Abdomen and Pelvis Anatomy

1. Overview and Surface Boundaries

• Superiorly: xiphoid process and costal margins (the costal margin separates the abdominal wall from the thoracic wall)
• Inferiorly: pubic tubercle, anterior superior iliac spine (ASIS), and iliac crest
• Laterally: the iliac crest separates the posterolateral abdominal wall from the gluteal region

Fig. 4.180 - Interior view showing palpable bony landmarks and position of the diaphragm (Gray's Anatomy for Students)

2. Abdominal Wall Layers

1. Skin
2. Superficial fascia (subcutaneous tissue)
3. Muscles and their deep fascias
4. Transversalis fascia (extraperitoneal fascia)
5. Parietal peritoneum

Fig. 4.24 - Layers of the abdominal wall (Gray's Anatomy for Students)

Superficial Fascia

• Camper's fascia (superficial fatty layer): contains fat, varies in thickness; continuous over the inguinal ligament with the superficial fascia of the thigh. In men, it continues into the scrotum forming dartos fascia. In women, it contributes to the labia majora.
• Scarpa's fascia (deep membranous layer): thin, membranous, little or no fat; fuses with fascia lata of the thigh just below the inguinal ligament; continues into the perineum as Colles' fascia (superficial perineal fascia).

Muscles of the Anterolateral Wall

• Rectus abdominis: long, flat, paired muscle extending the full length of the anterior wall; separated by the linea alba; enclosed in the rectus sheath
• Pyramidalis: small, triangular; lies in the rectus sheath inferior to rectus abdominis; tenses the linea alba

Transversalis Fascia

Innervation of the Abdominal Wall

• T7-T9: supply skin above the umbilicus
• T10: supplies the umbilical region
• T11, T12, L1: supply skin from just below the umbilicus to the pubic region
• The ilio-inguinal nerve (L1) also supplies the anterior scrotum or labia majora

Blood Supply

Fig. 4.39 - Arterial supply to the anterolateral abdominal wall (Gray's Anatomy for Students)

• Superior part: branches from the musculophrenic artery (branch of internal thoracic)
• Inferior part: superficial epigastric and superficial circumflex iliac arteries (branches of femoral artery)

• Superior: superior epigastric artery (terminal branch of internal thoracic)
• Lateral: branches of 10th and 11th intercostal arteries and the subcostal artery
• Inferior: inferior epigastric and deep circumflex iliac arteries (branches of external iliac artery)

Lymphatic Drainage

• Superficial lymphatics above the umbilicus drain superiorly to axillary nodes
• Superficial lymphatics below the umbilicus drain inferiorly to superficial inguinal nodes

3. Peritoneum and Peritoneal Cavity

• Parietal peritoneum: lines the walls
• Visceral peritoneum: covers the viscera

• Closed in men
• Semi-closed in women (the uterine tubes provide two small openings to the outside)

Innervation of the Peritoneum

• Parietal peritoneum: innervated by somatic afferents (spinal nerves) - sensitive to well-localized pain
• Visceral peritoneum: innervated by visceral afferents with autonomic nerves - produces poorly-localized, referred discomfort

Intraperitoneal vs. Retroperitoneal

• Intraperitoneal organs: suspended in the peritoneal cavity by folds of peritoneum (mesenteries), e.g., stomach, jejunum, ileum, transverse colon, liver, spleen
• Retroperitoneal organs: outside the peritoneal cavity, only partially covered by peritoneum on one surface, e.g., kidneys, aorta, IVC, pancreas, ascending/descending colon, duodenum (most parts)

Clinical: Ascites and Peritoneal Dialysis

4. The Pelvis

Bony Framework

• Two hip bones (each formed by ilium, ischium, pubis fused at the acetabulum)
• Sacrum
• Coccyx

• Iliac crest: separates the lateral abdominal wall from the gluteal region
• ASIS (anterior superior iliac spine): attachment of the inguinal ligament
• Pubic tubercle: medial attachment of the inguinal ligament
• Ischial tuberosities: define the lateral corners of the diamond-shaped perineum
• Pubic symphysis: anterior midline articulation

Orientation

Fig. 5.81 - Lateral view of the pelvic area showing bony features and planes of the urogenital and anal triangles (A = female, B = male; Gray's Anatomy for Students)

Pelvic Inlet and Outlet

• Pelvic inlet (brim): bounded by the sacral promontory, ala of sacrum, arcuate lines, pectineal lines, and symphysis pubis
• Pelvic outlet: bounded by the pubic symphysis anteriorly, ischial tuberosities laterally, and the tip of the coccyx posteriorly
• Greater (false) pelvis: above the pelvic inlet; contains lower abdominal viscera
• Lesser (true) pelvis: below the pelvic inlet; contains pelvic organs (bladder, rectum, uterus/prostate)

5. Pelvic Floor (Pelvic Diaphragm)

• Pubococcygeus (with the pubovaginalis in women / puboprostaticus in men)
• Puborectalis: forms a sling around the anorectal junction
• Iliococcygeus

6. Perineum

• Anteriorly: pubic symphysis
• Laterally: ischial tuberosities
• Posteriorly: tip of coccyx

• Urogenital triangle (anterior): contains external genitalia and urethral opening
• Anal triangle (posterior): contains the anal canal and external anal sphincter

The Perineal Body

• Levator ani muscles of the pelvic diaphragm
• Muscles of both perineal triangles - including sphincters of the urethra, vagina, and anus

Fig. 5.17 - Perineal body and surrounding muscles (female perineum; Gray's Anatomy for Students)

The Urethra

• Female urethra: short, passes inferiorly from the bladder directly through the pelvic floor to open in the perineum
• Male urethra: passes through the prostate, then through the deep perineal pouch and perineal membrane, then becomes enclosed in the erectile tissues of the penis; has a fixed anterior bend at the root of the penis after exiting the perineal membrane (clinically relevant for catheterization)

7. Key Clinical Correlations