Structure and Location
The central tendon occupies the middle of the unpaired dome-shaped muscle—the diaphragm—which seals the inferior thoracic aperture and separates the thoracic and abdominal cavities.
- Shape: The structure appears as a triangular, pale plate resembling a trefoil.
- Feature: The center contains no muscle fibers.
The height of the diaphragmatic domes depends on several factors:
- Dome Projection: The right dome sits higher than the left—at the level of the 4th intercostal space, whereas the left sits at the upper border of the 5th rib anteriorly. Posteriorly, the projection corresponds to the 9th intercostal space.
- Thoracic Shape: In a short and wide thorax, the diaphragm sits high; in a long and narrow thorax, it sits low.
- Age and Pathology: With age, as well as in the presence of pleural or peritoneal effusion, the diaphragmatic dome flattens.
Relationship with Diaphragmatic Muscular Parts
Muscle fibers from all three parts of the diaphragm converge on the central tendon:
- Sternal part (pars sternalis) — a narrow segment originating from the posterior surface of the xiphoid process.
- Costal part (pars costalis) — the largest part, forming the lateral walls of the domes. It originates from the inner surfaces of the lower six ribs and their cartilages.
- Lumbar part (pars lumbalis) — consists of right and left crura (crus dextrum et sinistrum), originating from the bodies of the upper lumbar vertebrae (L1–L3/L4), as well as arcuate ligaments.
What Passes Through the Central Tendon?
The central tendon houses one of the most critical apertures of the diaphragm:
- Vena caval foramen (foramen venae cavae).
- Vertebral level: Th8.
- Contents: Transmits the inferior vena cava (v. cava inferior) and frequently branches of the right phrenic nerve (n. phrenicus).
- Mechanism of action: The margins of the aperture are formed by the fibrous tissue of the central tendon. Consequently, when the diaphragm contracts during inspiration, the foramen does not constrict, facilitating venous blood return.
Unlike the vena caval foramen located within the central tendon, the other major diaphragmatic openings—the esophageal hiatus at Th10 and the aortic hiatus at Th12—are formed by muscular fibers of the crura or lie posterior to the diaphragm. The muscularly bounded esophageal hiatus functions as a sphincter.
Topography and Serous Coverings
The central tendon contacts organs of both the thoracic and abdominal cavities:
- Anteriorly: The central tendon relates to the pericardium.
- Thoracic surface: Covered by the parietal pleura via the endothoracic fascia (fascia endothoracica).
- Abdominal surface: Covered by peritoneum via the subserous fascia. Exceptions include areas devoid of peritoneal covering: the bare area of the liver (area nuda), along with contact areas for the kidneys, suprarenal glands, and esophagus.
Not to Be Confused: Perineal Body
The term "central tendon" is also applied to a pelvic floor structure—the perineal body (centrum tendineum perinei). This is a central fibromuscular node where fibers of several perineal muscles converge, intertwine, and decussate:
- Bulbospongiosus muscle (m. bulbospongiosus — posterior part);
- External anal sphincter (m. sphincter ani externus);
- Superficial transverse perineal muscle (m. transversus perinei superficialis).
This area is reinforced by tendinous fibers, ensures the functional interdependence of these muscles, and serves as a vital surgical landmark.