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Hypaxial muscles


The hypaxial muscles develop from the myotomal hypomere of each somite and form the lateral and ventral muscle wall of the trunk; that is, the neck, the thorax, the abdomen, and the pelvis. As the somitic hypomeres migrate to form this muscular trunk wall, a distinct pattern emerges at each of the somitic segments. This common muscle pattern is present in the anterior and lateral muscles of the neck, the thorax, the abdomen, and in a modified form in the muscular floor of the pelvis. Each somite contributes six basic muscles, per side, to the trunk wall. The six muscles are a strap-like ventral muscle, a series of four superficial to deep sheet-like muscles laterally, and a cord-like subvertebral muscle. This simple, eloquent design runs the entire length of the trunk. Understanding and recognizing this pattern of design not only clarifies trunk wall anatomy, but also helps simplify the task of learning the myriad of trunk hypaxial muscles. Remember, there are six basic muscles in the anterior and lateral wall of the embryonic trunk that develop to form the muscle wall of the adult trunk — a ventral muscle that spans from the chin to the pubis, a four-layered lateral muscle wall present in all regions (The outermost layer, named the supracostal musculature, is adapted and modified in mammalian vertebrates in a variety of ways. In the cervical and upper thoracic region, the upper limb annexes the outer muscle layer to support the pectoral girdle and bind it to the trunk. Because of the modifications that arise in this outer layer, it will not be grouped as one of the distinct layers of the hypaxial trunk wall.), and a subvertebral muscle on the ventral surface of the vertebral column. All of the named hypaxial muscles in each trunk region develop from one of these six basic embryonic muscles. These hypaxial trunk muscles are the flexors and rotators of the vertebral column. They also support the internal viscera of the abdomen and thorax and play important roles in respiration, vocalization, urination, and defecation. The anterior (ventral) ramus of each spinal nerve is the nerve supply to all of the hypaxial muscles.


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