Pelvis and perineum
Introduced| English | Meaning |
|---|---|
| pelvic floor | pelvic floor |
| levator ani | levator ani |
| pudendal nerve | pudendal nerve |
The body's floor
- The pelvis is the body's floor and its outlet: a bowl of bone, a sling of muscle, and nerves that share space with the birth canal and the rectum.
- Its anatomy explains continence, childbirth and some of the most feared nerve injuries.
A muscle floor
- The pelvic floor 盆底 is mainly the levator ani 肛提肌 with coccygeus, slung from the pubis to the coccyx.
- Above it sit the pelvic organs (bladder in front, uterus and rectum behind). Below it lies the perineum.
Which muscle forms most of the pelvic floor?
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Levator ani, with coccygeus, slings from pubis to coccyx and carries the pelvic organs.
Put these in order from top to bottom.
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The floor separates the pelvic cavity above from the perineum below.
The pudendal nerve
- The pudendal nerve 阴部神经 (S2–4) leaves the pelvis, hooks around the ischial spine and comes back in to supply the perineum.
- The ischial spine can be felt from inside, so it is the landmark for a pudendal nerve block.
Which landmark guides a pudendal nerve block?
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The pudendal nerve hooks around the ischial spine, which can be felt through the vaginal wall.
Continence: pressure and two switches
- The floor holds up the organs against abdominal pressure for decades. S2–4 nerves drive both the bladder wall (detrusor, parasympathetic) and the external sphincter (somatic, by the pudendal nerve).
- So continence can fail two ways: a stretched floor after childbirth, or a nerve lesion that breaks the switches. Each needs a different treatment.
Match each part of continence to what controls it.
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Two neural switches and one mechanical support; each fails in a different way.
Worked example: the S2–4 core
- S2–4 control bladder contraction, the sphincters, sensation of the perineum and the bulbocavernosus reflex.
- One short stretch of cord, one bedside examination: a lesion at S2–4 has a pattern you can test directly.
A lesion of the S2–4 cord segments can stop the bladder emptying.
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The detrusor is driven by parasympathetic S2–4 fibres; without them the bladder fills but does not contract well.
Worked example: two injuries in one birth
- (Fictional delivery) A long second stage: the baby's head presses the pudendal nerve at the ischial spines, causing temporary numbness of the perineum.
- A third-degree tear crosses the external anal sphincter, risking incontinence. One event, two anatomies: the nerve usually recovers, the sphincter needs repair.
A third-degree perineal tear is serious mainly because it —
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Sphincter damage risks faecal incontinence and needs repair; pudendal compression, by contrast, usually recovers.
Three common slips
- Sexing a pelvis by one feature. Check both the subpubic angle and the shape of the greater sciatic notch.
- The floor as the outlet. The floor is the support; the perineum below it is the outlet region.
- Forgetting the nerves beside the prostate. The nerves for erection run next to it, which is why prostate surgery puts them at risk.
Recap
- Levator ani floor carries the organs; the perineum lies below.
- The pudendal nerve (S2–4) hooks around the ischial spine.
- Continence needs a firm floor and working S2–4 switches; childbirth can injure either.