ATLAS OF HUMAN ANATOMY - G.L. Bilich - 2014

Muscular System

MUSCLES OF THE PERINEUM

The Perineum is a complex of Tissues closing the outlet of the lesser pelvis and forming the inferior wall of the Abdominal cavity. Formed by Skin, subcutaneous tissue, Muscles, and fasciae, the perineum is located between the external genitalia anteriorly and the anus posteriorly. This region corresponds to the central tendon of the perineum (centrum tendineum perinei). In clinical practice, the perineum in women is considered to be the area situated between the posterior margin of the pudendal cleft anteriorly and the anterior margin of the anus posteriorly, while in men, it lies between the posterior margin of the Scrotum and the anterior margin of the anus. The perineal raphe (raphe perinealis) extends anteroposteriorly through the skin of the perineum, continuing anteriorly into the scrotal raphe in men. The perineum is bounded anteriorly by the inferior margin of the Pubic Symphysis, posteriorly by the apex of the coccyx, and laterally by the inferior rami of the pubic and ischial bones and the ischial tuberosities. When viewed from below, the perineum has a diamond shape. A transverse line connecting the ischial tuberosities divides the perineum into two triangular regions: the anterosuperior part forms the urogenital region, or urogenital Diaphragm (regio urogenitalis), and the posteroinferior part forms the anal region, or pelvic diaphragm (regio analis).

The muscular framework of the perineum is formed by two layers of muscles covered by fasciae, designated as the superficial and deep Muscle layers (Table 45). The Muscles of the urogenital diaphragm and pelvic diaphragm are covered from the pelvic cavity side by the parietal pelvic fascia and externally (inferiorly) by the superficial perineal fascia. In addition to these two fasciae, the urogenital diaphragm possesses two proper fasciae—the superior and inferior Fasciae of the urogenital diaphragm—between which its deep muscle layer is located.

The urogenital diaphragm (diaphragma urogenitalis), occupying the anterior part of the perineum, is triangular in shape, with its apex directed toward the pubic symphysis. Its sides are formed by the inferior rami of the pubic and ischial bones, and its base is formed by a line connecting the ischial tuberosities. The urogenital diaphragm is traversed by the Urethra in men, and by the urethra and Vagina in women.

The muscles of the urogenital diaphragm are subdivided into superficial and deep groups. The superficial group comprises three muscles: the superficial transverse perineal muscle, the ischiocavernosus, and the bulbospongiosus (Figs. 297, 298, 299, 300).

The superficial transverse perineal muscle (m. transversus perinei superficialis), also known as Lesgaft's muscle (syn.: Theile's muscle), is a paired, thin strip of muscle lying beneath the superficial perineal fascia (Friedrich Wilhelm Theile, 1801–1879, a German anatomist; F. Theile described it almost 20 years later than P.F. Lesgaft). The muscle originates on the superior ramus of the ischium near the ischial tuberosity, runs transversely toward the corresponding muscle of the opposite side, and inserts into the central tendon of the perineum, which is formed by the thin, flat tendons of these muscles. A portion of the fibers of this muscle blends into the external anal sphincter and the contralateral bulbospongiosus.

Function: The superficial transverse perineal muscles strengthen the central tendon of the perineum and assist in the erection of the Penis (or Clitoris).

Blood supply: Perineal artery.

Innervation: Pudendal nerve (S—S_IV).

The ischiocavernosus muscle (m. ischiocavernosus), also known as Jarjavay's muscle or Macalister's muscle, is a paired, thin muscle strip that originates on the inferior ramus of the ischium, lies laterally adjacent to the ROOT of the penis (in men) or the clitoris (in women), runs anteriorly and medially, and inserts via a thin tendinous expansion into the tunica albuginea of the corpus cavernosum of the penis (or clitoris) (Jean François Jarjavay, 1815–1868, a French anatomist and surgeon; Alexander Macalister, 1844–1919, an Irish anatomist).

Function: Upon contraction, the ischiocavernosus muscle promotes the erection of the penis (or clitoris).

Blood supply: Perineal artery.

Innervation: Pudendal nerve (S_I—S_IV).

