Atlas of Human Anatomy by Netter

FACTS & HINTS

High-Yield Facts

Anatomic Points

Mechanism of Defecation (Emptying of Bowel)

Occurs as the result of action of the pelvic splanchnic nerves-increase peristaltic action of rectum

Feces move through anal canal

Pressure in rectum causes reflex contraction of external anal sphincter

Contraction of external anal sphincter causes relaxation of peristaltic contraction

External anal sphincter relaxes and defecation occurs

Clinical Points
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Anal Hemorrhoids (Varicosities)

Can be internal or external

Internal

Are prolapses of mucosal lining containing internal venous plexus

Can prolapse through anal canal, become strangulated by sphincters, and ulcerate

Are painless

External

Are blood clots in external venous plexus

Are painful

Are caused by

Anything that increases intra-abdominal pressure, such as chronic constipation (straining) or pregnancy

Portal hypertension (cirrhosis of the liver)

Varicosities of the venous plexuses are normal

Internal hemorrhages can occur without portal hypertension

Ischiorectal and Perianal Abscesses

A number of potential tissue spaces surround the anal canal and rectum and may become infected because of

Inflammation of anal sinuses

Extension of an infection from the pelvis

Tear in the mucous membrane of the anal canal

Abscesses are classified according to the space occupied:

submucosal

peri-anal

intersphincteric

ischiorectal

perirectal

Perianal and ischiorectal account for approximately 80% of all abscesses

Rectal prolapse

Eversion of the lower part of the rectum and anal canal

Occurs as a result of weakness of pelvic floor muscles and breakdown of pelvic fascia

Common in elderly (older than 80 years)

More common in women, since perineum is weakened by childbirth

Digital Rectal Examination

Performed with patient in left lateral position

Perianal region is initially inspected for fistulae, fissures, skin tags, and prolapsed hemorrhoids

Ninety percent of rectal cancers palpable

Surfaces of sacrum and coccyx and ischial spines and tuberosities can be palpated

Enlarged internal iliac lymph nodes and ischiorectal (ischioanal abscesses can also be palpated)

In men: prostate and seminal vesicles can be palpated anteriorly

In women: cervix can be palpated anteriorly