In adult

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s, trunk muscles can be broadly divided into hypaxial muscles, which lie

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

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 The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

and epaxial muscles, which lie

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

to the septum. Hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles. The

serratus posterior inferior The serratus posterior inferior muscle, also known as the posterior serratus muscle, is a muscle of the human body. Structure The muscle is situated at the junction of the thoracic and lumbar regions. It has an irregularly quadrilateral form, ...

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 The ventral ramus (pl. ''rami'') (Latin for ''branch'') is the anterior division of a spinal nerve. The ventral rami supply the antero-lateral parts of the trunk and the limbs. They are mainly larger than the dorsal rami. Shortly after a spinal n ...

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

transversospinales The transversospinales are a group of muscles of the human back. Their combined action is rotation and extension of the vertebral column. These muscles are small and have a poor mechanical advantage for contributing to motion. They include: * the ...

(including the

multifidus The multifidus (multifidus spinae : ''pl. multifidi'' ) muscle consists of a number of fleshy and tendinous fasciculi, which fill up the groove on either side of the spinous processes of the vertebrae, from the sacrum to the axis. While very th ...

, semispinalis and rotatores), the

splenius The splenius muscles are: *Splenius capitis muscle *Splenius cervicis muscle Their origins are in the upper thoracic and lower cervical spinous process The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are ...

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 A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

by the

ventral ramus The ventral ramus (pl. ''rami'') (Latin for ''branch'') is the anterior division of a spinal nerve. The ventral rami supply the antero-lateral parts of the trunk and the limbs. They are mainly larger than the dorsal rami. Shortly after a spinal n ...

(branch) of the spinal nerves, while the epaxial muscles are innervated by the

dorsal ramus The dorsal ramus of spinal nerve (or posterior ramus of spinal nerve, or posterior primary division) is the posterior division of a spinal nerve. The dorsal ramus (Latin for branch, plural ''rami'' ) is the dorsal branch of a spinal nerve that form ...

References

Muscular system {{Muscle-stub