Skin.—In the middle line of the posterior part of the perineum and about 4 cm. in front of the tip of the coccyx is the anal orifice. The junction of the mucous membrane of the anal canal with the skin of the perineum is marked by a white line which indicates also the line of contact of the external and internal Sphincters. In the anterior part of the perineum the external genital organs are situated. The skin covering the scrotum is rough and corrugated, but over the penis it is smooth; extending forward from the anus on to the scrotum and penis is a median ridge which indicates the scrotal raphé. In the female are seen the skin reduplications forming the labia majora and minora laterally, the frenulum of the labia behind, and the prepuce of the clitoris in front; still more anteriorly is the mons pubis.
Bones.—In the antero-lateral boundaries of the perineum, the whole outline of the pubic arch can be readily traced ending in the ischial tuberosities. Behind in the middle line is the tip of the coccyx.
Muscles and Ligaments.—The margin of the Glutæus maximus forms the postero-lateral boundary, and in thin subjects, by pressing deeply, the sacrotuberous ligament can be felt through the muscle. The only other muscles influencing surface form are the Ischiocavernosus covering the crus penis, which lies on the side of the pubic arch, and the Sphincter ani externus, which, in action, closes the anal orifice and causes a puckering of the skin around it.


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