Anatomy of abdomen

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Anatomy of the Abdomen

Source: Gray's Anatomy for Students

1. Boundaries and Surface Landmarks

The abdomen is the region between the thorax above and the pelvis below. Its surface boundaries are:

Superior: xiphoid process and costal margins (separates abdomen from thorax)
Posterior: vertebral column
Inferior: pubic tubercle, anterior superior iliac spine (ASIS), and iliac crest

The inguinal ligament runs between the ASIS and the pubic tubercle, forming the boundary between the anterior abdominal wall and the thigh.

The upper part of the abdominal cavity projects above the costal margin to the diaphragm - abdominal viscera in this region are protected by the thoracic wall. The dome of the right diaphragm can reach as high as the 4th costal cartilage during forced expiration.

Surface boundaries of the abdomen, showing the diaphragm, costal margin, iliac crest, inguinal ligament, and pubic tubercle

Fig. 4.180 - Surface landmarks of the abdominal region (Gray's Anatomy for Students)

2. Layers of the Abdominal Wall

From superficial to deep, the anterolateral abdominal wall has the following layers:

Skin
Superficial fascia (subcutaneous tissue) - two layers below the umbilicus:
- Camper's fascia - superficial fatty layer; continuous with superficial fascia of the thigh and perineum
- Scarpa's fascia - deeper membranous layer with little fat; fuses with fascia lata just below the inguinal ligament
Muscles and their aponeuroses
Transversalis fascia
Extraperitoneal fascia
Parietal peritoneum

Layers of the abdominal wall showing skin, Camper's fascia, Scarpa's fascia, external oblique, internal oblique, transversus abdominis, transversalis fascia, extraperitoneal fascia, and parietal peritoneum

Fig. 4.24 - Layers of the Abdominal Wall (Gray's Anatomy for Students)

Transverse section showing all abdominal wall layers with parietal and visceral peritoneum

Fig. 4.34 - Transverse section showing layers including parietal and visceral peritoneum

3. Muscles of the Anterolateral Abdominal Wall

Three flat muscles and one vertical muscle form the anterolateral wall:

Flat Muscles (lateral to medial)

Muscle	Fiber Direction	Key Features
External oblique	Inferomedial	Most superficial flat muscle; aponeurosis forms the inguinal ligament
Internal oblique	Superomedial	Smaller and thinner than external oblique
Transversus abdominis	Transverse	Deepest flat muscle; ends in anterior aponeurosis

Internal oblique muscle and its aponeurosis blending into the linea alba

Fig. 4.30 - Internal Oblique Muscle and Aponeurosis

Transversus abdominis muscle with its aponeurosis and the linea alba

Fig. 4.31 - Transversus Abdominis Muscle and Aponeurosis

Vertical Muscle

Rectus abdominis: Paired vertical muscles running from the pubis to the costal cartilages of ribs 5-7, enclosed in the rectus sheath formed by the aponeuroses of the three flat muscles.

The linea alba is a midline fibrous band formed by the interlacing aponeuroses of the three flat muscles on both sides.

Transversalis Fascia

A continuous layer of deep fascia lining the entire abdominal cavity; it continues into the pelvis and is continuous with the inferior surface of the diaphragm. Below the iliac crest, it becomes the parietal pelvic (endopelvic) fascia.

4. Innervation of the Abdominal Wall

The skin, muscles, and parietal peritoneum of the anterolateral wall are supplied by T7 to T12 and L1 spinal nerves. These anterior rami pass inferiorly and medially, giving off:

Lateral cutaneous branches (T7-T12)
Anterior cutaneous branches (T7-T12)

L1 contributes via:

Iliohypogastric nerve (L1)
Ilio-inguinal nerve (L1)

The intercostal nerves (T7-T11) leave their intercostal spaces, pass between the internal oblique and transversus abdominis, enter the rectus sheath, and pierce through as anterior cutaneous branches.

Innervation of the anterolateral abdominal wall showing lateral and anterior cutaneous branches T7-T12, iliohypogastric and ilio-inguinal nerves at L1

Fig. 4.36 - Innervation of the Anterolateral Abdominal Wall

5. Peritoneum

The peritoneum is a thin serous membrane with two layers:

Parietal peritoneum - lines the walls of the abdominal cavity; supplied by somatic afferents (well-localized pain)
Visceral peritoneum - covers the abdominal viscera; supplied by visceral afferents (poorly localized, referred pain)

The peritoneal cavity is the potential space between the two layers. It is:

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

Intraperitoneal vs. Retroperitoneal Organs

Intraperitoneal: suspended in the cavity by mesenteries (e.g., stomach, small intestine, transverse colon, spleen)
Retroperitoneal: outside the peritoneal cavity, only partially covered by peritoneum (e.g., kidneys, duodenum 2nd-4th parts, pancreas, ascending/descending colon, aorta, IVC)

Omenta and Mesenteries

Greater omentum: large apron-like peritoneal fold hanging from the greater curvature of the stomach; derived from the dorsal mesentery
Lesser omentum: connects the liver to the stomach and the first part of the duodenum; derived from the ventral mesentery
Mesentery of the small intestine: connects the jejunum and ileum to the posterior abdominal wall
Transverse mesocolon: connects the transverse colon to the posterior wall
Sigmoid mesocolon: connects the sigmoid colon to the posterior wall

Peritoneal ligaments connect organs to each other or to the body wall (e.g., splenorenal ligament, gastrophrenic ligament).

