Learning Objectives
By the end of this section, you will be able to:
Identify the following muscles and give their origins, insertions, actions and innervations:
- Axial muscles of the abdominal wall and thorax
AXIAL MUSCLES OF THE ABDOMINAL WALL AND THORAX
It is a complex job to balance the body on two feet and walk upright. The muscles of the vertebral column, thorax, and abdominal wall extend, flex, and stabilize different parts of the body’s trunk. The deep muscles of the body’s core help maintain posture as well as provide stability for movement of the limbs.
Muscles of the Abdomen
There are four pairs of abdominal muscles that make up the abdominal wall: the rectus abdominis, the external abdominal obliques, the internal abdominal obliques and the transverse abdominis (Figure 11.4.9 and Table 11.6).
| Muscles of the Abdomen (Table 11.6) | |||||
|---|---|---|---|---|---|
| Movement | Target | Target motion direction | Prime mover | Origin | Insertion |
| Twisting at waist; also bending to the side | Vertebral column | Supination; lateral flexion | External obliques; internal obliques | Ribs 5–12; ilium | Ribs 7–10; linea alba; ilium |
| Squeezing abdomen during forceful exhalations, defecation, urination, and childbirth | Abdominal cavity | Compression | Transversus abdominus | Ilium; ribs 5–10 | Sternum; linea alba; pubis |
| Sitting up | Vertebral column | Flexion | Rectus abdominis | Pubis | Sternum; ribs 5 and 7 |
| Bending to the side | Vertebral column | Lateral flexion | Quadratus lumborum | Ilium; ribs 5–10 | Rib 12; vertebrae L1–L4 |
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There are three flat skeletal muscles in the antero-lateral wall of the abdomen. The external oblique, closest to the surface, extend inferiorly and medially, in the direction of sliding one’s four fingers into pants pockets. Perpendicular to it is the intermediate internal oblique, extending superiorly and medially, the direction the thumbs usually go when the other fingers are in the pants pocket. The deep muscle, the transverse abdominis, is arranged transversely around the abdomen, similar to a belt. This arrangement of three bands of muscles in different orientations allows various movements and rotations of the trunk. The three layers of muscle also help to protect the internal abdominal organs in an area where there is no bone.
The linea alba is a white, fibrous band that is made of the bilateral rectus sheaths (see Figure 11.4.9) that join at the anterior midline of the body. These enclose the rectus abdominis muscles that originate at the pubic crest and symphysis, and extend the length of the body’s trunk. Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections resulting in the look of “six-pack abs”.
The posterior abdominal wall is formed by the lumbar vertebrae, parts of the ilia of the hip bones, psoas major and iliacus muscles, and quadratus lumborum muscle. This part of the core plays a key role in stabilizing the rest of the