In this form of laceration the sphincter ani muscle usually escapes injury; the tear is directed toward the ischio-rectal fossa, and the rectum and anus are pushed to one side. The structures of importance that are injured are the fascia, the levator ani muscle, the sphincter muscle of the vagina, and perhaps the transverse perineal muscle. All the supporting structures of the perineum and of the posterior vaginal wall are injured. If the laceration be bilateral, complete loss of support of the perineum and the posterior vaginal wall results, and if the condition be untreated, all the disastrous consequences of loss of support of the perineum occur--prolapse of the vagina, of the uterus, and of the other pelvic organs. It is unusual that this form of laceration is entirely limited to one sulcus, though one is usually more involved than the other. When the injury is limited to one side, the perineum is still supported by the muscles and fascia upon the other side, and the tendency to prolapse is not so marked. The nature of this injury may always be detected by examination after labor. The anterior vaginal wall should be elevated by a retractor, and the posterior wall should be carefully examined. An external tear of the skin, generally in the median line, usually accompanies laceration in the sulci; that is, the lacerations in the sulci converge toward the fourchette. The immediate operation should always be performed. The torn sulci should be closed by sutures introduced on the posterior vaginal wall (Fig. 43), and the external tear should be closed by sutures introduced as in the first form of injury to the perineum, already described. [Illustration: FIG. 43.--Sutures introduced for the closure of a recent perineal laceration in the sulci.] If this form of perineal injury is not repaired by the immediate operation, cicatrization takes place, and the tears in the mucous membrane and in the skin become healed. The fascia retracts, and the integrity of the supporting planes of fascia is destroyed. The torn muscles, the inner fibers of the levator ani and the sphincter vaginæ, also retract and cease to furnish any support to the perineum. In health these muscles embrace the lower portion of the posterior vaginal wall like a sling, drawing it toward the symphysis pubis; after laceration in the sulci the support of one or both of the arms of the sling is destroyed.