We cannot claim to know much definitely as to the exact method in which this disease arises. Four or five vegetable germs and at least two small parasites belonging to the animal kingdom have been found, but none of these are present in all cases, and many of them may be quite commonly discovered in the interior economy of quite healthy persons, so that either the true germ remains to be discovered, or those we know of have only a secondary importance, becoming harmful only when they find themselves in contact with an irritated bowel. As a matter of fact, there are a good many kinds of dysentery, but to enter into their various characteristics would only confuse the lay reader.
The common characteristic is the discharge of frequent scanty motions, with much pain, and an intolerable sensation that more is to come. The material voided is always extremely offensive, of a mucous consistence, and wanting in the natural bilious colour. In severer cases, the mucus becomes streaked with blood, and sometimes little else will be seen, and the actual loss of blood itself may become a serious element of danger.
The disease does not usually occur as an epidemic, though something very like one is not unfrequently to be met with among bodies of men subjected to severe hardships and privations, as for example among soldiers during an arduous campaign in extreme climates. Some predisposing cause capable of causing irritation of the intestine seems to be essential to enable the germs, known or unknown, to take action. This irritant may be mechanical, such as coarsely ground, ill-cleaned grain; or chemical, as in the dysentery that is apt to appear among persons feeding on too newly reaped barley, or from foul or saline water; but the commonest of all causes appears to be the decomposition of the contents of the bowel which almost inevitably occurs when, from any reason, the production of the bile is arrested.
The peculiar yellowish-green secretion of the liver known as the bile appears not only to assist in the solution and digestion of the food, but to act as a natural antiseptic, which checks the too great multiplication of the various germs which are naturally always to be found in the intestine. It is comparatively rarely that the liver itself strikes work, but what does very commonly happen is that a chill, or a mechanical or chemical irritation of the bowel, may extend to the bile duct, and by causing swelling, or spasmodic action of its muscles, prevent the contents of the gall bladder, in which the bile secreted by the liver is stored up, from passing on into the intestine. The chill or irritant that thus stops the flow of the bile necessarily at the same time produces a greater or less amount of catarrh and inflammation of the lower bowel, which, lying as it does next to the wall of the abdomen, is most easily affected by cold; but stoppage of the flow of bile into the intestine seems an essential element in the production of dysentery, as a more or less complete absence of bile from the motions is a universal symptom of the condition, and to restore the action of the liver is, practically speaking, in cases taken sufficiently early, equivalent to curing the disease.