The bulbospongiosus muscle (m. bulbospongiosus) is a thin, flat, paired muscle consisting of two parts. In men, they originate on the Inferior surface of the penile bulb, curve around the bulb and corpus spongiosum of the penis on the right and left sides, and attach to its tunica albuginea and to the superficial fascia on the dorsum of the penis. The posterior Deep Layer of the bulbospongiosus is known as Kobelt's muscle (Georg Ludwig Kobelt, 1804–1857, an Austrian physician and anatomist).

Class="center">Table 45. Muscles of the Perineum

In women, the bulbospongiosus originates at the central tendon of the perineum and the external anal sphincter, runs anteriorly, lies inferiorly adjacent to the vestibular glands, and attaches to the dorsal surface of the clitoris, blending into its tunica albuginea.

Function: upon contraction, the muscle compresses the bulb, corpora cavernosa, and dorsal vein of the penis, as well as the bulbourethral glands, and participates in erection. In women, the paired bulbospongiosus surrounds the external orifice of the vagina, acting as its functional sphincter. When contracting, the bulbospongiosus narrows the vaginal opening, compresses the greater vestibular gland (thereby facilitating the release of its secretion), and compresses the vestibular bulb along with its draining veins. Compression of the veins causes blood to engorge the venous plexuses of the vestibular bulb, leading to its Swelling.

Fig. 297. Muscles of the female perineum after removal of fasciae, inferior view:

1 — Anococcygeal body; Anococcygeal ligament; 2 — Levator ani; 3 — Sacrospinous ligament; 4 — Ischial spine; 5 — Ischial tuberosity; 6 — Acetabulum; 7 — Anterior superior iliac spine; 8 — Anterior inferior iliac spine; 9 — Ramus; 10 — Deep transverse perineal muscle; 11— Superior pubic ramus; 12 — Glans of clitoris; 13 — External urethral orifice; External urinary meatus; 14 — Vestibule; 15 — Bulbospongiosus; 16 — Ischiocavernosus; 17— Superficial transverse perineal muscle; 18— Obturator internus; 19— Piriformis; 20 — Perineal body; 21 — Sacrotuberous ligament; 22 — External anal sphincter; 23 — Coccyx [coccygeal vertebrae CoI—CoIV]

Fig. 298. Muscles of the female perineum after removal of the sphincters (except the urethral sphincter),

bottom view:

1 — Coccyx [coccygeal vertebrae Col-ColV]; 2 — Coccygeus; 3 — Levator ani; 4 — Sacrospinous ligament; 5 — Ischial tuber; 6 — Ischiocavernosus; 7— Pubic symphysis; 8— External urethral sphincter; 9— Perineal body; 10— Deep transverse perineal muscle; 11 — Superficial transverse perineal muscle; 12 — Obturator internus; 13 — Piriformis; 14 — Sacrotuberous ligament; 15 — Anococcygeal body; Anococcygeal ligament

Fig. 299. MUSCLES AND FASCIAE of the male perineum, bottom view:

1 — Anus; 2 — Levator ani; 3 — Gluteus maximus; 4 — Obturator fascia; 5 — Sacrotuberous ligament; 6 — Fascia; 7 — Ischial tuber; 8 — Superficial transverse pcrineal muscle; 9 —- Deep transverse perineal muscle; 10 — Bulbo-urethral gland; 11 — Perineal raphe; 12— Dartos fascia; Supcrficial fascia of scrotum; Dartos muscle; 13 — Corpus spongiosum penis; 14 — Bulbospongiosus; 15 — Ischiocavernosus; 16 — Gracilis; 17— Perineal fascia; Superficial investing fascia of perineum; Deep perineal fascia; 18 — Ischio-anal fossa; 19 — External anal sphincter; 20 — Anococcygeal body; Anococcygeal ligament; 21 — Coccyx [coccygeal vertebrae CoI—CoIV]

Blood supply: perineal artery.

Innervation: pudendal nerve (S_I—S_IV).

