- Email: [email protected]
Anatomy of the female pelvis and perineum in relation to labor
ANATOMY
OF
THE
FEMALE RELATION
(From
T
PELVIS
AND
PERINEUM
IN
TO LABOR
BY CALVIN R. HANNAH, M.D., F.A.C.S., DALLAS, TEXAS the Obstetrid Dcparrtmemt of Baylor Univer.tity, College of iKedi&e)
HE term pelvis, as we shall use it in this paper, will be applied to
the true pelvis or the pelvis minor, which is that portion of the abdominal cavity lying below the pelvic brim or linea terminalis and above the pelvic diaphragm. For purposes of anatomic completeness it would be necessary to discuss in connection with this space a floor and anterior, posterior and lateral walls; but for the purposes of this paper we deem it necessary to discuss only the floor. The pelvic floor or pelvic diaphragm is composed of both muscle and fibrous elements. The fibrous element is the so-called endopelvic fascia. The endopelvic fascia is a continuation of the general fascial lining of the abdominal cavity bein g continuous over the iliopectineal line with the fascia descending over the psoas major muscle. This fascia passes down the lateral mall of the peIvis, over the obturator internus muscle to a line extending from the back of the body pubis backward to the spine of the ischium, being attached to both of these points. Along this line, which is known as the arcus tendinous, the fascia divides into two layers, and from the angle thus formed the greater portion of the levator ani muscles take their origin. The inferior layer of this fascia again divides into two layers, one of which clothes or covers the inferior surfaces of the levator ani muscles, thus forming the medial wall of the ischio-rectal fossa. The superior or medial layer of this pelvic fascia covers the inner or medial or superior surface of the levator ani and coccygei muscles becoming continuous across the midline with a similar layer of the opposite side thus forming a hammock-like fascial structure supported on each side by the corresponding arcus tendinous. This layer of fascia is known as the visceral layer of the pelvic fascia because it is the layer that is so intimately associated w&h the pelvic viscera. We might think of the lower portion of the pelvic viscera, namely, the bladder, the urethra, the vagina, and the rectum as piercing this hammock-like structure. Thus, we see this layer of visceral fascia serving as a very important support for these organs. It might be advisable to mention some special thickenings of this fascia. Thus, we have anteriorly the pubovesical thickening which is attached both to the body of the pubis and lower portion of the bladder. Then another thickening known as uterovesical is found binding 228
HANNAH:
ANATOMY
OF
THE
FE,MALE
PELVIS
229
the lower portion of the uterus and upper portion of the vagina very firmly to the lower part of the urinary bladder. Still farther posteriorly is found another portion of the visceral fascia known as the rectouterine thickening which firmly binds the lower portion of the uterus and upper portion of the vagina to the mall of the rectum. That portion of this fascia through which the cervix passes is very firmly united with the fibrous portion of the cervix. The parietal layer of the pelvic fascia passes around the anterior border of the levator ani muscles, blending in the midline, forming the superior fascia of the urogenital diaphragm. The muscular layer consists of the levator ani and coccygei muscles, the muscular fibers being enclosed by the two layers of the endopelvic fascia, as just described. The coccygei muscles take their origin from the pelvic surfaces of the ischial spines and sacrospinous ligaments and are inserted into the lateral borders of the lower two pieces of the sacrum and upper two pieces of the coccyx. The levator The anteroinferior fibers take ani muscles have a threefold origin. origin from the posterior aspect of the body and superior rami of the pubes. The posterior superior fibers take their origin from the pelvic surfaces of the ischial spines. The intermediate fibers take their origin from the angle formed by the two layers of the endopelvie fascia, as previously described, or from the arcus tendinous or white line. The insertion is as follows: the anterior fibers pass downward and backward, a few of them are inserted into the central point of the perineum; others are inserted into the wall of the anal canal between the external and internal sphincters. The intermediate fibers sweep around into the angle between the posterior wall of the rectum and upper end of the anal canal where they unite with their fellows of the opposite side and form a strong muscular collar about the gut. The posterior fibers pass backward and medially and are inserted into the median anococcygeal raphe and into the lower part of the coccyx. The perineum which is the space lyin g below the pelvic diaphragm may be subdivided into two parts, the anal triangle and the urogenital triangle, the dividing line extending through the perineal body between the ischial tuberosities just anterior to the anus. The anal triangle has passing through its center t.he anal canal, having a space on either side known as the ischiorectal fossa which contains an extensive pad of fat, pyramidal in shape, through which extend vessels and nerves for the supply of the anal canal. The urogenital triangle which is more important than the anal triangle to the obstetrician will be detailed more. This space contains the so-called superficial and deep pouches. The superficial pouch is the space enclosed between Colles’ fascia superficially and the inferior fascia of the urogenital diaphragm deeply. Within this space we have the bulbocavernosus muscle, the ischiocavernosus muscles and the
