the other hand, the bacillus described by him has been found, by several observers, in the tissues, and in the sputa of persons whose conditions of disease would have suygested to any clinician, of ordinary experience, the probability of the presence of tubercle in the patient. It must, then, be admitted, that we have now to deal

* The substance of this chapter first appeared in a paper by the author, which was read before the Glasgow Medico-Chirurgical Society on December 1, 1882, and published in the Glasgow Medical Journal for February 1883.

with a new fact which characterises, by the presence of this organism, certain cases of disease of wellknown type. For those who find themselves justified in accepting Koch's results as true, all difficulties about the nature of these cases must cease, as soon as it is found that the patients concerned harbour in their tissues, or in their secretions, or excretions, this bacillus of tubercle.

The bacillus is demonstrated in tissues by employing a staining process first described by Professor Ehrlich. Koch adopted this process in preference to the one devised by himself, and with the aid of which he worked out all his early experiments. Ehrlich's

process will be found fully described in the British Medical Journal of October 14, 1882, and Professor Vignal makes some useful remarks upon the process in the same Journal on October 28 of that year. It is, for these reasons, unnecessary here to touch upon

the method of investigation required for the detection of the bacillus. There is one error in the remarks that appear in the Journal of October 14. It ought to have been there stated that the bacillus of leprosy gives precisely similar results with those shown by the bacillus of tubercle, when these two organisms are submitted to the process of staining devised by Ehrlich. There are, however, some differences in form, as Koch points out, between the two bacilli. The bacillus of leprosy is "more slender and more pointed at the ends” than that of tubercle. They are also distinguished from one another by the colour test of Weigert, to which the bacillus of leprosy responds; that of tubercle, on the contrary, is by it unaffected.

Koch thus describes the appearance of the bacillus in tuberculous tissues : “ In all cases where the tuberculous process is in its early stage and progressing rapidly, the bacilli are to be seen in great numbers. They then lie thickly, and often in groups or small bundles inside the cells, and in some places give the same appearances as the bacilli of leprosy when they are found in cells. Near these (groups or bundles) are found numerous free bacilli. Especially on the borders of large cheesy deposits crowds of bacilli appear, which are not shut up in cells.”

“As soon as the highest point of the tubercular eruption is overstepped the bacilli become rarer, or are only to be found in little groups or singly at the edges of the tuberculous deposits, and lying near them are bacilli su faintly coloured as scarcely to be recognisable ; these are, presumably, already dead or in the act of dying. Finally, they may quite disappear, although they are rarely altogether absent, and then only in such places as those in which the tuberculous process has come to a standstill.”

In his lecture, Dr. Koch lays emphasis upon the connexion which appears to exist between the presence of the bacillus and of the giant cell. “If," he says, “in the tuberculous texture giant cells appear,

then the bacilli lie by preference in these structures. In cases of very slowly progressing tuberculous processes, the inside of those giant cells is generally the only place where the bacilli are to be found.”

Koch has a theory about the connexion between the giant cell and the bacillus, and it is this : “ It is to be concluded from the size and position of the giant cells containing bacilli, that these cells are the

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