Villemin's observations have been repeatedly confirmed and extended; although subjected to the severest criticism and control, their results are so constant that the law of the inoculability of tubercle is almost universally regarded as fixed. Its value as a test is evident from the statement of Cohnheim,54 who regards as tuberculous only that which produces tuberculosis when transferred to suitable animals. The transfer may be made in various ways. Chauveau and others were successful in producing an intestinal tuberculosis by the introduction of tuberculous material into the intestinal canal of animals, especially the Herbivora. Tappeiner55 succeeded in producing pulmonary tuberculosis, with or without general tuberculosis, in dogs, by compelling them to breathe air in which were contained minute particles of sputa from tuberculous pulmonary cavities.
54 Die Tuberkulose vom Standpunkte der Infections-Lehre, 1880, 13.
55 Virchow's Archiv, 1878, lxxiv. 393.
The production of a tuberculosis of the iris, as well as of remote organs, by the inoculation of tuberculous material into the anterior chamber of the eye, was an ingenious method devised by Cohnheim and Salomonsen.56 It permitted the direct observation of the several steps in the process of absorption of the inoculated material and development of the tubercles.
56 Cohnheim's Vorlesungen über Allgemeine Pathologie, 2te Auflage, 1882, i. 707.
The objections to the various experiments above alluded to are based upon the assumption that the results of the inoculation are not tubercles, but inflammatory products resembling tubercles. It is further advocated that the inoculation of indifferent material, as bits of glass or hairs, as well as other foreign substances, will produce the so-called artificial tuberculosis, especially in rabbits and guinea-pigs. It is admitted that these animals readily become tuberculous when exposed to simple inflammatory irritants, the local action of which frequently results in the production of cheesy material. This termination is now regarded as due to faults in the method of experimentation, the animals not being thoroughly protected from the influence of the virus of tuberculosis.
The objection on the ground of structure loses its force in connection with the well known differences in the structure of miliary tubercles in the human body, already mentioned. The tubercles resulting from inoculation often resemble in structure the meningeal tubercles of the brain rather than the type presented by tubercles in lymphatic glands. The development of tubercles in the iris may take place without any permanent inflammatory reaction. The association of evidences of inflammation with the development of the tubercle is therefore unnecessary.
The experiments of Villemin have not only demonstrated the infectious nature of tuberculosis, but have also led to a more accurate knowledge of the relation between tuberculosis and its allied affections, scrofula and pearly distemper.
The anatomical characteristics of scrofula have obviously proved insufficient in determining the relation presented by this affection to tuberculosis. The tendency to cheesy degeneration of its inflammatory products was the feature of chief importance. Villemin showed that portions of a scrofulous (cheesy) gland when inoculated were followed by tuberculosis, and that the inoculation of cheesy material from non-tuberculous or non-scrofulous sources was not followed by this result. The assumption of Buhl, that the absorption of cheesy material, as such, was the cause of tuberculosis, was thus disproved. The frequency with which the inoculation of cheesy material, from what were regarded as scrofulous sources, was followed by tuberculosis, led to more exact studies concerning the anatomical peculiarities of scrofulous inflammation. Köster57 called attention to the regularity of the occurrence of miliary tubercles in the fungous granulations of the inflamed joints of scrofulous and tuberculous individuals. Wagner58 and Schüppel59 discovered that scrofulous glands, in most if not in all instances, were tuberculous glands. The regularity of the presence of tubercles in scrofulous abscesses and ulcers of the skin and in scrofulous caries was shown by Friedländer.60 This observer likewise called attention to the presence of agglomerated tubercles as the chief constituent of the new formation of lupus. These anatomical discoveries resulted in uniting more closely the affections scrofula and tuberculosis from the histological standpoint, and the union has become more firmly cemented from the etiological investigations.
57 Virchow's Archiv, 1869, xlviii. 95.