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 the disease. Another serum has been prepared by Lustig and Galeotti.

Morbid Anatomy:—(1) Bubonic cases. A bubo is found to consist of a chain of enlarged glands, surrounded by a mass of engorged connective tissue, coagulated blood and serum. Nearly all the lymphatic glands in the body are a little swollen, but the lymphatic vessels show little or no change. The spleen and liver are always enlarged, the former to sometimes twice or thrice its natural size. The lungs are engorged and oedematous, and often show haemorrhages. The kidneys are enlarged and congested. The serous membranes show petechiae and haemorrhages. The right side of the heart is frequently dilated, with clots in the cavities. The heart muscle is normal, or soft and friable. The substance of the brain, spinal cord and nerve-trunks is normal, but the membranes are engorged. (2) Pneumonic cases. The lymphatic glands are hardly affected. There is general engorgement and oedema of the lungs, with pneumonia patches varying in size and irregularly distributed. (3) Septicaemic cases. Nearly all the lymphatic glands in the body are involved, and have a characteristic appearance. They are enlarged to the size of an almond, rounded, firm and pink; there is some engorgement and oedema on section; the substance is rather soft, and can be scraped off with a knife. The surrounding tissue is not engorged or oedematous. The description of the other organs given under (1) applies also to (2) and (3).

Dissemination.—Given the bacillus, the questions arise, How is it disseminated? and What are the conditions that favour its propagation? That it is conveyed from person to person is an undoubted fact, proved by innumerable cases, and tacitly implied by the word “infectious,” which is universally allowed. The sick are a source of danger and one means of dissemination, and, since the illness may be so slight as to pass unrecognized, an obviously insidious one. The ambulatory plague patient goes far to explain the spread of the disease without leaving any track. But there is evidence that persons may carry the infection and give it to others without being ill at all themselves. One such case occurred at Glasgow, and another at Oporto. In the Glasgow case the wife of a laundryman employed in handling plague linen contracted the disease. She was brought into connexion with it in no other way, and there can be no doubt that she took it from her husband, though he was not ill at all himself. The Oporto instance is still more conclusive. Two little girls had plague at Argoncilhe, a suburb some miles from Oporto, and were the only cases which occurred in that place. Their father was a riverside labourer, who lodged during the week in Oporto, but went home for Sunday. He was not ill, but several cases of plague occurred in the house in which he lodged. How the poison passes from one person to another is less clear. In pneumonia cases patients no doubt spread it around them by coughing, and others may take it up through the air-passages or the skin; but even then the range of infection is small, and such cases are comparatively rare. In the vast majority of cases the bacilli are in the lymphatic or the circulatory system, and aerial convection, even for a short distance, seems highly improbable. This view is borne out by the experience in hospitals and with “contacts,” which goes to show that with reasonable care and under fair conditions the risk of infection from ordinary plague patients is very small. When persons live crowded together in closing contact, and when they are careless with regard to discharges of all kinds from patients, the risk is obviously much increased. Discharges-vomited matters, sputa, urine and faeces-are possible media by which plague is spread from person to person. They also contaminate clothing, which thus becomes another means of dissemination capable of acting at a distance. This is the most probable explanation of the two cases of indirect infection related above. Failure to catch or induce plague from clothing that has been worn by plague patients proves nothing Such cloth.ng is not necessarily infectious; indeed, the probability is that it is not, unless contaminated by discharges. There is no evidence that merchandise and foodstuffs are means of dissemination, but a great deal of evidence against such a theory. Then we come to the lower animals. Attention has been concentrated on rats, and some observers seem disposed to lay upon them the whole blame for the propagation and spread of plague, which is held to be essentially a rat-borne disease. The susceptibility of rats has been noted from remote times and in many countries, particularly in China, but it has never attracted so much attention as during the recent prevalence of plague. From one place after another a great mortality among rats was reported, and the broad fact that they do die of plague IS incontestable. It is therefore easily intelligible that they may play an important part in multiplying and fixing the poison on a locality. As to how they convey it from man to man the greatest probability is in favour of the fiea as an intermediary. Mortality among rats is said to precede the appearance of human plague, but the evidence of this is always retrospective and of a very loose character. At Sydney a careful investigation was made; and the conclusion reached by Dr Tidswell was that “there was no ground for even a suspicion that our epidemic was being maintained by any process of direct contagion between man and man,” but that rats were the carriers. In Glasgow the experience was just the contrary. Personal connexion was traced in every case, and rats excluded; there was no mortality among them, and of 300 caught and examined none had plague (Chalmers). Similarly, at Oporto, personal connexion was traced in all the earlier cases; there was no mortality among rats, and no evidence to connect them with the outbreak (Jorge). Again, a comparison between rat infested and rat-free districts in Bombay showed a much higher incidence of plague in the latter. A campaign against rats in Bombay, by which 50,000 or 60,000 were killed in a short time, had no effect in checking the disease. Plague-rats have rarely been found in ships sailing from infected ports; and though millions of these animals must have been carried backwards and forwards from quay to quay between Hong-Kong, Bombay and the great European ports, they have not brought the disease ashore.

By far the most important communication on the role of rats in the spread of plague is formed by the “Report on the Plague Investigations in India” (Journal of Hygiene, vol. vi. No. 4; vol. vii. No. 3, 1907). The chief conclusions arrived at in the report as the result of experiments are the following:—

1. Healthy rats contracted plague from infected rats when the only apparent means of communication between the two was the rat flea (pulex cheopis).

2. In 21 experiments out of 38, 55% of healthy rats living in flea-proof cages have contracted plague after receiving fleas collected from rats either dead or dying of septicaemic plague; consequently it is proved the rat flea can transmit plague from rat to rat.

3. Close and continuous contact of plague-infected animals with health ones does not infect the latter if fleas are excluded.

4. Should fleas be present an epizootic at once starts and spreads in proportion to the number of fleas present.

5. Guinea-pigs set free in plague-infected houses become infected with the rat flea and develop plague in a certain percentage.

6. Fleas caught on plague-infected rats are able to infect rats placed in flea-proof cages.

7. Guinea-pigs placed in plague-infected houses do not contract plague if they are protected from fleas; those placed in cages protected by a border of sticky paper at least six inches in radius, which the fleas cannot jump over, do not contract plague; the Others not similarly protected, do.

8. Chronic plague may prevail in rats.

On this report it may, therefore, be taken that aerial infection, except, perhaps, in pneumonic cases, may be excluded, and that the chief source of infection is the flea. It was also shown that animals may become infected through the faeces of a Bea which has been fed on plague-infected rats. This may serve to explain the manner in which plague-infected linen and clothing may convey the disease. The report also considers it proved that the bacillus pestis multiplies in the stomach of a flea and may remain a considerable time within its host.