6. Abdominal Viscera

Gastrointestinal Tract

Abdominal esophagus Emerges through the right crus of the diaphragm at vertebra TX; arterial supply from the left gastric artery (celiac trunk) and left inferior phrenic artery.

Stomach J-shaped, most dilated GI structure; located in epigastric, umbilical, and left hypochondrium regions. Divided into:

Cardia (near esophageal inlet)
Fundus (dome-shaped upper portion)
Body (main portion)
Pyloric part (exits into duodenum via pyloric orifice)

Duodenum C-shaped, 20-25 cm long, the first part of the small intestine. Divided into four parts:

Superior (1st part): at vertebra LI; duodenal cap (most common site of duodenal ulcers)
Descending (2nd part): to the right of midline, L1-L3; contains the major duodenal papilla (entry of bile and pancreatic ducts) and minor duodenal papilla; foregut-midgut junction just below the major papilla
Inferior (3rd part): crosses the IVC, aorta, and vertebral column; crossed anteriorly by superior mesenteric vessels
Ascending (4th part): ascends to the left of the aorta, terminates at the duodenojejunal flexure (supported by the ligament of Treitz)

Most of the duodenum is retroperitoneal (except the beginning, which is intraperitoneal).

Jejunum and Ileum: Intraperitoneal; arterial supply from branches of the superior mesenteric artery.

Overview of the abdominal viscera showing the duodenum, pancreas, kidneys, spleen, gallbladder, and major vessels at vertebral levels L1-L3

Fig. 4.67 - Abdominal Viscera Overview (note the four parts of the duodenum and adjacent structures)

7. Key Vascular Anatomy

Arterial Supply

The abdominal aorta enters through the aortic hiatus of the diaphragm at T12 and bifurcates into the common iliac arteries at L4. Its major branches supply the abdominal organs:

Artery	Vertebral Level	Territory
Celiac trunk	T12/L1	Foregut (distal esophagus to 2nd part duodenum, liver, gallbladder, pancreas, spleen)
Superior mesenteric	L1	Midgut (distal duodenum to 2/3 transverse colon)
Renal arteries	L1/L2	Kidneys and suprarenal glands
Inferior mesenteric	L3	Hindgut (distal transverse colon to rectum)

Venous Drainage

The portal vein drains blood from the GI tract, spleen, pancreas, and gallbladder to the liver
The inferior vena cava receives hepatic veins, renal veins, and drains below the diaphragm

8. Regions of the Abdomen

The abdomen is divided into 9 regions by two horizontal and two vertical planes:

	Left	Central	Right
Upper	Left hypochondrium	Epigastric	Right hypochondrium
Middle	Left lumbar (lateral)	Umbilical	Right lumbar (lateral)
Lower	Left iliac (inguinal)	Hypogastric (pubic)	Right iliac (inguinal)

Alternatively, two planes divide it into 4 quadrants: right upper, left upper, right lower, left lower.

Summary of Key Clinical Points

Umbilical hernias: usually congenital (failure of bowel to return from umbilical cord); most close in the first year of life
Incisional hernias: through a defect in a surgical scar; wide necks, low risk of strangulation
Spigelian hernia: passes through the arcuate line into the lateral border of the lower posterior rectus sheath
Perforated duodenal ulcer: releases gas into the peritoneal cavity visible as subdiaphragmatic gas on erect chest X-ray
Peritoneal dialysis: uses the large surface area of the peritoneum as a dialysis membrane
Referred pain: visceral peritoneum pain is poorly localized (visceral afferents); parietal peritoneum pain is sharp and well-localized (somatic afferents)

All content sourced from Gray's Anatomy for Students and the Color Atlas of Human Anatomy, Vol. 2 (Internal Organs) - from the medical textbook library.

3d view of pelvic girdle

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Highly detailed 3D anatomical illustration of the human pelvic girdle, showing the complete bony pelvis from a slightly anterior-superior oblique (3/4 view) perspective. Include and label all key structures: ilium (iliac crest, iliac fossa, anterior superior iliac spine, anterior inferior iliac spine, posterior superior iliac spine, posterior inferior iliac spine, greater sciatic notch), ischium (ischial tuberosity, ischial spine, lesser sciatic notch, ischial ramus), pubis (pubic symphysis, superior pubic ramus, inferior pubic ramus, pubic tubercle), sacrum (sacral promontory, sacral foramina, sacral canal, ala of sacrum), coccyx, acetabulum, obturator foramen, sacroiliac joint, and arcuate line (pelvic brim). Show the true pelvis (lesser pelvis) and false pelvis (greater pelvis) clearly. Medical textbook quality, photorealistic bone texture, clean white background, all labels with leader lines.

