Anatomy of the pelvic cavity

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Introduction0:00–0:51

The pelvic cavity is a funnel-shaped space surrounded by pelvic bones and it contains organs, such as the urinary bladder, rectum, and pelvic genitals, to name a few.
The pelvic cavity and the abdominal cavity together form the larger abdominopelvic cavity. The pelvic cavity is bounded superiorly by the pelvic inlet, which connects it to the abdominal cavity, and inferiorly, it’s bounded by a group of muscles and the fascia surrounding them, called the pelvic floor or pelvic diaphragm which is suspended above the pelvic outlet, with its central portion dipping below it.
The pelvic cavity also has an anteroinferior wall, two lateral walls, and a posterior wall. So let’s start with the anteroinferior wall, formed by the bodies and rami of pubic bones, and the pubic symphysis.

Anteroinferior wall0:51–1:35

Each pubic bone has three parts: a body and a superior and inferior ramus. The body of the pubic bone is the thickest, most anterior part of the pubic bone.
Anteriorly, the bodies of the right and left pubic bones are joined together in the median plane to form the pubic symphysis.
Now, in an anatomical position, the pelvis is tilted in the anterior-posterior plane, making the pubic bones and pubic symphysis act more like a weight bearing floor than an anterior wall.

Lateral walls1:35–3:00

The lateral walls of the pelvic cavity are formed by the right and left hip bones. Each hip bone has an inferior opening called the obturator foramen, which is formed by the pubis and the ischium.
The foramen is partially covered by a membrane called the obturator membrane. This membrane and the pelvic surfaces of the ilium and ischium give origin to a muscle called the obturator internus which pads most of the lateral pelvic wall.
Now, the fibers of the obturator internus muscle fibers converge posteriorly to become tendinous, and then travel through a foramen called the lesser sciatic foramen.
Finally, they attach to a large area on the lateral side of the femur called the greater trochanter. The medial surfaces of the obturator internus is covered by a fascia called the obturator fascia.
This fascia covers most of the muscle, and becomes thicker medially to form a tendinous arch. This arch gives origin to one of the muscles forming the pelvic floor or pelvic diaphragm.
Additionally, the attachment of the pelvic diaphragm to the obturator fascia divides the obturator internus into a superior pelvic portion, and an inferior peroneal portion.
Now let’s move on to the posterior wall! The posterior wall has a bony and a musculoligamentous part.

Posterior wall3:00–5:28

The bony part is formed by the sacrum and coccyx, which are the final parts of the vertebral column. The sacrum is a wedge-shaped bone composed of five fused sacral vertebrae.
It forms the posterosuperior wall of the pelvic cavity, and has a superior base and an inferior apex. Also, the sacrum connects to the hip bones on both sides forming the sacro-iliac joints.
The anterior and posterior sacro-iliac ligaments support this joint anteriorly and posteriorly. Next, there’s the coccyx, also called the tailbone, which is a small triangular bone formed by the fusion of 4 vertebrae.
Inferiorly, it attaches to the apex of the sacrum, and forms the last part of the vertebral column. Wonder why it’s called the tailbone?
Because the coccyx is the remnant of a tail-like structure found in human embryos between weeks four and eight, called the caudal eminence.
But it’s not just an embryonic remnant - multiple structures attach to the coccyx. These include parts of the gluteus maximus and coccygeus muscles, and a ligament called the anococcygeal ligament.
Now, as the name suggests, the musculoligamentous part of the posterior wall is composed of muscles and ligaments. The piriformis is the main muscle which forms the posterolateral walls of the pelvic cavity.
It originates from the superior part of the sacrum, goes through a foramen called the greater sciatic foramen, and finally attaches to the superior border of the greater trochanter of the femur.
The ligaments that form the posterior wall are the anterior sacro-iliac, sacrotuberous, and sacrospinous ligaments. The anterior sacro-iliac ligament is one of the ligaments that support the sacro-iliac joints anteriorly.
The sacrotuberous ligament passes from the posterior surface of the ilium and the lateral surfaces of the sacrum and coccyx, to the ischial tuberosity.
Finally, the sacrospinous ligament passes from the lateral surface of the sacrum and the coccyx to the ischial spine. The pelvic floor or pelvic diaphragm is musculofascial, meaning that it is composed of a group of muscles and the fascia covering them, called the fascia of the pelvic floor.

