The axial muscles fall into logical groups on the basis of location, function, or both. The groups do not always have distinct anatomical boundaries. For example, a function such as extension of the vertebral column involves muscles along its entire length and flexion at each of the intervertebral joints. We will discuss the axial muscles in four groups:
- The muscles of the head and neck. This group includes muscles that move the face, tongue, and larynx. They are therefore responsible for verbal and nonverbal communication—laughing, talking, frowning, smiling, whistling, and so on. You also use this group of muscles when you eat—especially in sucking and chewing—and even when you look for something to eat, by controlling your eye movements. This group does not include muscles of the neck that are involved with movements of the vertebral column.
- The muscles of the spine. This group includes numerous flexors, extensors, and rotators of the vertebral column.
- The oblique and rectus muscles. This group forms the muscular walls of the thoracic and abdominopelvic cavities between the first thoracic vertebra and the pelvis. In the thoracic area these muscles are partitioned by the ribs, but over the abdominal surface they form broad muscular sheets. The neck also has oblique and rectus muscles. Although they do not form a complete muscular wall, they share a common developmental origin with the oblique and rectus muscles of the trunk.
 EMBRYOLOGY SUMMARY 9: The Development of the Muscular System
- The muscles of the pelvic floor. These muscles extend between the sacrum and pelvic girdle. This group forms the perineum, a muscular sheet that closes the pelvic outlet.
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