In adult vertebrates, trunk muscles can be broadly divided into hypaxial muscles, which lie ventral to the horizontal

septum In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interatri ...

of the vertebrae and epaxial muscles, which lie dorsal to the septum. Hypaxial muscles include some vertebral muscles, the

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

, the abdominal muscles, and all limb muscles. The serratus posterior inferior and

serratus posterior superior The serratus posterior superior muscle is a thin, quadrilateral muscle. It is situated at the upper back part of the thorax, deep to the rhomboid muscles. Structure The serratus posterior superior muscle arises by an aponeurosis from the l ...

are innervated by the ventral primary ramus and are hypaxial muscles. Epaxial muscles include other (dorsal) muscles associated with the vertebrae, ribs, and base of the skull. In humans, the

erector spinae The erector spinae ( ) or spinal erectors is a set of muscles that straighten and rotate the back. The spinal erectors work together with the glutes (gluteus maximus, gluteus medius and gluteus minimus) to maintain stable posture standing or sittin ...

, the transversospinales (including the multifidus, semispinalis and

rotatores The rotatores muscles (rotatores spinae muscles) lie beneath the multifidus and are present in all spinal regions but are most prominent in the thoracic region; they are eleven in number on either side. Each muscle is small and somewhat quadrila ...

), the splenius and suboccipital muscles are the only epaxial muscles. Hypaxial and epaxial muscles develop directly from somitic cells. Differentiation of hypaxial and epaxial muscles is postulated to have evolved as a new trait in vertebrate animals.

Location

The hypaxial muscles are located on the ventral side of the body, often below the horizontal

in many species (primarily fish and amphibians). In all species, the hypaxial muscles are innervated by the ventral ramus (branch) of the spinal nerves, while the epaxial muscles are innervated by the dorsal ramus.

References

Muscular system {{Muscle-stub