Muscles of the Trunk

V. THE MUSCLES OF THE PELVIS

Obturator internus Levator ani
Piriformis Coccygeus

The muscles within the pelvis may be divided into two groups: (1) the Piriformis and the Obturator internus. which are described with the muscles of the lower limb: (2) the Levator ani and the Coccygeus, which, with the corresponding muscles of the opposite side, form the pelvic diaphragm, The classification of the two groups under a common heading is convenient in connexion with the fascia investing the muscles. These fasciae are closely related to one another and to the deep fascia, of the perineum, and in addition are connected with the fascial coverings of the pelvic viscera; it is customary therefore to describe them together under the term pelvic fascia.

Pelvic fascia.—The fascia of the pelvis may be resolved into: (A)-the fascial sheaths of the pelvic muscles; (B) the fascial sheaths of the pelvic viscera (see section on Splanchnology).

The fascia of the Obturator internus covers the pelvic surface of the muscle, and is attached round the margin of its origin. Above, it is connected to the posterior part of the arcuate line of the hip-bone, and is there continuous with the iliac fascia. In front of this, as it follows the line of origin of the Obturator internus, it gradually separates from the iliac fascia, and the continuity between. the two is retained only through the periosteurn. It arches below the obturator vessels and nerve, completing the obturator canal, and at the front of the pelvis is attached, to the back of the body of the pubis. The lower part of the obturator fascia forms the lateral wall of the ischiorectal fossa, and is attached inferiorly to the falciform process of the saero-tuberous ligament and to the pubic arch; at the pubic arch it is continuous with the fascia on the deep surface of the Deep transverses perinei and the Sphincter urethrae. Behind, it is indirectly continuous with the fascia of the Piriformis.

The internal pudendal vessels and their accompanying nerves are placed in the lateral wall of the ischiorectal fossa, and are enclosed in a special sheath of the fascia, named the pudendal canal.

The fascia of the Piriformis is very thin and is attached to the front of the sacrum around the margins of the anterior sacral foramina. At its sacral attachment it comes into intimate association with and ensheathes the nerves emerging from these foramina; hence the sacral nerves are frequently described as lying behind the fascia. The internal iliac (hypogastric) vessels and their branches, on the other hand, lie in the subperitoneal connective tissue in front of the fascia, and the branches of these vessels to the gluteal region emerge in special sheaths of this tissue, above and below the Piriformis muscle.

The fascia of the pelvic diaphragm (fig. 612) covers both surfaces of the Levatores ani. That on the inferior surface of the muscle is very thin, and known as the anal fascia; it forms the medial wall of the ischiorectal fossa, and above is continuous with the obturator fascia along the line of origin of the Levator ani; it is continuous below with the fascia on the Sphincter urethra and the Sphincter ani externus. On the lateral side the layer covering the upper surface of the Levator ani follows the line of origin of the muscle, and is therefore somewhat variable. In front it is attached to the back of the symphysis pubis about 2 cm. above its lower border, and can be traced laterally across the back of the superior ramus of the pubis for a distance of 1.25 cm., when it reaches the obturator fascia. It blends with this fascia along a line which pursues a some what irregular course to the spine of the ischium. The irregularity of this line is explained by the fact that whereas in lower mammals the Levator ani arises posteriorly from the pelvic brim, in man it has descended to a lower level and arises from the obturator fascia. In some cases tendinous fibers of origin extend up towards, and may reach, the pelvic brim; carrying the fascia with them. Internally the fascia covering the upper surface of the pelvic diaphragm blends with the fascial sheaths of the pelvic viscera. The fascia covering that part of the Obturator interntis which lies above the origin of the Levator ani is therefore a: composite structure and includes, (a) the obturator fascia, (b) the fascia of the Levator ani, and (c) the degenerated fibers of origin of the Levator ani.

At the level of a line extending from the lower part of the symphysis pubis to the spine of the ischium there is a thickened whitish band in this upper layer of the fascia of the pelvic diaphragm. It is termed the tendinous arch of the pelvic fascia, and marks the line of attachment of the lateral true ligament of the urinary bladder. Anteriorly the fascia forms two thickened bands, named the puboprostatic ligaments, one on each side of the median plane.

