Although the parasite is undoubtedly transmitted from man to man by the agency of mosquitoes, it is not certain if this is the only mode of propagation of the disease. It may be that the malarial organism has an extracorporeal existence other than that in the mosquito. It is difficult to speak dogmatically on this subject, but certain considerations are against such an hypothesis. Thus, it seems hardly likely that such a delicate organism as the malarial parasite, which requires for its development not one but two distinct hosts, should be able to thrive indifferently in air, soil, or other media. Besides, wherever anopheles have been entirely suppressed malaria has almost vanished, which shows that the mosquitoes are apparently the sole carriers of infection from man to man. We may,

therefore, conclude that turning up the soil and impure water play no etiological rôle in the production of malaria, except in so far as they offer facilities for the breeding of anopheles.

Prevention. The measures for the prevention of malaria have for their object either the prevention of the infection of man by the mosquito, or the prevention of the infection of the mosquito by man. The principal methods are as follows:

(i.) The destruction of the parasite in man by the systematic use of quinine. Koch has obtained

marvellous results by this method, but it is scarcely applicable to large communities. (ii.) The prevention of the access of the mosquito to man by the use of mosquito nets, or such means as sleeping in the upper storeys of houses, camp fires, etc.

(iii.) Extermination of the mosquito. Some good can be done by certain insecticides, such as chrysanthemum powder, sulphur, or kerosene oil, and

by abolition of their breeding places by means of proper drainage. The eggs, larvæ, or the adult insect may also be directly destroyed; but the method is too costly and laborious to be of much use.

A more efficient means for their suppression would be the pitting against them of some of their natural enemies. It is known that the larvæ of mosquitoes are swallowed by many aquatic insects, and the adults are killed off by bats, lizards, and dragon flies. It is not impossible that in the near future we may discover some plant that would prevent the breeding of the anopheles, or some innocuous animal which would destroy or displace this species.

Relation to Kala-Azár.-Kala-azár is a very fatal disease largely prevalent in Assam. The nature of this affection has not been sufficiently investigated, and conflicting views are held with regard to its etiology. According to Giles, it is the result of ankylostomiasis, either by itself, or in some cases complicated by a coincident malarial infection. Ross, on the contrary, regards it as an aggravated form of malaria, from which, however, it is distinguished by a rapid occurrence of cachexia, greater resistance to quinine, and by the fact that it frequently attacks several members of one family.

DYSENTERY.

As is well known, a number of pathological lesions situated at the lower end of the large intestine are empirically included under the name of dysentery. The various types of the disease do not appear to have a common source of origin, and their etiology has not been satisfactorily worked

out.

For our purpose, however, dysentery may be divided into the bacillary and amabic varieties. The former has been ascribed by Celli to the bacillus coli communis taking on a virulent function when in the presence of the pyogenic cocci. It is also suggested that a mere catarrhal condition of the bowels (as induced by malaria, bad food, exposure, etc.) may enable the colon bacilli to become virulent and promote dysentery. But it seems difficult to imagine that so severe a disease as dysentery could be due to bacteria which are the normal constituents of the alimentary canal.

Shiga, in 1898, isolated an organism which he suggested as bearing a causal relation to the disease. It resembles in form and growth the typhoid rather than the colon bacillus. It is constantly present in the dejecta in acute cases, and gives the "agglutination reaction" when mixed with the serum of patients suffering from the disease. When introduced into the stomach of cats, characteristic

changes are produced in the lower intestine. It appears, therefore, that Shiga's bacillus is the undoubted cause of acute dysentery.

Amabic Dysentery.-This variety is characterised by the presence of a definite species of amoeba both at the seat of lesion and in the stools. If a piece of flocculent mucus be picked out from the fresh stools and examined under the microscope, the amaba coli can usually be observed. They throw out characteristic pseudo-podia, change their shape, and slowly move across the field

of the microscope. When at rest the parasite is rounded in form and about four or five times the length of a red-blood corpuscle. There is a central nucleus, and the protoplasm frequently contains vacuoles, bacteria, red corpuscles, etc.

Outside the body, the amoeba immediately breaks up, and cannot be made to grow on artificial media. Rectal injections of stools containing amoeba give rise, in cats, to hæmorrhagic enteritis, the amoeba being present in the

dejecta and frequently invading the mucous membrane of the intestine.

But these results cannot be regarded as conclusive. The ambæ have not been used in pure cultures, and the results following the introduction of so complex a material as stools cannot, by any means, determine the properties of any single constituent. Then, again, the cat is an animal very susceptible to catarrhal inflammations of the bowels, which can be readily produced by the use of non-specific irritants.

However, the constant presence of the amoeba in the submucous tissue, in the ulcers and in their spreading zone, cannot be regarded as an accidental coincidence. Further, the presence of amoeba coli in the pus of liver abscess (a condition frequently associated with tropical dysentery), where they may exist without the pus cocci, shows that they have migrated into the portal circulation from the dysenteric ulcer and thence carried to the liver. This pus, moreover, when injected into animals, gives rise to lesions precisely similar to those produced by the injection of dysenteric stools. On the whole, then, we may conclude that a definite form of dysentery does exist in which the amoeba coli plays an all-important etiological rôle.