230
THE
AMERICAN
JOURNAL
OF
OBSTETRICS
AND
GYNEXOLOGP
superficial transverse perineal muscles and the vessels and nerves that supply these structures and the labia majora. In addition to these structures we have in this space the clitoris, the bulb of vestibule, Bartholin’s glands and the terminal portion of the urethra and vagina. The deep pouch lies between the superior and the inferior fascia of the urogenital diaphragm. The superior fascia, which is a part of the endopelvie fascia, has been previously mentioned. The inferior fascia is the fascia bridging between the inferior rami of the pubes and lies in the morphologic plane as the obturator membrane. Within this pouch we have the sphincter of the membranous part of the urethra, the deep transverse perineal muscles, the deep branches of the perineal nerves, dorsal nerves of the clitoris and the trunk of the internal pudendal arteries and a portion of the urethra and the vagina. The lack of knowledge of the structure of the female pelvis is one of the fundamental causes of not recognizing complications during labor, the inability to correct this condition and to adequately repair injured tissues. To visualize the progress of labor one should know the muscular structure, the lymph and blood vessels of the uterus, the morphologic difference between the upper and lower uterine segments, the ligaments that attach the uterus to the walls of the pelvis, the parietal and visceral fascia which may blend or join with the muscular fibers of the cervix and uterus. IIow can one visualize the mechanism of labor without a knowledge of the levator ani muscles and fascia and their origin and insertion and their relation to the pelvic organs? Without this knowledge how can one hope to prevent injuries to the muscles and fascia of the superficial and deep structures of the perineum, and, if injured, repair them? The physiology and mechanism of the first stage of labor is better understood when it is known that the oblique muscular structures of the uterus are arranged in the form of a figureof-eight around the blood vessels and are capable of powerful constriction, and, when contracted, produce intrauterine pressure. The visceral fascia of the levator ani muscles is a part of the pelvic floor. This fascia in its morphologic development moors the uterus by the cervix to the pelvic wall. In this structural handiwork the pubovesical, the uterovesical and the uterosacral ligaments are mere thickenings of the visceral fascia and together attach or anchor the uterus to the bony structure. The visceral fascia joins or blends with the fascia of the fibers of the cervix. The visceral fascia attaches the bladder to the anterior surface of the cervix and lower part of the uterus and is considered with the uterus and rectum as pelvic organs. The blending of the visceral fascia with the cervical fibers of the uterus and their fascia forms an important structure in the physiologic and mechanical part of labor. The fibers of the oblique or middle muscular layer of the uterus pass obliquely from the peritoneal surface of
HANNAH
:
ANATOMY
OF
THE
FEM.ALE
PELVIS
231
the uterus to the endometrium. E. Hastings Tweedy says, “As these fibers approach the internal OS, they turn almost at right angles, passing out and attaching into the pelvic wall as muscular tendons.” He further says, “The circular fibers of the cervix likewise terminate in muscular tendons and they, too, largely attach themselves to the bony wall of the pelvis.” Tweedy emphasizes the fact that “Involuntary muscular fibers may have muscular tendons as well as any other muscle, and if this were not so during the first stage of labor, the visceral fascia of the pelvic floor would be completely destroyed.” Dr. W. W. Looney, Professor of Anatomy, Baylor University, College of Medicine, says, “The visceral pelvic fascia serves as an aponeurosis for the oblique and cervical fibers instead of these muscle fibers having independent muscular tendons, which, Tweedy states, ‘are inserted into the bony pelvic surface.’ Probably as the visceral fascia passes to the uterus and cervix it blends with the fascial covering of the muscle fibers of these structures. This explanation appears more plausible when the visceral fascia is considered a part of an exAnatomists say that tension of the fascia covering the psoas muscle.” the dissecting of the individual fibers of the nonpregnant uterus is a difficult task, and that probably as the oblique fibers approach the eervix, these may become more circular and form what is known during labor as the contraction, retraction or Bandl’s ring.