The Deep Muscles of the urogenital diaphragm include the deep transverse perineal muscle and the external urethral sphincter. The deep transverse perineal muscle (m. transversus perinei profundus) — Hyrtl's urogenital diaphragm — is a paired, thin lamina located in the anterosuperior part of the pelvic floor between the inferior rami of the pubic and ischial bones. The muscle originates on the rami of the ischium and pubis, running medially and transversely toward the corresponding muscle of the opposite side. Along the midline of the perineum, both muscles join via their flat tendons, contributing to The formation of the anterior part of the perineal body. Both muscles reinforce the urogenital diaphragm, constrict the urethra, and, in females, also constrict the vagina. The arcuate fibers of the deep transverse perineal muscle, which surround the membranous part of the urethra and compress it, form Wilson's muscle (syn.: Guthrie's muscle, urethral sphincter, m. sphincter urethrae) (Wilson, James, 1765–1821 — English surgeon and anatomist; Guthrie, George James, 1785–1856 — English surgeon and anatomist).

Function: compresses the urethra, and in females also the vagina; reinforces the urogenital diaphragm.

Blood supply: perineal artery.

Innervation: pudendal nerve (S_I—S_IV).

The external urethral sphincter (m. sphincter urethrae externus) is an unpaired, flattened circular muscle. Circular bundles predominate in this muscle, surrounding the urethra, and in males, its membranous part. In females, this muscle also surrounds the vagina. Radial fibers originate on the inferior rami of the pubic bones and blend into the prostate capsule in males and the vaginal walls in females. The muscle acts as a voluntary constrictor of the urethra, and in females, of the vagina as well. When contracted independently, the radial fibers facilitate the dilation of the previously constricted membranous part of the urethra.

Fig. 300. Muscles and fasciae of the female perineum, bottom view:

1 — Superficial transverse perineal muscle; Deep transverse perineal niusele; 2 — Gluteus maximus; 3 — Ischio-anal fossa; 4 - Obturator fascia; 5 — Sacrotuberous ligament; 6 — Ischial tuber; 7 — Perineal raphe; 8 — Bulbospongiosus; 9 — Ischiocavernosus; 10 — Gracilis; 11 — Labium minus; 12 — External urethral orificc; External urinary meatus; 13 — Prepucc of clitoris; 14 — Glans of clitoris; 15 — Vaginal orifice; 16 —Vaginal rugae; 17— Greater vestibulargland; 18— Perineal fascia; Superficial investing fascia of perineum; Deep perineal fascia; 19 — Superficial transverse perineal muscle; 20 — External anal sphincter; 21 — Levator ani; 22 — Anus; 23 — Anococcygeal body; Anococcygeal ligament; 24— Coccyx [coccygeal vertebrae CoI—ColV]

Function: compresses the urethra, and in females also the vagina; reinforces the urogenital diaphragm.

Blood supply: perineal artery.

Innervation: pudendal nerve (S_I—S_IV).

The urogenital diaphragm in females is broader than in males, transmitting both the urethra and the vagina. However, its muscles are less pronounced than in males. The paired superficial transverse perineal muscle is frequently absent in females altogether, and the deep transverse perineal muscle is poorly developed. Conversely, the superior and inferior fasciae of the urogenital diaphragm are stronger in females than in males. The muscle bundles of the female urethral sphincter also encompass the vagina, blending into its walls. The perineal body, situated between the vagina and the anus, consists of interweaving tendinous and elastic fibers.

The pelvic diaphragm (diaphragma pelvis) occupies the posterior part of the perineum and has a triangular shape with its apex directed toward the coccyx. The angles of this triangle point toward the ischial tuberosities. The terminal part of the rectum passes through the pelvic diaphragm in both males and females. The pelvic diaphragm comprises two muscle layers: superficial and deep (see Table 45; see Figs. 297, 298, 299).

The superficial muscle layer of the pelvic diaphragm contains the unpaired external anal sphincter (m. sphincter ani externus), a thickened circular muscle located beneath the superficial perineal fascia and surrounding the terminal part of the rectum. The Muscle consists of several parts. The bundles of the deep part surround the upper portion of the rectum and lie adjacent to the levator ani muscle. The bundles originating at the tip of the coccyx — the superficial part — encircle the anus and blend into the perineal body. The most superficial bundles (the subcutaneous part) blend into the subcutaneous adipose tissue and superficial fascia.