Pelvic floor5:28–6:00

The pelvic floor separates the pelvic cavity superiorly from the perineum inferiorly, which is the space between the anus and scrotum in males, and between the anus and the vulva in females.
The muscles that form the pelvic diaphragm are the coccygeus and the levator ani. The coccygeus are triangular muscles that run from the ischial spine to the lateral surfaces of the sacrum and coccyx.

Coccygeus muscle6:00–6:23

These muscles are innervated by branches of spinal nerves S4 and S5, and each coccygeus muscle flexes the coccyx anteriorly, helping it bear the body weight while sitting.

Levator ani muscle6:23–8:13

Now, the levator ani is the larger muscle forming the pelvic floor. Each levator ani originates anteriorly from the body of pubic bones, and to the ischial spines posteriorly.
Between these two bony origins, each levator ani muscle attaches to the tendinous arch of the levator ani, which is medial thickening of the obturator fascia called the tendinous arch of the levator ani.
The pelvic diaphragm spans between the anterior, lateral, and posterior walls of the pelvis, closing most of the pelvic girdle ring, but leaves a small gap anteriorly just behind the pubic bones, called the urogenital hiatus.
In females, the vagina passes through this gap, as well as the urethra in both females and males. The levator ani forms a dynamic floor that supports the abdominopelvic viscera, and most of the time it’s contracted to assist in maintaining urinary and fecal continence.
So, basically, it helps us hold it in until we get to the bathroom! Or a bush when no one is looking.
It also contracts during forced expiration, sneezing, coughing, and vomiting to increase viscera support during times of increased intra-abdominal pressure.
And finally, it relaxes in order for urination or defecation. Alright, now the levator ani muscle consists of three parts: puborectalis, pubococcygeus, and iliococcygeus.
All three are innervated by the nerve to levator ani coming from branches of S4, the inferior anal nerve, and coccygeal plexus.

Puborectalis part of levator ani8:13–9:09

The puborectalis is the thicker, narrower, medial part of the levator ani, and it bounds the urogenital hiatus from both sides.
And its name kinda gives away the whole story. Pubo- means that this part attaches to the posterior surfaces of the bodies of the pubic bones anteriorly, and -rectalis means that this part forms a sling around the anorectal junction.
Easy, right? The puborectalis is U-shaped, with the tips of the U attached anteriorly to the posterior surfaces of the bodies of both pubic bones.
The bowl of the U forms a puborectal sling around the junction between the rectum and the anus, called the anorectal junction or flexure.
So when all is said and done, the puborectalis plays a major role in maintaining fecal continence. Next, there’s the pubococcygeus, which is the wider, thinner intermediate part of the levator ani muscle.

Pubococcygeus part of levator ani9:09–10:13

This part arises lateral to the puborectalis, from the posterior surfaces of the bodies of pubic bones, in addition to the anterior tendinous arch.
The lateral muscle fibers go posteriorly to attach to the coccyx, hence its name - the pubococcygeus, The medial muscle fibers of the paired pubococcygeus muscles fuse in midline to form a median tendinous raphe, which forms part of the anococcygeal body or ligament between the anus and coccyx, often referred to as the levator plate.
This raphe unites with the external anal sphincter, forming a ligament that extends between the coccyx and the anus, called the anococcygeal body or ligament.
The main action of the pubococcygeus muscles is to maintain urinary continence and help in male ejaculation. Finally, there’s the iliococcygeus, which is the posterolateral part of the levator ani.

Iliococcygeus part of levator ani10:13–10:35

This part arises from the ischial spines and tendinous arch, and inserts on the coccyx and the anococcygeal body. It is usually thin and poorly developed, and it usually looks like a fibrous tissue rather than muscular fibers.
Alright, as a quick recap, the pelvic cavity is a funnel-shaped cavity bounded superiorly by the pelvic inlet, inferiorly by the pelvic floor or diaphragm.

Review10:35–11:25

It has an anteroinferior wall, two lateral wall, and a posterior wall. The anteroinferior wall is formed by the bodies and rami of pubic bones, the lateral wall is formed by the obturator membrane and the obturator internus muscle.
Finally, the posterior wall is formed by the sacrum and coccyx, and the anterior sacro-iliac, sacrotuberous, and sacrospinous ligaments.
The pelvic floor contains two muscles, the coccygeus and the levator ani. Those muscles support the pelvic viscera and help in maintaining