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Thoracic hypaxial muscles
The muscles of the thoracic cage develop from the ventral portions of the thoracic somites of the embryo. Like the neck and abdomen they clearly demonstrate the muscle pattern of the vertebrate body wall. Unlike the neck and abdomen, however, the thoracic body wall has well-developed ribs that dominate the wall and limit the movements of the vertebral column. Because of the well-developed segmental ribs, the muscles of the thoracic wall retain their segmental origins. Primitively, these intercostal muscles were the major muscles of locomotion in vertebrate animals. They generated the lateral undulating movements of the vertebral column that early vertebrates used to propel themselves forward. In the human body these muscles have a variety of functions. They help to stiffen the thoracic cage and they help to produce the rib movements that increase and decrease the thoracic volume, activities, both of which, help increase respiratory efficiency. The uniquely mammalian diaphragm muscle is a complex member of this group that plays an important role in respiration. The outermost layer of the lateral muscle wall is well developed in the thorax. Some portions of this muscle layer remain associated with the ribs, while the large serratus anterior muscle migrates onto the scapula to become a principal anchor of the upper limb to the trunk. For more information about the serratus anterior, see the muscles of the superior limb. The ventral ramus of each of the thoracic spinal nerves innervate s all of the muscles of their associated segment.