.
The uterus requires more space for the growing embryo about the seventh month of pregnancy. This space is supplied by the pushing down of the contents of the pregnant uterus against the internal OS which causes these oblique fibers to dilate and retract, producing what is later known as the lower uterine segment. This segment is not necessarily taken from the cervix, for in the nonpregnant uterus the oblique and circular fibers are closely associated with each other as if they were compressed, and it is difficult to dissect them. A study of the anatomy of the uterus suggests that the lower uterine segment probably is developed from the fibers of the middle layer of the uterus which are present just superior to the internal OS and bounded superiorly by Bandl’s ring. This anatomic space of the uterus is the potential lower uterine or dilating segment. The lower uterine segment is formed before the onset of labor. During the first stage of labor as the uterus contracts and forces the fetus against the internal OS, the fibers of the lower uterine segment pull out and up on the circular fibers of the cervix, causing them to dilate. As the cervix dilates.” This has been said, “As the uterus contracts, causes the shortening of the cervix which previously has not been very perceptible, yet the cervix may have been shortened by the irregular and painless contraction of the uterus before the onset of labor. As the cervix dilates, beginning at the internal OS and extending to the external OS,this progress is spoken of as effacement, and the gradual
232
THE
AMERICAN
JOURSAL
OF
OBSTETRIC’S
Ah-D
GYNECOLOGY
progress is considered favorable during labor. The cervix may be completely obliterated or effaced before the external OS is perceptibly opened. Especially in a primipara is this progress more favorable in the prevention of lacerations of the cervix than a thick or long cervix opening up in its entirety. During the first stage of labor, the fetus has not been pushed through the cervix, but rather the cervix has been pulled back over the presenting part, taking with it the bladder. As effacement takes place, the cervix is retracted over the presenting part, carying with it the bladder, which previously has been a pelvic organ and now becomes and later will be an abdominal organ. This is an important point in the diagnosis of the close of the first stage of labor and the entering into of, and during the secoud stage of labor. This is a point to be considered if catheterization at this time is necessary. A full bladder during any period in labor may prevent full contractility of the uterine fibers. Rearing down while the bladder is a pelvic organ should be prevented to avoid tears of the uterovesical fascia and injury to the trigone and sphincter uret,hrae and urethra, which may cause a cystocele. The pelvic floor is composed of the levator ani muscles and the visceral fascia which extends across the lower part of the pelvis, hammock-like in shape, and supports the nonpregnant uterus, and, also, during pregnancy, the head during labor. The course of the head during labor from the pelvic floor is downward and upward. A definite rule in the mechanism of labor is that the lowest point of the head is rotated to the front under the pubes by the levator ani muscles and the visceral fascia. The pelvic floor pushes the lower part of the head to the. front, and the suboccipitobregmatic diamet,er which was in the oblique diameter now lies in the anterior posterior diameter of the outlet. It is the levator ani muscles, their shape and contractility which cause rotation during labor. The levator ani muscles and the uterine muscles oppose each other as they contract, which is clinically demonstrated as the head passes over the perineum and underneath the symphysis. Extension takes place while the uterine muscles contract; and, when ended, the head is flexed by the levator ani muscles pushing the head back. Probably the levator ani muscles lose their power to flex the head after the biparietal diameter has passed through. The perineum is t.he structure that receives the force of labor after the head has passed through the pelvic floor. Lacerations can occur anywhere in the birth canal and may be snbmueus in character, located in the sulci or in and around the perineum, even including the sphincter ani. If the levator ani muscles and their fascia are not congenitally weak, or have not been overstretched, or have not been torn, these structures will support the uterus and prevent its prolapse. The uterus is supported by the pelvic floor and is not suspended from above.
