There are several reasons why rectal abscesses so frequently degenerate into fistulæ. One is, that, owing to an internal opening within the bowel, small particles of fecal matter find their way into the sinus, and, acting as foreign bodies, prevent the healing; another, that, owing to the frequent movement of the parts by the sphincter muscles, sufficient rest is not obtained for the completion of the reparative process; and, finally, the vessels near the rectum are not well supported, and the veins have no valves, hence there is a tendency to stasis, which is unfavorable to rapid granulation. According to the authority of Mr. Harrison Cripps,[33] if the fistula be divided its surface will be seen to be lined with a smooth, gelatinous membrane, which when examined under the microscope is found to consist of granulation-tissue exactly analogous to that which lines the interior of a chronic abscess. The leucocytes constituting the outer wall of this membrane are but loosely adherent, and constantly becoming free they form the chief part of the pus which drains from the fistula. RELATIVE FREQUENCY OF THIS AFFECTION.—In point of frequency, compared with other rectal diseases, fistula is next to hemorrhoids. This statement is contrary to the showing made by the published statistics of St. Mark's Hospital, as quoted by Allingham.[34] This table shows that out of four thousand cases taken consecutively from the out-patient department of the hospital there were one thousand and fifty-seven persons suffering from fistula and one hundred and ninety-six from abscess, of which latter number one hundred and fifty-one subsequently became fistulæ, so that more than one-fourth of the whole number of cases treated were fistulæ. Allingham also states that a recent examination of the records of the in-patients of the same institution, covering a period of several years, shows that two-thirds of those operated upon were cases of fistula. Mr. Allingham[35] justly calls attention to one source of error in drawing conclusions from statistics—namely, the fact that many patients suffer from more than one malady. He states that it constantly happens that a fistula is found in connection with hemorrhoids, either as the substantive disease or as a complication. Again, a fissure or circular ulcer often has a sinus running from it, so that it may fairly be considered as the opening of an internal fistula, and the case called a fistula; or the sinus is not detected, and the case is called ulcer or fissure.