brought into close proximity. And thus the infection of the system becomes complete.

It remains yet a question open for further inquiry, what is the influence in the inoculating matter or open wound which appears thus to affect the vitality of the sarcophytes in the part, and make them proliferate in a concrete perishable form, and communicate a like tendency to other sarcophytes in the adenoid tissue of the body through which they circulate? From its infective power it would appear to be of a zymotic nature (whatever that may be, whether a chemical catalytic force or an organic spore or germ). And the notion has been propounded even recently by Dr. Madden, of Torquay, and Dr. W. Budd, of Bristol, that tubercles are the result of a specific poison, like scarlatina, typhoid fever, and other zymotic diseases. We are, perhaps, as yet hardly in a position to accept or to reject this notion, for there are many facts for and against it, which we cannot afford space to consider at length; but at present it seems to me more probable that the influence which causes tubercle is something more common than a specific poison-something more analogous to putrefactive matter, which may proceed from various materials, and even from the decomposition of a part of the body itself. It may, indeed, be said that putrefaction and other kinds of decomposition are promoted, if not produced, by the presence of vibriones, bacteria, and other septic organisms; but if so, it is in the way of common corruption, and not through any specific agency engendering or engendered by disease. If I am correct in my inference that tubercles arise from a degraded vitality and concrete state of the sarcophytes, whereby they lose their mobility and plasticity, although they retain their proliferating power, it can be easily conceived that, without assuming the existence of any specific poison, various noxious agencies may be capable of so

NATURE OF INFECTION.

99

injuring them. The observation of Max Schultze has been already noticed, that heat above 104° Fahr. is sufficient to stop the movements of the sarcophytes and make them harden. It may be a question how far this power of heat is concerned in developing tubercles in exanthematous fevers, especially measles and scarlatina, which are not unfrequently followed by tuberculosis. This power of heat to injure the sarcophytes may also have some share in the difficult healing of wounds from burns and scalds, and in the very cacoplastic character of the scars which they leave behind them. But both eruptive fevers and burns have other effects besides this direct operation of temperature; and it is easy to perceive that foulness or tendency to corruption of any kind may similarly injure the vital properties of the bioplasm, and degrade its constituent corpuscles to the condition of tubercle.

And if we revert to the miscellaneous matters found capable of producing tubercle by inoculation (for example, in Dr. Fox's experiments, tubercle; red, grey, and cheesy pneumonic matter; foul pus; pycemic pus; lardaceous liver; putrid muscle; vaccine matter; and, less successfully, seton wounds, &c.), we find further proof that the cause seems to be some common corrupting influence, rather than any specific poison.

Again, we must not lose sight of the different susceptibility to the morbific operation of this cause in different individuals, and still more in different species of animals. In most carnivorous and herbivorous animals, inoculation commonly produces mere inflammation and suppuration, and very rarely tubercles. In rabbits, inflammation and suppuration are as commonly excited as tubercle, and often prevent the production of tubercle. In guinea-pigs, on the other hand, suppuration is rare, and tuberculisation is the common result. In their indisposition to suppuration they resemble birds and reptiles, in which Mr.

Gulliver doubted that true pus is ever produced. This must depend on a natural difference in the sarcophytes of these respective classes of animals, and suggests the probability of similar variations between different individuals of those animals capable of both suppuration and tuberculisation.

We are now in a position to consider the spontaneous occurrence of miliary tubercles in the human subject, and the light which it may derive from the above experiments. The grey miliary tubercle of Laennec, of size varying from a millet seed to a hemp seed, remarkable for almost cartilaginous hardness and some degree of translucency, occurring scattered through the pulmonary tissue or gathered into groups, which in relation to the bronchioles resemble little bunches of berries, is the production which most distinctly characterises the tuberculous form of pulmonary consumption. Laennec noticed that these little bodies tend to become opaque and to soften, just as diffused consolidation do, and, therefore, he concluded all, granular and diffused, under the common term tubercle, which presents the different conditions of grey, crude yellow, mature and softened, as successive stages of the same accidental production. Whilst, with Laennec, we acknowledge the identity of the granular and diffused consolidations in their composition and changes to the opaque and softened states, we reject his notion that they are accidental or heterologous growths like those of Cancer, following special laws of their own; and we adopt the much simpler and more obvious view that they are only results of the degradation of the ordinary material and process of histogenesis or textural nutrition, and that their changes are no other than the successive steps of decay, the natural consequence of gradual declension of life and organisation.

With regard to the diffused consolidations of the lung

ANALOGIES IN PRODUCTION OF TUBERCLE.

101

in phthisis, we found reasons for regarding them as the results of inflammation, and have endeavoured to trace them from the observed operation of that process in its different varieties. Miliary tubercles may probably also sometimes originate in inflammatory irritation; for example, when they break out during the febrile stage of an exanthem, measles or scarlatina, from the febrile poison acting as an irritant, in points of the pulmonary tissue, as it does outwardly in the eruption in the skin. Probably we may also refer a certain number of cases of miliary tubercle to prolonged or repeated bronchial inflammation, under circumstances, which deteriorate the blood plasma, such as poor living, damp air, and close habitations.

But in the greater number of instances miliary tubercles seem to arise independently of inflammation, by a spontaneous dissemination through the lungs and other textures, as if scattered by means of the blood-vessels or lymphatics. These are the cases that are so precisely imitated by the experiments of artificial tuberculisation that we can hardly avoid the conclusion that the process is similar in the two cases. Let us trace the analogous cases of tubercle spontaneously arising from infecting causes in the body.

1. We are abundantly taught by clinical experience that miliary tubercles form in the lung after various kinds of unhealthy local suppurations: for example, after scrofulous suppuration of the cervical and other lymphatic glands; after abscesses and unhealthy wounds connected with diseased bones; after fistula in ano; after ulceration of the intestines from typhoid fever; and after empyema and pyoperitonitis. 2. Miliary tubercles form in the lungs and other organs also after caseation taking place in the lungs; in the bronchial, cervical or other lymphatic glands; in the liver, kidneys,

spleen, or serous cavities. (Buhl and other German writers have taken too partial a view in limiting the paternity of miliary tubercles to this cause.) 3. Miliary tubercles form also in consequence of the presence of various consolidations in the lung or other organs when these consolidations have a decaying tendency. Thus various chronic indurations of the lung (among which may be included pre-existing miliary tubercles), lardaceous liver, granular kidneys, and disintegrating fibrinous emboli in blood-vessels, may become causes of new miliary tubercles. 4. And lastly, we learn from Drs. Sanderson's and Fox's experiments that common wounds in the integuments, such as that of a seton or issue, may, in rare cases, determine the production of miliary tubercles in the internal organs.

But as in the experiments, so in the natural development of disease, it is not in every case that tubercles are produced from their supposed causes. If all kinds of animals are included, inoculation with the most potent materials, such as tubercle itself, or the matter of pyœmia, had no effect in many instances, and in others caused suppuration and not tubercle. Even in guinea-pigs, which these experiments prove to be naturally disposed to tubercle, the effect was not certain when matters less allied to tubercle, or less foul, were used; and no result was obtained from healthy pus or other products of healthy inflammation. So in clinical experience we find many instances of the presence of even scrofulous suppuration, of caseation, of chronic indurations of organs, including tubercle, without any further speading or infecting the body; and quite exceptional are the cases of a common wound or healthy suppuration being followed by internal tuberculisation.

There must, therefore, be some co-operating cause which renders inoculation in the one case, and the products of