Figure 612
Male pelvis, prostate coronal section - Figure 612
The Levator ani (fig. 614) is a broad, thin muscle; it is attached to the inner surface of the side of the false (lesser) pelvis, and unites with its fellow of the opposite side to form the greater part of the floor of the pelvic cavity. It arises, in front, from the pelvic surface of the body of the pubis lateral to the symphysis; behind, from the inner surface of the spine of the ischiurn and between these two points, from the obturator fascia. Posteriorly this origin from the obturator fascia corresponds, more or less closely, with the tendinous arch of the pelvic fascia, but in front, the muscle arises from the fascia at a varying distance above the arch, in some cases reaching nearly as high as the canal for the obturator vessels and nerve. The fibers pass towards the median plane with varying degrees of obliquity. (a) The most anterior fibers sweep backwards and downwards across the side of the prostate to be inserted into the perineal body (central point of the perineum). They constitute the Levator prostate in the male, but in the female they cross the side of the vagina to reach their insertion, and so constitute an additional and important sphincter for that structure. (b) The succeeding fibers pass backwards and downwards across the side of the prostate and the upper end of the anal canal, and turn medially at the anorectal flexure to become continuous with the corresponding fibers of the opposite side, but a number of them are lost in the wall of the anal canal. This part of the muscle is termed the puborectalis. (c) The remaining fibers are inserted into the side of the last two segments of the coccyx and into a median fibrous raphe which stretches between the coccyx and the anorectal flexure.

Morphologically, the Levator ani may be divided into Iliococcygeus and Pubococcygeus. The Iliococcygeus arises from the ischial spine and from the posterior part of the tendinous arch of the pelvic fascia, and is attached to the coccyx and the median raphe; it is usually thin, and may fail entirely or be replaced largely by fibrous tissue. An accessory slip at its posterior part is sometimes named the Iliosacralis. The Pubococcygeus arises from the back of the pubis and from the anterior part of the obturator fascia, and is directed backwards almost horizontally along the side of the anal canal. Between the coccyx and the anal canal the Pubococcygei come together and form a thick, fibromuscular layer lying on the raphe formed by the Iliococcygei. In lower mammals both muscles are inserted only into the caudal vertebra. The iliococeygeus is responsible for side to side movements of the tail, and the Pubococcygeus draws it downwards and forwards between the hind limbs. The gradual disappearance of the tail sets free these muscles to meet the demands for a more complete pelvic floor made by the gradual adoption of the erect attitude.

Figure 613
Male pelvis median sagittal section - Figure 613
Relations.–The upper or pelvic surface of the Levator ani is separated by its covering fascia from the bladder, prostate, rectum, and peritoneum. Its lower or perineal surface forms the medial boundary of the ischiorectal fossa, and is covered by the anal fascia. Its posterior border is free and separated from the Coccygeus muscle by areolar tissue. Its anterior border is separated from the muscle of the opposite side by a triangular space, through which the urethra and, in the female, the vagina pass from the pelvis.

Nerve-supply.-The Levator ani is supplied by a branch from the fourth sacral nerve and by a branch which arises either from the perineal, or from the inferior hemmorrhoidal, division of the pudendal nerve.

Actions.–The Levatores ani constrict the lower end of the rectum and vagina and steady the perineal body (central point of the perineum). Together with the Coccygei they form the muscular diaphragm which supports the pelvic viscera and opposes itself to the downward thrust: produced by any increase in the intra-abdominal pressure.

The Coccygeus (fig. 614) is situated behind the Levator ani. It is a triangular sheet of muscular and tendinous fibers, arising by its apex from the pelvic surface of the spine of the ischium and from the sacrospinous ligament, and inserted by its base into the margin of the coccyx and into the side of the lowest piece of the sacrum. It assists the Levator ani and Piriformis in closing the posterior part of the outlet of the pelvis.

Figure 614
Male pelvis muscles median sagittal section - Figure 614
Nerve-supply.-The Coccygeus is supplied by a branch from the fourth and fifth sacral nerves.

Actions.–The Coccygei pull forward and support the coccyx, after it has been pressed backwards during defecation or parturition.

Applied Anatomy.- Injury to the muscles forming the pelvic floor occurs not infrequently during parturition. When the perineal body has been torn through, and has not been repaired satisfactorily, the contraction of the anterior fibers of the Levator ani increases instead of diminishing the normal gap in the pelvic floor, and prolapse of the uterus results; in severe cases the ovaries, bladder and rectum may also prolapse.

 


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