Osteology

The thoracic cage covers all or parts of the upper abdominal organs (e.g., liver stomach, spleen, kidneys). The abdominal skeleton consists primarily of the lumbar vertebrae, the margin of the thoracic cage, and the iliac and pubic bone elements of the pelvis (Fig. 6.1). These structures provide attachment sites for the abdominal wall muscles and limited support for abdominal organs, but little direct protection for those organs.

Lumbar Vertebrae

The anatomy of the lumbar vertebrae is described in detail in Chapter 2. The body of each vertebra (Fig. 6.2) is massive and kidney shaped and bears the greater part of the body weight. The 5th lumbar vertebra articulates with the base of the sacrum at the lumbosacral joint. The transverse processes are long and slender and provide sites of attachment for several abdominal wall muscles.

The intervertebral discs (Fig. 6.3) in the lumbar region are thicker than in other regions of the vertebral column. They are wedge shaped and are responsible for the normal posterior concavity (lordosis) in the curvature of the vertebral column in the lumbar region.

Thoracic Cage

The anatomy of the thoracic cage is described in detail in Chapter 4. The components most pertinent to the abdominal wall include the xiphoid process of the sternum, costal margin (costal cartilages 7 to 10), and lower ribs (see Fig. 6.1). The head of the 12th rib is different from most others in that it has a single facet for articulation with the body of the 12th thoracic vertebra. The anterior end is pointed and has a small costal cartilage, which is embedded in the musculature of the anterior abdominal wall. In many people, it is so short that it fails to protrude beyond the lateral border of the erector spinae muscle on the back.

Pelvis

The os coxae (hip bone) is composed of the ilium, ischium, and pubis (Fig. 6.4; also see Fig. 6.1). The medial surface of the ilium is divided into two parts by the arcuate line. The iliac fossa is a concave surface above the arcuate line. A flattened surface that is continuous with the medial surfaces of the pubis and ischium lies below the arcuate line. Note that the arcuate line of the ilium forms the posterior part of the iliopectineal line, whereas the pectineal line of the pubis forms the anterior part of the iliopectineal line. The iliopectineal line demarcates the false from the true pelvis. See Chapter 8 for further details on the structure of the hip bone.

ANTEROLATERAL ABDOMINAL WALL

The anterolateral abdominal wall is made up of seven layers: Skin Superficial fascia Deep fascia Muscles Transversalis fascia Extraperitoneal fascia Parietal peritoneum

General Appearance of the Abdominal Wall

The normal abdominal wall is soft and pliable and undergoes inward and outward excursion with respiration. The contour is subject to considerable variation and depends on the tone of its muscles and the amount of fat in the subcutaneous tissue. Well-developed muscles or an abundance of fat can prove to be a severe obstacle to the palpation of the abdominal viscera.