Rectus abdominis muscle
The rectus abdominis muscle (musculus rectus abdominis) is a long and powerful muscular band positioned vertically on either side of the midline.
Attachment points:
- Origin (Origo): muscle fascicles originate from the anterior surface of the pubic bone (in the region of the pubic crest) and from the pubic symphysis.
- Insertion (Insertio): passing superiorly, the fibers attach to the xiphoid process of the sternum as well as to the cartilaginous parts of the 5th, 6th, and 7th ribs.
Structural features: A specific characteristic of this muscle is the presence of three or four tendinous intersections (intersectiones tendineae). These connective tissue bands cross the muscle belly transversely. Most commonly, they are localized at the level of the costal margin, the umbilical ring, and midway between these two landmarks. Throughout its entire length, the rectus muscle is enclosed in a specialized tendinous sheath.
Biomechanics and functions
The rectus abdominis muscle is a crucial element of the trunk's biomechanical chain. Upon contraction, it performs the following actions:
- Executes trunk flexion by powerfully pulling the thoracic cage toward the pelvic ring.
- When the rib cage is fixed, it stabilizes the pelvis, preventing its excessive extension (backward tilt).
- Actively participates in increasing intra-abdominal pressure, thereby enhancing abdominal muscle tone.
- Acts as an accessory respiratory muscle, facilitating forced expiration.
Blood supply and innervation
Neural regulation of the rectus abdominis is provided by branches of the intercostal nerves (nn. intercostales) originating from the thoracic segments of the spinal cord (Th7–Th12), as well as the iliohypogastric nerve (n. iliohypogastricus), which is formed from fibers of Th12 and the first lumbar segment (L1).
The arterial bed is represented by two large vessels:
- The superior epigastric artery (a. epigastrica superior), which is a continuation of the internal thoracic artery (a. thoracica interna).
- The inferior epigastric artery (a. epigastrica inferior), branching off from the external iliac artery (a. iliaca externa).
Both of these arteries penetrate into the thickness of the muscle fibers, where they form a prominent vascular anastomosis with each other.
Pyramidalis muscle
The pyramidalis muscle (musculus pyramidalis) is a small triangular muscle located at the very bottom of the anterior abdominal wall, anterior to the lower part of the rectus abdominis. It is worth noting that this structure is variable and may be absent in some individuals.
- Origin: pubic bone (anterior to the origin of the rectus abdominis).
- Insertion: muscle fascicles blend into the linea alba (linea alba).
- Innervation: supplied by the lower branches of the intercostal nerves (nn. intercostales).
- Function: upon contraction, the muscle tensions the linea alba. In clinical practice, it serves as an important anatomical landmark for surgical approaches in the lower abdomen.
Briefly: answers to topical questions
How are the anterior and posterior walls of the rectus sheath formed?
- Above the umbilicus (above the linea arcuata):
- Anterior wall: aponeurosis of the external oblique and the anterior lamina of the internal oblique aponeurosis.
- Posterior wall: posterior lamina of the internal oblique aponeurosis and the aponeurosis of the transversus abdominis.
- Below the umbilicus (below the linea arcuata):
- Anterior wall: aponeuroses of all three flat muscles (external oblique, internal oblique, transversus abdominis) pass anterior to the rectus muscle.
- Posterior wall: the posterior wall of the sheath is absent; the muscle lies directly against the transversalis fascia and peritoneum.
What anatomical structures form the linea alba?
- The linea alba (linea alba) is a tendinous plate and fibrous cord formed by the interlacing of tendinous fibers from the aponeuroses of all three pairs of flat abdominal muscles: the external oblique, internal oblique, and transversus abdominis. It is located between the rectus abdominis muscles and connects the xiphoid process to the pubic symphysis.
Where does the linea semilunaris (Spigelian line) run?
- The linea semilunaris (linea semilunaris) is a C-shaped line marking the transition of the muscular part of the transversus abdominis into its aponeurotic part. Clinically and anatomically, it represents the lateral border of the rectus sheath.
To memorize the arteries of the rectus muscle: "The superior came from the chest (a. thoracica interna), the inferior ascended from the pelvis (a. iliaca externa) — and they met in the muscle."
Class and exam questions
Where are the tendinous intersections of the rectus abdominis normally located?
Tendinous intersections (intersectiones tendineae) are most commonly projected at the level of the costal margin, the umbilicus, and midway between them.
Which vessels form the anastomosis within the rectus muscle?
Within the thickness of the muscle, the superior epigastric artery (from the internal thoracic artery system) and the inferior epigastric artery (a branch of the external iliac artery) anastomose.
Which structure is tensioned upon contraction of the pyramidalis muscle?
The pyramidalis muscle inserts into the linea alba and tensions it upon contraction.
What else to study on this topic
- Structure of the rectus sheath
- Formation of the linea alba
- Clinical significance of the epigastric artery anastomosis
- Topography of intercostal nerves on the anterior abdominal wall
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