HANNAH
:
ANATOMY
OF
THE
FEMALE
PELVIS
233
9 tear in t.he sulci probably will involve the anterior fibers and a Few intermediate fibers of the levator ani muscles and their fascia. A destruction of the anterior and intermediate fibers and their fascia which are inserted into the central point of the perineum and anal canal and posterior to the rectum will deprive the rectum of this support. With the loss of this support and by the contraction of the sphincter ani the lower part of the rectum is pulled back forming a more acute angle which enlarges the ampulla, and this, in turn, will cause, during defecation, the feces to go against the posterior wall of the vagina rather than follow the anal canal and form or cause a rectocele. If the tear is in the anterior sulcus, the anterior fibers of the levator ani muscles with their fascia will be involved and destroy the support for the bladder and anterior vaginal wall and cause the base of the bladder together with the anterior wall of the vagina to sag and form a cystocele. The tear may begin in the vagina above the perineum, extending outward separating the muscles and fascial structures down to and including the sphincter ani. A median laceration through the perineal body does not involve the fibers of the levator ani muscles and their fascia except a few of the anterior fibers attached in the central point, and this would injure the perineal diaphragm but little. This tear would only partly destroy the supporting action of the superficial transverse perineal muscles, the bulbocavernosus and the urogenital diaphragm. Rupture of perineum alone will not cause prolapse. Perineotomy or central incision of the perineum will prevent ,or control extensive lacerations. In a perineotomy only a few tendinous insertions of the levator ani muscles are divided, the muscular fibers themselves escaping because they are superior to the central point of the perineum. Lateral episiotomy may be preferred by some; but the danger of severing, if extensive, the fibers of the levator ani muscles and their nerves might later cause atrophy of the muscles. The stretching or “ironing out” of the perineum, the levator ani muscles and their fascia to prevent a tear is illogical and not a surgical procedure, and t,his is ineompatible with the new era in obstetrics, that of prophylaxis. We teach a technic that will prevent infection. We emphasize the value of rectal examinations as preferable to the vaginal in order to prevent infections and to ascertain t,he progress of labor. I sometimes fear we are overzealous in making rectal examinations and consider these examinations as totally void of danger, yet there is a potential danger. Rectal examinations are safer than the vaginal, but it is not logical to advocate rectal examinations instead of the vaginal and then practice “ironing out” or stretching the perineum to avoid lacerations. If vaginal examinations are a source or an avenue for infections, then surely manual dilatation of the vaginal tissues is poor surgical procedure and probably more hazardous than several
234
THE
AMERICAN
JOURNAL
OF
OBSTETRICS
AND
GYNECOLOGY
vaginal examinations, and not only is this method a source of infection, but may rupture the fibers and fascia of the levator ani muscles and cause prolapse, cystocele and rectocele. Since the uterus is supported by the pelvic floor rather than suspended from above, this overstretching or breaking of the muscular fibers may cause weakening of the support of the uterus and result in a prolapse, while perineotomy causes less injury to the tissues and is not so great a source of infection. CONCLUSIONS
1. The lack of knowledge of the structures of the pelvic tissues is one of the fundamental causes of not recognizing complications during labor, the inability to correct these conditions and to adequately repair injured tissues. 2. The visceral fascia serves as an aponeurosis for the oblique and cervical fibers of the uterus and cervix. As the visceral fascia passes to the uterus and cervix, it probably blends with the fascial covering of the muscle fibers of these structures. 3. During the first stage of labor the bladder is a pelvic organ; and after effacement, complete dilatation and retraction the bladder becomes an abdominal organ. Bearing down while the bladder is a pelvic organ may cause tears of the uterovesical fascia and injury to the trigone, sphincter urethrae and urethra of the bladder, and may cause cystocele. An injury of the uterovesical fascia may cause prolapse of the base of the bladder; and injury to the sphincter urethrae or Bell’s muscles may cause urine to collect anteriorly to the base of the trigone. 4. A tear in the sulci will probably involve the anterior intermediate fibers of the levator ani muscles with their deprive the rectum of this support and cause rectocele. 1 am indebted to J. C. Haley, M.D., of the Department of University, College of Medicine, for assistance in the preparation discussion in this paper.
and a few fascia and
Anatomy of the
of Baylor anatomical
REFERENCES (Ih‘?mhgleam:
Manual
Wrench, and Solom0n.3: University MEDICAL
Press,
1925.
