The anterior and lateral walls of the abdomen are reinforced by four pairs of sheetlike muscles that support the viscera, stabilize the vertebral column during heavy lifting, and aid in respiration, urination, defecation, vomiting, and childbirth. They are the rectus abdominis, external abdominal oblique, internal abdominal oblique, and transversus abdominis (table 10.6; figs. 10.14-10.16).
The rectus38 abdominis is a medial straplike muscle extending vertically from the pubis to the sternum. It is separated into four segments by fibrous tendinous intersections that give the abdomen a segmented appearance in well-muscled individuals. The rectus abdominis is enclosed in a fibrous sleeve called the rectus sheath, and the right and left muscles are separated by a vertical fibrous strip called the linea alba.39
The external abdominal oblique is the most superficial muscle of the lateral abdominal wall. Its fascicles run anteriorly and downward. Deep to it is the internal abdominal oblique, whose fascicles run anteriorly and upward. Deepest of all is the transversus abdominis, whose fascicles run horizontally across the abdomen. Unlike the thoracic cavity, the abdominal cavity lacks a protective bony enclosure. However, the wall formed by these three muscle layers is strengthened by the way their fascicles run in different directions like layers of plywood.
The tendons of the abdominal muscles are aponeu-roses. They continue medially to form the rectus sheath and terminate at the linea alba. At its inferior margin, the
38rect = straight
Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
Chapter 10 The Muscular System 347
Table 10.6 Muscles of the Abdomen (see figs. 10.14 and 10.15)
O = origin, I = insertion, N = innervation (n. = nerve, nn. = nerves) Rectus Abdominis (ab-DOM-ih-niss)
Supports abdominal viscera; flexes waist as in sit-ups; depresses ribs; stabilizes pelvis during walking; increases intra-abdominal pressure to aid in urination, defecation, and childbirth O: pubis I: xiphoid process, costal cartilages 5-7 N: intercostal nn. 7-12
External Abdominal Oblique
Flexes waist as in sit-ups; flexes and rotates vertebral column
O: ribs 5-12 I: xiphoid process, linea alba N: intercostal nn. 8-12, iliohypogastric n., ilioinguinal n.
Internal Abdominal Oblique
Similar to external oblique
O: inguinal ligament, iliac crest, I: xiphoid process, linea alba, pubis, ribs 10-12 N: same as external oblique thoracolumbar fascia
Compresses abdomen, increases intra-abdominal pressure, flexes vertebral column O: inguinal ligament, iliac crest, thoracolumbar I: xiphoid process, linea alba, pubis, fascia, costal cartilages 7-12 inguinal ligament
N: intercostal nn. 8-12, iliohypogastric n., ilioinguinal n.
aponeurosis of the external abdominal oblique forms a strong, cordlike inguinal ligament that extends from the pubis to the anterior superior spine of the ilium.
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