Canker, a form of aphthous ulceration of the mucous membrane of the mouth, most commonly seen in children, and usually connected with derangement of the digestive apparatus. The ulcers are small, circular, superficial, filled with a white thick exudation, sometimes surrounded by a circle of inflammation, and very sensitive; they originate in small, hard, red, and painful prominences, which are soon changed into vesicles, whence the name " vesicular stomatitis." When the ulcers are few in number they quickly disappear, their cicatrization being hastened by astringent or caustic applications, and by the exhibition of gentle aperients. In unhealthy children the ulcers are apt to be confluent, and tend to spread to the oesophagus and stomach; in such cases there may be considerable constitutional disturbances, requiring tonics and alteratives. The predisposing cause of aphthae is anything that enfeebles the system, and the exciting cause any irritation in the mouth from foreign bodies, decaying teeth, or acrid food. The usual seat is on the inside of the lower lip and cheeks, and on the tongue, though they may occur on almost any part of the mucous membrane.

Billard represents them as ulcerations of the muciparous glands or follicles, but in many cases they are too superficial to admit of this explanation. When they occur in debilitated constitutions, in the course of other diseases, they form a painful and dangerous complication, from their liability to extend and to take on a gangrenous aspect. They are generally only a local affection, and require for their removal only local applications. The best of these are nitrate of silver, applied either in the solid form or in solution, and borate of soda, which is often used dissolved in honey. Internal remedies are rarely required. The return of the ulcers may be prevented by attention to the general rules of hygiene, and especially to the diet, which should be simple, nutritious, and easily digested.