ARTS BUILDING.
of Practical Anatomy, ed. 7, 2: pp. 147-197. Practical Obstetrics, ed. 5, pp. 7-9 and 591-595,
Tweedy, Oxford
OF
THE
FEMALE RELATION
(From
T
PELVIS
AND
PERINEUM
IN
TO LABOR
BY CALVIN R. HANNAH, M.D., F.A.C.S., DALLAS, TEXAS the Obstetrid Dcparrtmemt of Baylor Univer.tity, College of iKedi&e)
HE term pelvis, as we shall use it in this paper, will be applied to
the true pelvis or the pelvis minor, which is that portion of the abdominal cavity lying below the pelvic brim or linea terminalis and above the pelvic diaphragm. For purposes of anatomic completeness it would be necessary to discuss in connection with this space a floor and anterior, posterior and lateral walls; but for the purposes of this paper we deem it necessary to discuss only the floor. The pelvic floor or pelvic diaphragm is composed of both muscle and fibrous elements. The fibrous element is the so-called endopelvic fascia. The endopelvic fascia is a continuation of the general fascial lining of the abdominal cavity bein g continuous over the iliopectineal line with the fascia descending over the psoas major muscle. This fascia passes down the lateral mall of the peIvis, over the obturator internus muscle to a line extending from the back of the body pubis backward to the spine of the ischium, being attached to both of these points. Along this line, which is known as the arcus tendinous, the fascia divides into two layers, and from the angle thus formed the greater portion of the levator ani muscles take their origin. The inferior layer of this fascia again divides into two layers, one of which clothes or covers the inferior surfaces of the levator ani muscles, thus forming the medial wall of the ischio-rectal fossa. The superior or medial layer of this pelvic fascia covers the inner or medial or superior surface of the levator ani and coccygei muscles becoming continuous across the midline with a similar layer of the opposite side thus forming a hammock-like fascial structure supported on each side by the corresponding arcus tendinous. This layer of fascia is known as the visceral layer of the pelvic fascia because it is the layer that is so intimately associated w&h the pelvic viscera. We might think of the lower portion of the pelvic viscera, namely, the bladder, the urethra, the vagina, and the rectum as piercing this hammock-like structure. Thus, we see this layer of visceral fascia serving as a very important support for these organs. It might be advisable to mention some special thickenings of this fascia. Thus, we have anteriorly the pubovesical thickening which is attached both to the body of the pubis and lower portion of the bladder. Then another thickening known as uterovesical is found binding 228
HANNAH:
ANATOMY
OF
THE
FE,MALE
PELVIS
229
the lower portion of the uterus and upper portion of the vagina very firmly to the lower part of the urinary bladder. Still farther posteriorly is found another portion of the visceral fascia known as the rectouterine thickening which firmly binds the lower portion of the uterus and upper portion of the vagina to the mall of the rectum. That portion of this fascia through which the cervix passes is very firmly united with the fibrous portion of the cervix. The parietal layer of the pelvic fascia passes around the anterior border of the levator ani muscles, blending in the midline, forming the superior fascia of the urogenital diaphragm. The muscular layer consists of the levator ani and coccygei muscles, the muscular fibers being enclosed by the two layers of the endopelvic fascia, as just described. The coccygei muscles take their origin from the pelvic surfaces of the ischial spines and sacrospinous ligaments and are inserted into the lateral borders of the lower two pieces of the sacrum and upper two pieces of the coccyx. The levator The anteroinferior fibers take ani muscles have a threefold origin. origin from the posterior aspect of the body and superior rami of the pubes. The posterior superior fibers take their origin from the pelvic surfaces of the ischial spines. The intermediate fibers take their origin from the angle formed by the two layers of the endopelvie fascia, as previously described, or from the arcus tendinous or white line. The insertion is as follows: the anterior fibers pass downward and backward, a few of them are inserted into the central point of the perineum; others are inserted into the wall of the anal canal between the external and internal sphincters. The intermediate fibers sweep around into the angle between the posterior wall of the rectum and upper end of the anal canal where they unite with their fellows of the opposite side and form a strong muscular collar about the gut. The posterior fibers pass backward and medially and are inserted into the median anococcygeal raphe and into the lower part of the coccyx. The perineum which is the space lyin g below the pelvic diaphragm may be subdivided into two parts, the anal triangle and the urogenital triangle, the dividing line extending through the perineal body between the ischial tuberosities just anterior to the anus. The anal triangle has passing through its center t.he anal canal, having a space on either side known as the ischiorectal fossa which contains an extensive pad of fat, pyramidal in shape, through which extend vessels and nerves for the supply of the anal canal. The urogenital triangle which is more important than the anal triangle to the obstetrician will be detailed more. This space contains the so-called superficial and deep pouches. The superficial pouch is the space enclosed between Colles’ fascia superficially and the inferior fascia of the urogenital diaphragm deeply. Within this space we have the bulbocavernosus muscle, the ischiocavernosus muscles and the
