ANATOMY AND MECHANISM OF THE PERINEUM.
An accurate knowledge of the anatomy and mechanism of the female perineum is essential to an understanding of the nature and treatment of injuries to this structure. The anatomical structures lying between the anus behind and the symphysis pubis in front are those that most directly interest the gynecologist. Proceeding from below upward, we find the following structures lying in superimposed planes: the skin, the superficial fascia, the deep layer of the superficial fascia, the transversus perinæi and the sphincter vaginæ muscles, the anterior layer of the triangular ligament, the posterior layer of the triangular ligament, the levator ani muscle ([Fig. 19]).
Fig. 18, A.—Superficial structures of the female perineum (Weisse).
Fig. 19.—Dissection of female perineum: on the left side the perineal muscles are exposed by the reflection of the perineal fascia; on the right side the muscles and the superficial layer of the triangular ligament have been removed, thereby exposing the deep layer of the ligament. S. V., Sphincter vaginæ muscle.
The vagina passes through these structures. They surround and support the ostium vaginæ as the fascia and muscles surround and support the opening of the rectum or the anus. The muscles and fasciæ are attached in the median line between the anus and the vagina, and therefore this part of the body, which is called the perineum, is supported or maintained in its proper position by these various structures. The transversus perinæi arises from the ramus of the ischium and is inserted in the perineum. The bulbo-cavernosus, or sphincter vaginæ, arises in the perineum and is inserted in and about the clitoris. The inner fibers of the levator ani arise from the symphysis pubis and are inserted in the perineum and the lower part of the vagina ([Fig. 20]). When these muscles contract, their action, therefore, is to draw the perineum upward and forward. At the same time the anus is drawn upward and forward, and so also is the posterior margin of the ostium vaginæ and the lower portion of the posterior vaginal wall.
Fig. 20.—Dissection of female perineum, showing the deeper structures after removal of the levator and sphincter ani muscles.
The vagina has no circular sphincter like the anus, but the vaginal month is kept closed by the action of the transversus perinæi, sphincter vaginæ, and levator ani muscles, which draw the perineum forward, and thus keep the posterior vaginal wall in apposition with the anterior wall.
Fig. 21.—Muscular floor of the pelvis seen from above.
This sling of muscles and fascia, which surrounds and supports the opening of the vagina, may readily be felt in the nulliparous woman by introducing the finger in the vagina and pressing backward and outward toward the ischio-rectal fossa. We then feel plainly, immediately within the ostium vaginæ, a firm resisting band of tissue, apparently about half an inch broad, embracing the posterior portion of the lower vagina. This band is formed by the inner edges of the various muscles and planes of fascia that have been described.
Fig. 22.—Sagittal section showing relations of the several layers of fascia within the pelvic floor (Dickinson).
The vagina extends, as a transverse slit in the pelvic floor, upward and backward, approximately in the direction of a line drawn from the ostium vaginæ to the fifth sacral vertebra. It is approximately parallel with the conjugate of the brim, so that when the woman is erect the long axis of the vagina is inclined at an angle of 60° to the horizon. The vagina is not a vertical open tube: it is a slit in the pelvic floor, in health always closed by the accurate apposition of the anterior and posterior walls ([Fig. 21]). The anterior vaginal wall is about 2½ inches long in a vertical mesial line. The posterior vaginal wall is about 3½ inches long. The vaginal walls are triangular in shape, being broader above than below. The shape of the normal vagina at the pelvic outlet is shown by [Fig. 23]. The section here shows the vaginal slit of the shape of the letter H. The portions of the slit extending backward and somewhat outward are called the vaginal sulci or furrows. They are directions of diminished resistance in which tears are liable to occur.
Fig. 23.—Section illustrating the characteristic form of the vaginal cleft (Henle): Ua, urethra; Va, vagina; L, levator ani; R, rectum.]