In addition to certain morphological and biological differences, an important point with regard to this organism is that it is pathogenic for fowls and harmless for man.

The two types of bacilli, however, do not form distinct species, but are only varieties modified by growth in the tissues of different hosts. This is now established by the experiments of Nocard, who, by growing human tubercle bacilli in the peritoneal cavities of a series of fowls, was able to convert them into the avian type.

(2) Bovine Tubercle Bacillus.-As has already been remarked, Koch recently stated that the bacilli of human and bovine tuberculosis are essentially different from each other. He, therefore, believes that human tuberculosis differs from the bovine, and cannot be transmitted to cattle. This is nowadays accepted by most authorities. But the converse of the proposition, i.e., the insusceptibility of man to bovine disease, has not been definitely proved. On the contrary, cases have frequently been recorded in which man has been accidentally inoculated with the bovine tubercle. The question, however, must be regarded as sub judice, although most workers are disinclined to accept Koch's observations.

(3) Recently, certain organisms have been obtained from various species of grass, milk and manure, which bear a remarkable resemblance to the tubercle bacillus. They present the same staining reactions (acid-fast), and on inoculation into guinea-pigs give rise to tubercle-like nodules. But unlike tubercle bacilli they grow quickly on artificial media, showing that these organisms are not parasites.

CHAPTER XVII.

LEPROSY.

THE essential cause of leprosy is an organism, the bacillus lepra, discovered by Hansen in 1879. It closely resembles the organism of tuberculosis in form and staining reactions, although differing from it in some essential particulars. Both bacilli are "acid-fast," and their pathological lesions are more or less alike. But the leprosy bacilli are somewhat thinner, stain more easily, and are usually arranged in bundles. They lie chiefly in the lepra cells, but may also be seen in the lymph spaces (Frontispiece, Fig. V.). They are never found free in the blood, a fact which may explain the chronic nature of the disease and the difficulty of its communication to healthy subjects.

The bacilli are present in the discharges from ulcers, and probably also in the nasal and salivary secretions. Nothing is known of their distribution outside the body. They have not been detected in the soils on which the lepers reside, or in the water in which they bathe. Examinations of fish have also given negative results. feeding on leprous ulcers fail to show the bacilli.

Flies

The leprosy bacillus has not been cultivated outside the body, and all attempts to reproduce the disease in animals. have failed. Some experiments have been made by inoculating with portions of leprous tissue the vascular combs of fowls and the anterior chambers of rabbits' eyes, but

beyond a local infiltration no other results followed. In no case a definite generalised leprosy has, thus far, been produced.

But this only shows that leprosy is essentially a human disease and does not occur in lower animals. The failure to cultivate the specific bacillus outside the body is probably due to the fact that the organism, being a strict parasite, requires a more specialised pabulum than that afforded by our artificial media. Although the exact proof of an etiological relationship is wanting, the universal presence of the bacilli in the tissues, its absence in healthy persons, and its intimate association with the pathological lesions, warrant the assumption that it is the essential cause of leprosy.

CHAPTER XVIII.

ACTINOMYCOSIS.

THIS disease, which occurs commonly in cattle, and not unfrequently in man, is caused by a specific organismthe streptothrix actinomyces, or the "ray fungus". This is not a true bacterium, but a streptothrix, and is characterised by a filamentous growth which gives rise to a felted mass of structure. The individual filaments are

[From Schenk's Bacteriology.

frequently branched, and at the free ends may show pearshaped swellings ("club formation"). Occasionally the threads segment into coccus-like bodies, from which new individuals develop.

The fungus grows on barley and other cereals, and by these means infects both men and animals. It gains

access to the tissues chiefly through abrasions of the mucous membrane of the mouth, but there is no tissue or organ which may not be attacked by the fungus. In cattle the disease is usually localised, and is of the formative type. A favourite seat of lesion is the tongue, which becomes extremely hard and indurated ("wooden tongue"), but the palate and skin may also be attacked.

In man the disease frequently affects the lower jaw, but the inflammatory tissue, instead of forming nodules, usually breaks down into pus and gives rise to abscesses.

Actinomyces grows in tissues in round yellowish masses, which are just visible to the naked eye, and appear like grains of iodoform. If one of these bodies be flattened out and examined under the microscope it is seen to be made up of a central network of branching filaments enclosing coccus-like bodies, and forming at the periphery a fringe of club-shaped processes. The clubs are formed by the swelling of the sheath of the fungus, as the result of degenerative changes. They are frequently calcified, and are more often met with in the bovine than in the human variety of the disease.

The fungus readily grows on artificial media. On agar the colonies are discrete, of a yellow beeswax colour, and usually very hard. On potato the growth is abundant and of a yellowish or brownish colour. Bread paste is also a favourable medium.

The results of inoculation are frequently unsatisfactory. The fungus has not been found outside the body, and nothing is known of its life-history.

Madura Disease. This is a chronic local affection, frequently affecting the foot, which is enlarged and shows numerous fistulous openings discharging a thin sanious liquid containing peculiar cellular bodies. These are sometimes of a yellowish and sometimes of a black