Borders: External and Internal
Anatomists strictly distinguish between the external contours of the abdomen and its internal space. A crucial feature is that the external and internal borders do not coincide.
External borders of the anterolateral wall:
- Superior: runs along the costal margin and the xiphoid process of the sternum.
- Inferior: formed by the iliac crests, inguinal ligaments, and pubic symphysis.
- Lateral: represented by conventional vertical lines descending from the anterior ends of the 11th (XI) ribs directly to the iliac crests. These lines visually demarcate the abdomen from the lumbar region of the back.
Internal borders (abdominal cavity):
- Superior: the dome of the diaphragm. It arches deeply into the thoracic cavity, causing a portion of the abdominal organs to be effectively concealed beneath the protection of the ribs.
- Inferior: the so-called terminal line (linea terminalis). It serves as the watershed between the abdominal cavity and the pelvic cavity.
Structure of the Abdominal Cavity
From the perspective of topographic anatomy, the abdomen consists of walls and the space enclosed within them. The abdominal cavity itself (cavitas abdominis or regio abdominis) is heterogeneous and divided into two major compartments:
- Peritoneal cavity (cavitas peritonei). This space contains internal organs covered to varying degrees by a serous membrane, the peritoneum. Organs may be located entirely within it (intraperitoneally), covered on three sides (mesoperitoneally), or covered on only one side (extraperitoneally).
- Retroperitoneal space (spatium retroperitoneale). Located posterior to the peritoneal cavity, it serves as the bed for retroperitoneal organs (organa retroperitonealia).
External Bony and Muscular Landmarks
To navigate the body surface, clinicians use several palpable structures:
- Costal margin (arcus costalis). Formed by the fusion of the cartilages of the 7th, 8th, 9th, and 10th ribs.
- Infrasternal angle (angulus infrasternalis). The convergence point of the right and left costal margins. At the apex of this angle, the xiphoid process (processus xiphoideus) is easily palpable.
- Iliac crest (crista iliaca) and its anterior superior iliac spine (spina iliaca anterior superior).
- Pubic symphysis (symphysis pubica) and the pubic tubercle (tuberculum pubicum) located slightly lateral to it.
- Umbilicus. Normally located precisely at the midpoint of the distance between the xiphoid process and the pubic symphysis.
- Inguinal fold. On the skin surface, it corresponds to the projection of the inguinal ligament. Clinically important: in patients with marked obesity, this skin fold may descend below the true position of the ligament, which must be considered when performing surgical incisions.
- Striae (striae gravidarum). Whitish stripes on the skin caused by stretching and micro-tears of fibers (often persisting after pregnancy).
Scheme of Division into Floors and Regions
To accurately describe the localization of pain or the position of organs, the anterior abdominal wall is delineated by conventional lines into 3 floors and 9 regions (on the principle of a tic-tac-toe grid).
Horizontal lines (forming floors):
- Intercostal line (linea bicostarum). Connects the lowest points of the 10th (X) ribs. Projects at the upper border of the third lumbar vertebra (L3). Separates the upper floor from the middle floor.
- Interspinous line (linea bispinarum). Runs between the two anterior superior iliac spines. Corresponds to the upper border of the second sacral vertebra (S2). Separates the middle floor from the lower floor.
These two lines divide the abdomen into the epigastrium (epigastrium), mesogastrium (mesogastrium), and hypogastrium (hypogastrium).
Vertical lines (forming regions): Drawn on both sides from the pubic tubercles strictly superiorly, running along the lateral borders of the rectus abdominis muscles up to their intersection with the costal margin.
As a result of the intersection of all lines, 9 regions are formed:
Unpaired (lying centrally, from top to bottom):
- Epigastric region (regio epigastrica).
- Umbilical region (regio umbilicalis).
- Pubic region (regio pubica).
Paired (located laterally, on the right and left):
- Hypochondriac regions (regiones hypochondricae dextra et sinistra).
- Lateral abdominal regions (regiones abdominales laterales dextra et sinistra).
- Inguinal regions (regiones inguinales dextra et sinistra).
Briefly: answers to topical questions
What is the difference between intraperitoneal organ placement and meso- and extraperitoneal placement?
The difference lies in the degree of coverage of the organs by the visceral layer of the peritoneum and the presence of a mesentery.
| Peritoneal relations | Degree of Coverage | Presence of Mesentery |
|---|---|---|
| Intraperitoneal | On all sides | Have a mesentery |
| Mesoperitoneal | On three sides | No |
| Extraperitoneal | On one side or not covered at all | No |
Which vessels and nerves pass through the retroperitoneal space?
The following neural structures and blood vessels pass through the retroperitoneal space.
Nerves and plexuses:
- Subcostal nerve (n. intercostalis XII)
- Iliohypogastric and ilioinguinal nerves (nn. iliohypogastricus et ilioinguinalis)
- Abdominal aortic plexus (plexus aorticus abdominalis)
- Genitofemoral nerve (n. genitofemoralis)
- Lateral cutaneous nerve of thigh (n. cutaneus femoris lateralis)
- Superior gluteal nerve (n. gluteus superior)
- Obturator nerve (n. obturatorius)
- Sacral plexus (plexus sacralis)
- Femoral nerve (n. femoralis)
- Lumbar ganglia of sympathetic trunk
Vessels:
- Femoral artery (a. femoralis)
- Femoral vein (v. femoralis)
Which muscles form the anterolateral abdominal wall and what are their attachment points?
The anterolateral abdominal wall is formed by five pairs of muscles, although attachment points are not described for all of them in sources.
- Rectus abdominis muscles
- Pyramidalis muscles (m. pyramidalis)
- Internal oblique muscle (m. obliquus internus abdominis)
- External oblique muscle (m. obliquus externus abdominis) — originates from the external surface of the thoracic cage (lower 8 ribs), attaches inferiorly.
- Transversus abdominis muscle (m. transversus abdominis) — originates from the internal surface of the cartilages of the lower 6 ribs, the deep layer of the thoracolumbar fascia, and the iliac crest.
To memorize the horizontal division lines, use vertebral landmarks: Intercostal line (Bicostarum) is L3 (the number 3 looks like rounded ribs). Interspinal line (Bispinarum) is S2 (both start with the letter S).
Class and exam questions
Why do the internal boundaries of the abdomen not coincide with the external ones?
Superiorly, the dome of the diaphragm extends deeply into the thoracic cavity, so some abdominal organs are hidden behind the ribs. Inferiorly, the abdominal cavity does not end at the level of the inguinal ligaments, but continues into the pelvic cavity past the pelvic brim (linea terminalis).
What is the difference between the abdominal cavity and the peritoneal cavity?
The abdominal cavity is the entire internal volume bounded by the walls. The peritoneal cavity is only a part of the abdominal cavity, representing a slit-like space lined by the peritoneum. Everything lying behind it (e.g., kidneys) belongs to the retroperitoneal space.
What should a surgeon rely on when locating the inguinal ligament in an obese patient?
You should rely on bony prominences (anterior superior iliac spine and pubic tubercle). The inguinal skin fold in such patients is displaced by gravity and does not correspond to the true anatomical projection of the ligament.
What else to study on this topic
- Projection of internal organs onto the 9 regions of the anterior abdominal wall
- Topography and layers of the anterolateral abdominal wall
- Weak spots of the abdominal wall and hernia anatomy
- Blood supply and innervation of the abdominal wall
- Relationship of internal organs to the peritoneum (intra-, meso-, and extraperitoneal)
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