Trunk muscles can be broadly divided into hypaxial muscle, which lie ventral to the horizontal septum of the vertebrae and epaxial muscles, which lie dorsal to the septum. Hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles. Epaxial muscles include other (dorsal) muscles associated with the vertebrae, ribs, and base of the skull. In humans, the erector spinae

Erector spinae

The Erector spinæ is a muscle group of the back in humans and animals. It is also known as sacrospinalis in older texts. A more modern term is extensor spinae, though this is not in widespread use. The name of the muscle is pronounced e-rec-tor speen-aye, or e-rec-tor spinae-ee.It is really not...

, the transversospinal muscles (including the multifidus, semispinalis and rotatores), the splenius and suboccipital muscles are the only epaxial muscles.

Location

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

Septum

In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...

 in many species (primarily fish and amphibians). In amniote

Amniote

The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s, the positional distinctions have been lost as the body's muscles have changed through evolution, but in all species, the hypaxial muscles are innervated by the ventral ramus of the spinal nerves, while the epaxial muscles are innervated by the dorsal ramus.