230
THE
AMERICAN
JOURNAL
OF
OBSTETRICS
AND
GYNEXOLOGP
superficial transverse perineal muscles and the vessels and nerves that supply these structures and the labia majora. In addition to these structures we have in this space the clitoris, the bulb of vestibule, Bartholin’s glands and the terminal portion of the urethra and vagina. The deep pouch lies between the superior and the inferior fascia of the urogenital diaphragm. The superior fascia, which is a part of the endopelvie fascia, has been previously mentioned. The inferior fascia is the fascia bridging between the inferior rami of the pubes and lies in the morphologic plane as the obturator membrane. Within this pouch we have the sphincter of the membranous part of the urethra, the deep transverse perineal muscles, the deep branches of the perineal nerves, dorsal nerves of the clitoris and the trunk of the internal pudendal arteries and a portion of the urethra and the vagina. The lack of knowledge of the structure of the female pelvis is one of the fundamental causes of not recognizing complications during labor, the inability to correct this condition and to adequately repair injured tissues. To visualize the progress of labor one should know the muscular structure, the lymph and blood vessels of the uterus, the morphologic difference between the upper and lower uterine segments, the ligaments that attach the uterus to the walls of the pelvis, the parietal and visceral fascia which may blend or join with the muscular fibers of the cervix and uterus. IIow can one visualize the mechanism of labor without a knowledge of the levator ani muscles and fascia and their origin and insertion and their relation to the pelvic organs? Without this knowledge how can one hope to prevent injuries to the muscles and fascia of the superficial and deep structures of the perineum, and, if injured, repair them? The physiology and mechanism of the first stage of labor is better understood when it is known that the oblique muscular structures of the uterus are arranged in the form of a figureof-eight around the blood vessels and are capable of powerful constriction, and, when contracted, produce intrauterine pressure. The visceral fascia of the levator ani muscles is a part of the pelvic floor. This fascia in its morphologic development moors the uterus by the cervix to the pelvic wall. In this structural handiwork the pubovesical, the uterovesical and the uterosacral ligaments are mere thickenings of the visceral fascia and together attach or anchor the uterus to the bony structure. The visceral fascia joins or blends with the fascia of the fibers of the cervix. The visceral fascia attaches the bladder to the anterior surface of the cervix and lower part of the uterus and is considered with the uterus and rectum as pelvic organs. The blending of the visceral fascia with the cervical fibers of the uterus and their fascia forms an important structure in the physiologic and mechanical part of labor. The fibers of the oblique or middle muscular layer of the uterus pass obliquely from the peritoneal surface of
HANNAH
:
ANATOMY
OF
THE
FEM.ALE
PELVIS
231
the uterus to the endometrium. E. Hastings Tweedy says, “As these fibers approach the internal OS, they turn almost at right angles, passing out and attaching into the pelvic wall as muscular tendons.” He further says, “The circular fibers of the cervix likewise terminate in muscular tendons and they, too, largely attach themselves to the bony wall of the pelvis.” Tweedy emphasizes the fact that “Involuntary muscular fibers may have muscular tendons as well as any other muscle, and if this were not so during the first stage of labor, the visceral fascia of the pelvic floor would be completely destroyed.” Dr. W. W. Looney, Professor of Anatomy, Baylor University, College of Medicine, says, “The visceral pelvic fascia serves as an aponeurosis for the oblique and cervical fibers instead of these muscle fibers having independent muscular tendons, which, Tweedy states, ‘are inserted into the bony pelvic surface.’ Probably as the visceral fascia passes to the uterus and cervix it blends with the fascial covering of the muscle fibers of these structures. This explanation appears more plausible when the visceral fascia is considered a part of an exAnatomists say that tension of the fascia covering the psoas muscle.” the dissecting of the individual fibers of the nonpregnant uterus is a difficult task, and that probably as the oblique fibers approach the eervix, these may become more circular and form what is known during labor as the contraction, retraction or Bandl’s ring.
