Another fallacious element in the statistics of Mr. Allingham, which should not be overlooked, is pointed out by Mr. Chas. B. Ball.[36] St. Mark's has a special reputation for the cure of fistula, so that many persons suffering from this disease go there, and in this way the records show an apparent greater frequency of fistula. Mr. Ball also states[37] that at the Dublin General Hospital, although fistula is common, it is by no means the commonest of rectal diseases; and in his own practice this affection has not furnished more than one-sixth of rectal operative cases. AGE AND SEX AFFECTED.—This disease is commonly met with during middle age, but it is by no means restricted to that period of life. Allingham states[38] that he has operated upon an infant in arms, and upon a man over eighty years old. Dr. Henry R. Wharton[39] mentions having seen a number of cases at the Children's Hospital, Philadelphia, among which he records one of complete fistula in a child a few months old. CAUSES.—Fistula in ano may originate in ulceration and perforation of the mucous membrane of the bowel—the result of the irritation produced by fecal accumulations (arising from any cause, such as atony of the intestines, irregularity of habits, rectal stricture, etc.), or by foreign bodies, such as fish- or rabbit-bones, grape- or fig-seeds, etc.; more frequently it owes its origin to an abscess caused by injuries, such as blows or kicks upon the anus, or by exposure to cold, as from sitting upon damp seats—especially after exercise, when the parts are hot and perspiring; it may also arise from excessive irritation of the rectum occasioned by the presence of any of the forms of parasites which infest the anus and its immediate neighborhood. Other predisposing causes are thrombosed veins and suppurating hemorrhoids. Abscess, and then fistula, may likewise supervene in fevers and certain depressed conditions of the blood, such as frequently give rise to boils or carbuncles.[40]