The anal and rectal canals are made up of circular and longitudinal muscular bands, which, when invaded by disease, lose their proper or normal sensibility and coöperative voluntary action. The excessive contraction of the circular muscles closes the calibre or bore of the gut, and the excessive contraction of the longitudinal muscles shortens the length of the gut, thus throwing the mucous membrane into abnormal folds which increase the depth of the sacculi, or cavities, between the fibrous folds. In the normal gut the sacculi and bands act as valves to control the descent of the feces. This valvular arrangement and the curvatures of the lower bowels conserve the energy of the involuntary and voluntary nerve force until there is a sufficient accumulation of feces to excite a normal desire for stool; otherwise the feces would rush upon the anus at once and occasion much inconvenience. Catarrhal inflammation of the mucous membrane of the anal canal will sooner or later penetrate the muscular structure of that canal, causing an abnormal irritability and contraction of the sphincter ani and the other tissues composing its structure. The contraction of the anal tissues becomes more permanent as the muscular tissues of the structure become cohered or bound together by the process of inflammation. The normal stimulus and sensation that should precede the act of defecation are perverted or destroyed by the excessively irritable contraction of the sphincter ani, which contraction is occasioned by the presence of feces and gases just above the seat of inflammation, that is, above the anal canal or at the lower end of the rectum. As the bulk of feces and gases lodged at this point increases, the anal contraction becomes firmer in grip, and as a consequence permits no hint of the imprisoned contents until the accumulating bulk is beyond the power of toleration by the organ. Daily a portion of the lodged feces, or some new addition to the mass, passes the anal canal, but the attending irritation or contraction of the muscles prevents any further exit of the imprisoned rectal contents. CHAPTER XIII. THE ETIOLOGY OF THE MOST COMMON FORM OF DIARRHEA, i.e., EXCESSIVE INTESTINAL PERISTALSIS.