Anatomy - Pelvis

1. What are the important obstetric measurements used to determine if a vaginal delivery will be possible?

1. Obstetric conjugate: anteroposterior diameter from the midpoint of the sacral promontory to the midpoint of the superior border of the pubic symphysis.

2. Transverse midplane diameter: distance between ischial spines (if less than 9.5cm, may require surgical intervention (Ceasarean delivery).

2. a. What is the urogentital hiatus?

A midline defect in the levator ani muscle through which pass the urethra (male, female) and the vagina (female).

b. How is this hiatus reinforced?

3. a. How do hemorroids develop in an alcoholic with a cirrhotic liver?

Veins of the submucosal plexus form extensive anastomoses with the middle rectal veins. Above the pectinate line, superior rectal veins drain into the inferior mesenteric artery into the hepatic portal system. Below the pectinate line, the inferior rectal veins drain into the systemic circulation via IVC. The middle rectal veins anastomose with the superior and inferior rectal veins creating a portocaval anastomoses. Therefore, any block in the valvelss hepatic portal system will be transmitted back into the rectal veins.

b. Why are “internal hemorroids” more dangerous than “external hemorrhoids”?

Internal: Above the pectinate line, the epithelium is innervated by visceral afferents (only pressure/stretching is felt). Varicosities covered by mucous membrane.

4. Boundaries of the Anal and Urigenital Triangles:

Anal: tip of the coccyx and ischial tuberosities.

6. What are the motor and sensory effects of a “saddle block”?

Insert needle between perineal body and ischial tuberosity and angle toward ischial spine to direct needle near pudendal canal. Alternative route involves vaginal palpation for the ischial spine followed by needle insertion through lateral vaginal wall.

Motor: Temporary urinary incontinence. Rectal continence preserved.

1. Why do the majority of pampiniform varices occur on the left side?

A full sigmoid colon compresses the left testicular vein. Contrary to wide acceptance, drainage of the left testicular vein into the left renal vein is not a cause of varicocele.

2. The pudendal nerve goes through the greater/lesser/both sciatic foramen.

Both

3. The posterior and anterior sacral foramina which rami?

Posterior primary rami and anterior primary rami, respectively.

4. What gender differences in pelvis outlet bone structure facilitate child birth?

Females have greater (almost 90 degrees) subpubic angle than males.

Females have a more vertically directed coccyx.

5. Which pelvic organs lie in the major pelvis?

None. (Some smart kid may say that a full bladder can rise all the way up into the abdomen).

6. What are the three components to fecal continence?

1. puborectalis muscle forms rectal sling
2. external anal spincter can voluntarily contract against a peristaltic wave.
3. ischioanal fat.

Note: the internal anal sphincter is a tube of involuntary muscle that encloses the entire anal canal. The sphincter is inhibited by the peristaltic waves and thus does not contribute to fecal continence.

7. Which erectile tissue in the penis is softer and why?

Corpus spongiosum, allows ejaculation.

8. The testes descend peritoneally/retroperitoneally and end up being peritoneal/retroperitoneal.

Descend retroperitoneally, are peritoneal due to presence of mesorchium.

9. Why doesn’t urine in the superficial pouch extravate into the anal triangle?

Colle’s (superficial of superficial) fuses with Scarpa’s (deep of superficial) forming the posterior boundary of the superficial pouch.

10. You’re on your third year Ob/Gyn rotation and a female patient presents with what looks like a cyst in the vestibule of her vagina. The attending says that it is a Barolinth’s cyst and asks the resident to explain what it is. He panics, and you bail him out by signing the answer to him from across the room. That it is….

It is a cyst of the greater vestibular gland.

Category: Anatomy Notes