VI. MUSCLES OF THE PERINEUM
The perineum corresponds to the outlet of the pelvis. Its deep boundaries are–in front, the pubic arch and the inferior pubic ligament; behind; the tip of the coccyx; and on each side the inferior ramus of the pubis and the ramus of the ischium, the ischial tuberosity and the sacrotuberous ligament. The space within these boundaries is somewhat lozenge-shaped. On the surface of the body the perineum is limited by the scrotum in front, the buttocks behind, and the medial sides of the thighs laterally. A line drawn transversely in front of the isebial tuberosities divides the space into two portions. The posterior contains the termination of the anal canal, and is known as the anal region; the anterior contains the external urogenital organs, and is termed the urogenital region.
The muscles of the perineum may therefore be divided into two groups
- Those of the anal region.
- Those of the urogenital region: A, In the male; B, In the female.
1. THE MUSCLES OF THE ANAL REGION
| Corrugator cutis ani | Sphincter ani externus | Sphincter anal internus |
The superficial fascia is very thick, areolar in texture, and contains much fat in its meshes. On each side a pad of fatty tissue extends deeply between the Levator ani and Obturator internus into a space known as the ischiorectal fossa.
The deep fascia forms the lining of the ischiorectal fossa; it comprises the anal fascia, and that part of the obturator fascia which lies below the origin of Levator ani.
Ischiorectal fossa.–The fossa is somewhat wedge-shaped, with its base directed to the surface of the perineum, and its thin edge at the line of meeting of the obturator and anal fasciae. It is bounded medially by the Sphincter ani externus and the anal fascia; laterally, by the tuberosity of the ischium and the obturator fascia; anteriorly, by the perineal membrane (inferior fascia of the urogenital diaphragm); posteriorly, by the Gluteus maximus and the sacrotuberous ligament. Crossing the space transversely are the inferior rectal vessels and the inferior hemorrhoidal nerve; at the posterior part are the perineal and perforating cutaneous branches of the sacral plexus; while from the anterior part the scrotal (or labial) vessels and nerves emerge. The internal pudendal vessels and pudendal nerve lie on the lateral wall of the fossa in the pudendal canal. The fossa is filled with fatty tissue, across which numerous fibrous bands extend.
The Corrugator cutis ani forms a thin stratum of involuntary muscular fibers which radiate from the anal orifice. Medially the fibers fade off into the subcutaneous tissue; laterally they blend with the true skin. When the muscle contracts the skin around the anus is raised into ridges.
The Sphincter ani externus (figs. 615, 617) is a flat sheet of muscular fibers, elliptical in shape and intimately adherent to the skin surrounding the anus. It measures from 8 to 10 cm in length and is about 2.5 cm in width opposite the anus. It consists of two portions, superficial and deep. The superficial portion constitutes the chief part of the muscle, and arises by a narrow tendinous band from the tip of the coccyx; the muscle consists of two flattened sheets which pass one on each side of the anus and meet in front to be inserted into the perineal body (central tendinous point of the perineum), joining with the Transversus perinei superficialis, the Levator ani, and the Bulbospongiosus (Bulbocavernosus). The deep portion, forms a complete sphincter to the axial canal. Its fibers surround the canal, closely applied to the Puborectalis and the Sphincter ani internus, and in front blend with the other muscles at the perineal body. In a considerable proportion of cases the fibers decussate in front of the anus, and are continuous with the Transversi perinei superficiales. Posteriorly they are not attached to the coccyx, but are continuous with those of the opposite side behind the anal canal. The upper edge of the muscle is ill-defined, since it receives numerous fibers from the Puborectalis.
Actions.-The Sphincter ani externus is normally in a state of tonic contraction, aid having no antagonistic muscle it keeps the anal canal and orifice closed. It can be put into a condition of greater contraction under the influence of the will, so as to occlude the anal aperture more firmly. Taking its fixed point at the coccyx, it helps to fix the perineal body (central point of the perineum).
The Sphincter ani internus is a muscular ring which surrounds about 2.5 cm. of the anal canal; its inferior border is in contact with, but distinct from, the deep surface of the Sphincter ani externus and, posteriorly and on each side, it is covered by the Puborectalis. It is about 5 mm. thick, and is formed by an aggregation of the involuntary circular fibers of the intestine. Its lower border is about 6 mm. from the orifice of the anus.
Actions — Its action is entirely involuntary. It helps the Sphincter ani externus and the Puborectalis to occlude the anal canal.
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