In this disease we have a violent inflammation determined by the presence of an intense irritant. The disease occurs in a sporadic and an endemic form. As already mentioned, the infective agent is believed to be an animal parasite, the amoeba coli (see page 378).

The virus is introduced chiefly by the drinking water, and it acts-on the mucous membrane directly. The locality of the lesion is apparently determined by the stagnation of the contents. It is essentially a disease of the large intestine, and is generally most intense in the rectum. It usually decreases in intensity from the rectum upwards, but not uniformly, there being more affected and less affected parts, the former corresponding usually with the flexures. In severe cases the whole colon is affected, and sometimes even the lower part of the ileum.

In the earlier stages the mucous membrane is swollen by serous exudation, soft and juicy, and it is thrown into folds, on the summits of which it is peculiarly hyperaemic. The surface is covered by a mucous or grumous material, consisting of shed epithelium with mucus and inflammatory exudation. The mucous membrane and submucous tissue are infiltrated with serous fluid and leucocytes in great abundance.

In higher degrees the mucous membrane is still more thickened and thrown into still more prominent folds. There is also considerable haemorrhage in its substance occasionally. The summits of the folds being specially exposed to mechanical irritation commonly undergo Necrosis, and the sloughs are generally distinguishable by the Brown colour which they assume from becoming stained with the bile pigment. The necrosis involves the mucous membrane to varying depths, sometimes very superficially, sometimes through its whole thickness, and, if the slough surrounds the gut, we may have a ring of necrosed tissue ultimately discharged by the anus.

These sloughs leave Ulcers behind, whose walls present great infiltration of leucocytes. But ulcers form also by processes similar to those in catarrh, namely, by inflammatory infiltration and molecular destruction of the mucous membrane, and by suppuration of and around the closed follicles.

In some cases the solitary follicles seem to be specially engaged, and some authors even distinguish a follicular form of dysentery, but the follicles are probably affected in the early stages of almost all cases, and partake in the general inflammation. By suppuration of the follicles there may be the formation of numerous ulcers with small apertures.

The contents of the intestine are in severe cases formed of dark decomposing material, mixed with blood. The mesenteric glands are always secondarily affected, being enlarged and hyperaemic.

If the patient survive the acute attack, the disease very commonly passes into Chronic dysentery. The ulcers formed in the various ways described above show little tendency to heal, but remain as open discharging sores, Sometimes they penetrate more deeply, and lead to abscesses in the surrounding tissue, especially of the rectum (Periproctitis). The remaining mucous membrane is swollen and in a state of catarrh. The intestinal wall is irregularly drawn in and adherent to the surrounding parts.

In some cases a tendency to Healing manifests itself. If the attack has been slight and the ulceration only superficial, there may be a complete restoration of the mucous membrane, with insignificant •cicatrices. But for the most part the cicatrices are of considerable superficial extent, perhaps surrounding the gut. The cicatrices of dysentery have usually a dark, almost a black colour. When the cicatrices in the usual fashion contract, they cause narrowing of the intestine, and this, in some cases, is very considerable. Alternating with the narrowing there is very commonly dilatation of the more healthy parts, so that a very remarkable pouching of the colon may result, the narrowed parts being probably adherent to the tissues around. The obstruction produced in this way is all the greater because the contraction often throws the remaining mucous membrane into folds which may act as valves to the constricted parte.

The symptoms of dysentery often continue, in the chronic form, after healing of all the ulcers has occurred. In that case along with the cicatricial contraction mentioned above, there is a very marked atrophy of the coats of the intestine, so that the wall is very thin and translucent. There will also be considerable adhesion to surrounding parts.