Perineal lithotomy-to remove a stone from the bladder, which here lies at a depth of 3 inches-is an operation seldom performed. It may be either lateral or median. In the lateral operation an incision 2½ inches long is made, commencing just to the left and behind the central point of the perineum, and carried down and outwards into the ischio-rectal fossa, to end at the junction of the outer and middle one-third of a line joining the tuber ischii and the anus. In addition to skin and superficial fascia, the transversus perinei muscle, artery, and nerve, lower edge of external layer of triangular ligament, and external haemorrhoidal vessel and nerves are cut. The scalpel is now entered through the exposed membranous urethra, and its point engaged in the groove of the staff, while the edge is directed toward the left tuber ischii, and in this position it is pushed along the groove into the bladder. In this incision the membranous and prostatic portions of the urethra, posterior layer of the triangular ligament, compressor urethrae, and anterior fibres of the levator ani, and left lateral lobe of the prostate are divided. In the first incision of this operation the bulb may be wounded if the incision be begun too far forward, or the staff is not drawn sufficiently up under the pubes. The rectum may be cut if the incision is carried too far back or the viscus is distended, and the pudic vessels might be damaged if the incision were carried right to the ramus. In the second incision the prostatic capsule and plexus of veins are necessarily cut ; but if the incision ' be carried too far forward, the visceral layer of the pelvic fascia might be cut, and the pelvic cavity opened ; this is more likely to occur in children, where the prostate is rudimentary.

In median lithotomy (Cock's operation) the knife is entered in the middle line just in front of the anus, and is directed to enter the median groove on the staff at the apex of the prostate, the membranous urethra being incised in withdrawing the knife, a wound 1¼ inches long being made in the median raphe. The finger is now introduced, the parts dilated, and the stone removed. Here the parts divided are skin and superficial fascia, sphincter ani, central point of perineum, base of the triangular ligament, the whole length of the membranous urethra, and compressor urethrae. As this operation is made through the avascular raphe, there is little bleeding, and the pelvic fascia is less likely to be opened by the dilating process ; on the other hand, the space obtained is small, the bulb is apt to be wounded (but median wounds of the bulb do not bleed much), and in children the process of separation would be very apt to tear the bladder from the urethra. Similar operations may be performed for removal of the prostate by the perineal route, or for making a simple incision into the bladder to afford free drainage in critical cases of enlarged prostate. The pudic nerve supplies sensation to the skin of the perineum and also of the penis, scrotum, and anus, while it also supplies the mucous membrane of the urethra and muscles of the penis. Thus, painful affections of the perineum and anus may cause priapism. The perineal branch of the small sciatic also supplies sensation to the perineum, and thus in perineal abscess pain is frequently referred to the gluteal region and posterior part of the thigh.

In the female the urogenital triangle is perforated by the vaginal orifice, the vulva forming practically a cleft between two halves of a rudimentary scrotum. The deep layer of superficial fascia runs through the labia majora to ascend-on to the abdomen. Rupture of the perineum occurs frequently in connection with labour, and may extend into the rectum. In such cases the pelvic organs are deprived of considerable support, and may project through the vulva, forming a vesicocele or rectocele.

The anal triangle, occupying the posterior portion of the perineum, contains the rectum and ischio-rectal fossae. The rectum and anus are situated centrally, being bounded on either side by the ischio-rectal fossae. Each ischio-rectal fossa is wedge-shaped, the base being directed downwards, while the apex, 2½ inches from the surface, corresponds to the position of the white line, where the anal and obturator fasciae join, and is directed upwards and backwards. The triangle is bounded by the levator ani, covered on its inferior surface by the anal fascia, and the external sphincter internally, while it is bounded externally by the obturator externus muscle, covered on its inner surface by the obturator fascia. Anteriorly the fossa is limited by the transversus perinei muscle and base of the triangular ligament, and posteriorly by the margin of the gluteus maximus muscle ; but two small extensions occur : an anterior (pubic recess), extending beneath the transversus perinei between the obturator internus and levator ani ; and a posterior between the gluteus maximus and great sacro-sciatic ligament and the coccyx to the level of the ischial spine and coccygeus muscle. The fossa is occupied by a quantity of fatty connective tissue continuous with the surrounding subcutaneous fatty tissue, and the two fossae communicate freely behind the anus with one another. This tissue by yielding permits of faecal dilatation of the rectum, while in labour the fossae become almost obliterated by distension of the vagina. As the edge of the gluteus maximus overlaps this pad, it may indirectly assist the levator ani. The inferior hemorrhoidal vessels and nerves pierce the obturator fascia near the posterior part of the space, and pass downwards and inwards toward the rectum. The perineal branch of the fourth sacral becomes superficial near the tip of the coccyx, and small branches of the small sciatic nerve and sciatic artery curve round the lower border of the gluteus maximus, while the superficial perineal vessels and nerves enter the anterior portion of the space, and immediately leave it again by piercing the triangular ligament.

Alcock's canal, situated in the outer wall of the ischiorectal fossa, is formed by a splitting of the obturator fascia, and contains from above downwards the dorsal nerve of the penis, third part of internal pudic artery with venae comités, and perineal division of pudic nerve.