The irritation in many such cases is not due to one agent only, the vegetable or other irritant may be the starting point, acting but as a temporary irritant, the action of which is supplemented and aggravated by the subsequent attacks of bacterial ferments on the inflamed, weakened or abraded tissues. The bacteria present in the mouth, food or water would have had no effect whatever upon the healthy mucosa, while they make serious inroads on the diseased. On the other hand the vegetable, mechanical or chemical irritant would have had but a transient effect, but for the supplementary action of the bacteria. In horses that have the bad habit of retaining masses of half masticated food in the cheeks the growth of cryptogams is greatly enhanced and such food often becomes violently irritating. Among other mechanical causes may be named pointed or barbed hairs or spines (barley awns, spikes, thorns, etc.) which, lodging in a gland orifice, or in a wound of the gum or mucosa, form a source of irritation or a centre for bacterial growth and abscess. Again, irritants of animal origin must be named. These are not taken by choice, but when lodged in fodder, or in the pastures they are taken in inadvertently with the food. In this way poisonous insects, and especially hairy caterpillars, cantharides, potato bugs, etc., gain access to the mouth. It must not be overlooked that stomatitis occurs as an extension, sympathetic affection or sequel of diseases of other organs. Gastritis is usually attended by redness and congestive tenderness of the tongue, especially of the tip and margins, and other parts of the buccal mucosa, notably the palate just back of the incisors, are often involved. In other cases it appears as a complication of pharyngitis, laryngitis, of affections of the lower air passages, of the teeth and periodontal membrane or of the salivary glands. It appears also in a specific form in certain fevers, as in horsepox, pustulous stomatitis, aphthous fever and even in strangles. Mercurial stomatitis, rarely seen at the present time, is one of the worst forms of the disease, and like the infectious forms will be treated separately. _Lesions and Symptoms._ At the outset and in the slighter forms of congestion there is merely heat and dryness of the buccal mucosa. Redness may show on the thinner and more delicate portions of the membrane, as under the tongue, on the frænum, and on the sublingual crest. But elsewhere it is hidden by the thickness of the epithelium, and the manifestations are merely those of suppressed secretion with local hyperthermia. As the congestion is increased there is seen, even at this early stage, a slight thickening or tumefaction of the mucosa, especially on the gums, lips, the sublingual area, the orifices of the salivary glands, and the palate back of the upper incisors. On the dorsum of the tongue, the cheeks and lips, generally the lack of loose connective tissue tends to prevent the swelling.