.
The uterus requires more space for the growing embryo about the seventh month of pregnancy. This space is supplied by the pushing down of the contents of the pregnant uterus against the internal OS which causes these oblique fibers to dilate and retract, producing what is later known as the lower uterine segment. This segment is not necessarily taken from the cervix, for in the nonpregnant uterus the oblique and circular fibers are closely associated with each other as if they were compressed, and it is difficult to dissect them. A study of the anatomy of the uterus suggests that the lower uterine segment probably is developed from the fibers of the middle layer of the uterus which are present just superior to the internal OS and bounded superiorly by Bandl’s ring. This anatomic space of the uterus is the potential lower uterine or dilating segment. The lower uterine segment is formed before the onset of labor. During the first stage of labor as the uterus contracts and forces the fetus against the internal OS, the fibers of the lower uterine segment pull out and up on the circular fibers of the cervix, causing them to dilate. As the cervix dilates.” This has been said, “As the uterus contracts, causes the shortening of the cervix which previously has not been very perceptible, yet the cervix may have been shortened by the irregular and painless contraction of the uterus before the onset of labor. As the cervix dilates, beginning at the internal OS and extending to the external OS,this progress is spoken of as effacement, and the gradual
232
THE
AMERICAN
JOURSAL
OF
OBSTETRIC’S
Ah-D
GYNECOLOGY
progress is considered favorable during labor. The cervix may be completely obliterated or effaced before the external OS is perceptibly opened. Especially in a primipara is this progress more favorable in the prevention of lacerations of the cervix than a thick or long cervix opening up in its entirety. During the first stage of labor, the fetus has not been pushed through the cervix, but rather the cervix has been pulled back over the presenting part, taking with it the bladder. As effacement takes place, the cervix is retracted over the presenting part, carying with it the bladder, which previously has been a pelvic organ and now becomes and later will be an abdominal organ. This is an important point in the diagnosis of the close of the first stage of labor and the entering into of, and during the secoud stage of labor. This is a point to be considered if catheterization at this time is necessary. A full bladder during any period in labor may prevent full contractility of the uterine fibers. Rearing down while the bladder is a pelvic organ should be prevented to avoid tears of the uterovesical fascia and injury to the trigone and sphincter uret,hrae and urethra, which may cause a cystocele. The pelvic floor is composed of the levator ani muscles and the visceral fascia which extends across the lower part of the pelvis, hammock-like in shape, and supports the nonpregnant uterus, and, also, during pregnancy, the head during labor. The course of the head during labor from the pelvic floor is downward and upward. A definite rule in the mechanism of labor is that the lowest point of the head is rotated to the front under the pubes by the levator ani muscles and the visceral fascia. The pelvic floor pushes the lower part of the head to the. front, and the suboccipitobregmatic diamet,er which was in the oblique diameter now lies in the anterior posterior diameter of the outlet. It is the levator ani muscles, their shape and contractility which cause rotation during labor. The levator ani muscles and the uterine muscles oppose each other as they contract, which is clinically demonstrated as the head passes over the perineum and underneath the symphysis. Extension takes place while the uterine muscles contract; and, when ended, the head is flexed by the levator ani muscles pushing the head back. Probably the levator ani muscles lose their power to flex the head after the biparietal diameter has passed through. The perineum is t.he structure that receives the force of labor after the head has passed through the pelvic floor. Lacerations can occur anywhere in the birth canal and may be snbmueus in character, located in the sulci or in and around the perineum, even including the sphincter ani. If the levator ani muscles and their fascia are not congenitally weak, or have not been overstretched, or have not been torn, these structures will support the uterus and prevent its prolapse. The uterus is supported by the pelvic floor and is not suspended from above.