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3D View of the Pelvic Girdle

Here is a detailed 3D anatomical illustration of the pelvic girdle with all key structures labeled:

3D anatomical illustration of the human pelvic girdle showing all bones, landmarks, joints, pelvic inlet, and foramina from an anterior-superior oblique view

Textbook Anatomy Views

Anterior View - The Pelvic Ring

The pelvic girdle (from the THIEME Atlas of General Anatomy and Musculoskeletal System):

Anterior view of the pelvic girdle showing the two hip bones (gray), sacrum (orange), sacroiliac joints, pubic symphysis, and arcuate line forming the pelvic inlet

The pelvic girdle is formed by three bones connected in a ring:

Two hip bones (os coxae) - one on each side
The sacrum posteriorly (shown in orange above)

They articulate at the sacroiliac joints posterolaterally and the pubic symphysis anteriorly. The arcuate line on each hip bone, together with the sacral promontory and pubic crest, forms the pelvic brim (inlet).

Sacrum and Coccyx - Anterior & Posterior Views

From the THIEME Atlas (actual specimen photographs):

Sacrum and coccyx - anterior (pelvic) view showing promontory, base of sacrum, superior articular processes, wing of sacrum, anterior sacral foramina, transverse lines, apex, sacrococcygeal joint, and coccyx; Posterior view showing sacral canal, lateral part, auricular surface, posterior sacral foramina, lateral sacral crest, median sacral crest, medial sacral crest, sacral hiatus, sacral cornua, coccygeal cornua

Acetabulum - The Three Components of the Hip Bone

CT scan lateral view of the right acetabulum showing the three bones - ilium (upper 2/5), ischium (lower 2/5), and pubis (1/5) - joined by the triradiate (Y-shaped) cartilage in a child

The acetabulum is formed by all three components of the hip bone: the ilium and ischium each contribute 2/5, and the pubis contributes 1/5. The triradiate cartilage fuses definitively between ages 14-16 years.

Pelvic Inlet - Gray's Anatomy View

Anterior view of the full pelvis showing the pelvic inlet (dashed red line), pelvic bones, ala of sacrum, sacroiliac joints, inguinal ligament, and the 5th lumbar vertebra (LV) above the sacrum (SI)

Bones of the Pelvic Girdle - Key Landmarks

Hip Bone (Os Coxae)

Each hip bone is formed by three bones that fuse during development:

Part	Key Landmarks
Ilium (largest)	Iliac crest, ASIS, AIIS, PSIS, PIIS, iliac fossa, greater sciatic notch, auricular surface
Ischium	Ischial tuberosity, ischial spine, lesser sciatic notch, ischial ramus, body
Pubis	Superior ramus, inferior ramus, pubic crest, pubic tubercle, pubic symphysis
Acetabulum	Cup-shaped socket at junction of all three; articulates with the femoral head
Obturator foramen	Large opening formed by ischium and pubis; mostly closed by the obturator membrane

Sacrum

Formed by fusion of 5 sacral vertebrae. Key features:

Sacral promontory - anterior projection of S1; important obstetric landmark
Ala (wings) of sacrum - lateral parts that articulate with the ilium
Anterior sacral foramina (4 pairs) - transmit ventral rami of S1-S4
Posterior sacral foramina (4 pairs) - transmit dorsal rami
Sacral canal - continuation of the vertebral canal; ends at the sacral hiatus
Sacral hiatus - used for caudal epidural injections
Auricular surface - articulates with the ilium at the sacroiliac joint
Median sacral crest - fused spinous processes
Lateral sacral crests - fused transverse processes

Coccyx

3-4 rudimentary vertebrae, connected to the sacrum at the sacrococcygeal joint
Moves passively during childbirth to increase the anteroposterior diameter of the pelvic outlet

Joints of the Pelvic Girdle

Joint	Type	Movement
Sacroiliac joint	Synovial (partly), partly fibrous	Very little; transfers trunk loads to the lower limb
Pubic symphysis	Cartilaginous (secondary)	Minimal; widens slightly during pregnancy
Sacrococcygeal joint	Cartilaginous	Passive flexion; important in childbirth

The pelvic ring is designed for stability to allow load transfer from the trunk to the lower limbs during gait. Very little motion is permitted.

True vs. False Pelvis

The pelvic brim (arcuate line, sacral promontory, pubic crest) divides the pelvis into two parts:

False (greater) pelvis - above the brim; bounded by the iliac fossae; contains lower abdominal organs (sigmoid colon, ileum, uterus in pregnancy)
True (lesser) pelvis - below the brim; contains the pelvic viscera (bladder, rectum, uterus/prostate)

The pelvic inlet (superiorly) and pelvic outlet (inferiorly, bounded by the coccyx, ischial tuberosities, and pubic arch) are key obstetric measurements.

Sources: Gray's Anatomy for Students; THIEME Atlas of General Anatomy and Musculoskeletal System

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