Function: upon contraction, all bundles of the external anal sphincter compress (close) the anal opening. The muscle relaxes voluntarily during defecation.

Blood supply: inferior rectal and perineal arteries.

Innervation: anococcygeal nerve (S_I—S_IV].

The deep muscles of the pelvic diaphragm include the levator ani and coccygeus muscles.

The levator ani muscle (m. levator ani) is a paired, thin, triangular sheet that combines with its counterpart to form a funnel-shaped Structure tapering downwards. It originates on the lateral wall of the lesser pelvis from the inner surface of the pubic bone branch and the tendinous arch of the levator ani muscle. Several distinct PARTS OF THE muscle are recognized:

— the pubococcygeus muscle (m. pubococcygeus), which in turn is subdivided into the puboperinealis muscle (m. puboperinealis); the puboprostaticus or levator prostatae muscle (in males) (m. puboprostaticus; m. levator prostatae); and the pubovaginalis muscle (m. pubovaginalis);

— the puboanalis muscle (m. puboanalis);

— the puborectalis muscle (m. puborectalis);

— the iliococcygeus muscle (m. iliococcygeus).

The anterior fibers originate from the inner surface of the inferior pubic ramus, and the lateral fibers arise from the arched thickening of the levator ani fascia known as the tendinous arch of the levator ani muscle (arcus tendineus musculi levatoris ani). The internal, or longitudinal, layer of the levator ani muscle (stratum internum musculi levator ani) is referred to as Lesgaft's muscle. These are muscle fibers originating from the superior pubic ramus, ischial spine, obturator fascia, and the tendinous arch of the levator ani, running downward and medially toward the rectum, where they transition into elastic fibers passing between individual bundles of the external anal sphincter to terminate in the subcutaneous tissue surrounding the anus. The posterior fibers of the levator ani muscle, extending from the ischial spine of the ischium to the coccyx, are known as Savage's muscle (syn. Browning's muscle) (Browning, William, 1855–1941, American physician and anatomist; Savage, Henry, 1810–1900, English physician and anatomist). A portion of the levator ani fibers—Albinus's muscle (syn.: compressor prostatae, m. compressor prostatae)—passes alongside the prostate, firmly adhering to it; in females, this muscle descends along the vagina, coming into contact with its walls (Albinus, Bernhard Siegfried, 1697–1770, German anatomist and physician).

The muscle bundles of the right and left levator ani muscles course inferiorly and posteriorly, uniting with each other to loop around the rectum. Some fibers of the levator ani blend into the wall of the Urinary Bladder, and in males into the prostate as well, while in females they merge with the vaginal wall.

Extending from the inferior pubic rami to the rectum within the fasciae of the pelvic diaphragm, between the middle portions of the levator ani muscle, lies a cleft known as Tandler's cleft (Tandler, Julius, 1869–1936, Austrian anatomist).

Function: Upon contraction, the levator ani muscles strengthen and elevate the pelvic floor, pulling the terminal part of the rectum forward and upward while compressing it. In females, the muscle also narrows the vaginal introitus and approximates the posterior vaginal wall to the anterior wall.

Blood supply: Internal pudendal and perineal arteries.

Innervation: Anococcygeal nerves (S_I).

The coccygeus muscle, or ischiococcygeus muscle (m. coccygeus, m. ischiococcygeus), is a quadrilateral, flat, paired, and often inconstant sheet originating from the ischial spine and sacrospinous ligament; it courses medially and posteriorly, lying medially adjacent to the sacrospinous ligament, into which bundles of this muscle blend. The muscle inserts laterally onto the margin of the coccyx and the apex of the sacrum, reinforcing the posterior part of the pelvic diaphragm.

Function: Reinforces the posterior part of the pelvic diaphragm.

Blood supply: Internal pudendal and perineal arteries.

Innervation: Anococcygeal nerves (S_I–S_IV].

Arising rarely from the pelvic surface of the sacrum is the rudimentary ventral sacrococcygeus muscle (m. sacrococcygeus ventralis)—Meckel's muscle—which attaches to the ventral sacrococcygeal ligament as far as the II–IV coccygeal vertebrae (Meckel, Johann Friedrich (junior) [Meckel Johann Friederich (junior)]).