HANNAH
:
ANATOMY
OF
THE
FEMALE
PELVIS
233
9 tear in t.he sulci probably will involve the anterior fibers and a Few intermediate fibers of the levator ani muscles and their fascia. A destruction of the anterior and intermediate fibers and their fascia which are inserted into the central point of the perineum and anal canal and posterior to the rectum will deprive the rectum of this support. With the loss of this support and by the contraction of the sphincter ani the lower part of the rectum is pulled back forming a more acute angle which enlarges the ampulla, and this, in turn, will cause, during defecation, the feces to go against the posterior wall of the vagina rather than follow the anal canal and form or cause a rectocele. If the tear is in the anterior sulcus, the anterior fibers of the levator ani muscles with their fascia will be involved and destroy the support for the bladder and anterior vaginal wall and cause the base of the bladder together with the anterior wall of the vagina to sag and form a cystocele. The tear may begin in the vagina above the perineum, extending outward separating the muscles and fascial structures down to and including the sphincter ani. A median laceration through the perineal body does not involve the fibers of the levator ani muscles and their fascia except a few of the anterior fibers attached in the central point, and this would injure the perineal diaphragm but little. This tear would only partly destroy the supporting action of the superficial transverse perineal muscles, the bulbocavernosus and the urogenital diaphragm. Rupture of perineum alone will not cause prolapse. Perineotomy or central incision of the perineum will prevent ,or control extensive lacerations. In a perineotomy only a few tendinous insertions of the levator ani muscles are divided, the muscular fibers themselves escaping because they are superior to the central point of the perineum. Lateral episiotomy may be preferred by some; but the danger of severing, if extensive, the fibers of the levator ani muscles and their nerves might later cause atrophy of the muscles. The stretching or “ironing out” of the perineum, the levator ani muscles and their fascia to prevent a tear is illogical and not a surgical procedure, and t,his is ineompatible with the new era in obstetrics, that of prophylaxis. We teach a technic that will prevent infection. We emphasize the value of rectal examinations as preferable to the vaginal in order to prevent infections and to ascertain t,he progress of labor. I sometimes fear we are overzealous in making rectal examinations and consider these examinations as totally void of danger, yet there is a potential danger. Rectal examinations are safer than the vaginal, but it is not logical to advocate rectal examinations instead of the vaginal and then practice “ironing out” or stretching the perineum to avoid lacerations. If vaginal examinations are a source or an avenue for infections, then surely manual dilatation of the vaginal tissues is poor surgical procedure and probably more hazardous than several
234
THE
AMERICAN
JOURNAL
OF
OBSTETRICS
AND
GYNECOLOGY
vaginal examinations, and not only is this method a source of infection, but may rupture the fibers and fascia of the levator ani muscles and cause prolapse, cystocele and rectocele. Since the uterus is supported by the pelvic floor rather than suspended from above, this overstretching or breaking of the muscular fibers may cause weakening of the support of the uterus and result in a prolapse, while perineotomy causes less injury to the tissues and is not so great a source of infection. CONCLUSIONS
1. The lack of knowledge of the structures of the pelvic tissues is one of the fundamental causes of not recognizing complications during labor, the inability to correct these conditions and to adequately repair injured tissues. 2. The visceral fascia serves as an aponeurosis for the oblique and cervical fibers of the uterus and cervix. As the visceral fascia passes to the uterus and cervix, it probably blends with the fascial covering of the muscle fibers of these structures. 3. During the first stage of labor the bladder is a pelvic organ; and after effacement, complete dilatation and retraction the bladder becomes an abdominal organ. Bearing down while the bladder is a pelvic organ may cause tears of the uterovesical fascia and injury to the trigone, sphincter urethrae and urethra of the bladder, and may cause cystocele. An injury of the uterovesical fascia may cause prolapse of the base of the bladder; and injury to the sphincter urethrae or Bell’s muscles may cause urine to collect anteriorly to the base of the trigone. 4. A tear in the sulci will probably involve the anterior intermediate fibers of the levator ani muscles with their deprive the rectum of this support and cause rectocele. 1 am indebted to J. C. Haley, M.D., of the Department of University, College of Medicine, for assistance in the preparation discussion in this paper.
and a few fascia and
Anatomy of the
of Baylor anatomical
REFERENCES (Ih‘?mhgleam:
Manual
Wrench, and Solom0n.3: University MEDICAL
Press,
1925.
ARTS BUILDING.
of Practical Anatomy, ed. 7, 2: pp. 147-197. Practical Obstetrics, ed. 5, pp. 7-9 and 591-595,
Tweedy, Oxford